GIFT  OF 
SEELEY  W.  MUDD 

and 

GEORGE  1.  COCHRAN     MEYER  ELSASSER 
DR.  JOHN  R.  HAYNES    WILLIAM  L.  HONNOLD 
JAMES  R.  MARTIN         MRS.  JOSEPH  F.  SARTORI 

to  tin 

UNIVERSITY  OF  CALIFORNIA 
SOUTHERN  BRANCH 


JOHN  FISKE 


This  book  is  DUE  on  the  last  date  stamped  below 

1 


THE    PRINCIPLES    OF 
BIOLOGY 


BY 
HERBERT    SPENCER 


IN   TWO    VOLUMES 
VOL.  II 

REVISED   AND   ENLARGED   EDITION 
1899 


NEW    YORK 

D.    APPLETON    AND    COMPANY 
1900 

9803 


COPYRIGHT,  1867,  1899, 
BY  D.  APPLETON  AND  COMPANY. 


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PREFACE 

TO  THE  REVISED  AND  ENLARGED  EDITION 
OF  VOL.  II. 


To  the  statements  made  in  the  preface  to  the  first  volume 
of  this  revised  edition,  there  must  here  be  added  a  few  hav- 
ing special  reference  to  this  second  volume. 

One  of  them  is  that  the  revision  has  not  been  carried 
out  in  quite  the  same  way,  but  in  a  way  somewhat  less  com- 
plete. When  reviewing  the  first  volume  a  friendly  critic, 
Prof.  Lloyd  Morgan,  said : — 

"  But  though  the  intellectual  weight  has  also  been  augmented,  it 
is  an  open  question  whether  it  would  not  have  been  wiser  to  leave 
intact  a  treatise,  &c.  .  .  .  relegating  corrections  and  additions  to 
notes  and  appendices." 

I  think  that  Prof.  Morgan  is  right.  Though  at  the  close 
of  the  preface  to  volume  I,  I  wrote  : — "in  all  sections  not 
marked  as  new,  the  essential  ideas  set  forth  are  the  same  as 
they  were  in  the  original  edition  of  1864,"  yet  the  reader 
who  has  not  read  this  statement,  or  does  not  bear  it  in  mind, 
will  suppose  that  all  or  most  of  the  enunciated  conceptions 
are  of  recent  date,  whereas  only  a  small  part  of  them  are. 
I  have  therefore  decided  to  follow,  in  this  second  volume,  a 
course  somewhat  like  that  suggested  by  Prof.  Morgan — 
somewhat  like,  I  say,  because  in  sundry  cases  the  amend- 
ments could  not  be  satisfactorily  made  by  appended  notes. 


vi  PREFACE  TO  THE  REVISED  EDITION. 

But  there  has  been  a  further  reason  for  this  change  of 
method.  An  invalid  who  is  nearly  eighty  cannot  with  pru- 
dence enter  upon  work  which  will  take  long  to  complete. 
Hence  I  have  thought  it  better  to  make  the  needful  altera- 
tions and  additions  in  ways  requiring  relatively  moderate 
time  and  labour. 

The  additions  made  to  this  volume  are  less  numerous  and 
less  important  than  those  made  to  the  first  volume.  A  new 
chapter  ending  Part  V,  on  "  The  Integration  of  the  Or- 
ganic World,"  serves  to  round  off  the  general  theory  of 
Evolution  in  its  application  to  living  things.  Beyond  a  new 
section  (§  289#)  and  the  various  foot-notes,  serving  chiefly 
the  purpose  of  elucidation,  there  are  notes  of  some  signifi- 
cance appended  to  Chapters  I,  III,  IV,  and  Y,  in  Part  IY, 
Chapters  Y  and  VIII,  in  Part  Y,  and  Chapters  IX,  X,  and 
XII  in  Part  VI.  Moreover  there  are  three  further  appen- 
dices, Ds,  F,  and  G,  which  have,  I  think,  considerable  sig- 
nificance as  serving  to  make  clearer  some  of  the  views 
expressed  in  the  body  of  the  work. 

Turning  from  the  additions  to  the  revisions,  I  have  to 
say  that  the  aid  needed  for  bringing  up  to  date  the  contents 
of  this  volume,  has  been  given  me  by  the  gentlemen  who 
gave  me  like  aid  in  revising  the  first  volume  :  omitting 
Prof.  Perkin,  within  whose  province  none  of  the  contents 
of  this  volume  fall.  Plant-Morphology  and  Plant-Physi- 
ology have  been  overseen  by  Mr.  A.  G.  Tansley.  Criti- 
cisms upon  parts  dealing  with  Animal  Morphology  I  owe 
to  Mr.  J.  T.  Cunningham  and  Prof.  E.  W.  MacBride. 
And  the  statements  included  under  Animal  Physiology 
have  been  checked  by  Mr.  W.  B.  Hardy. 


PREFACE  TO   THE  REVISED   EDITION.  vii 

For  reasons  like  those  named  in  the  preface  to  the  first 
volume,  I  have  not  submitted  the  proofs  of  this  revised 
second  volume  to  these  gentlemen :  a  fact  which  it  is  need- 
ful to  name,  since  one  or  other  of  them  might  else  be  held 
responsible  for  some  error  which  is  not  his  but  mine.  It  is 
the  more  requisite  to  say  this  because  while,  in  respect  of 
matters  of  fact,  I  have,  save  in  one  or  two  cases,  accepted 
their  corrections  as  not  to  be  questioned,  I  have  not  always 
done  this  in  respect  of  matters  of  inference,  but  in  sundry 
places  have  adhered  to  my  own  interpretations. 

Perhaps  I  may  be  excused  for  expressing  some  satisfac- 
tion that  I  have  not  been  obliged  to  relinquish  the  views  set 
forth  in  1864-7.  The  hypothesis  of  physiological  units — or, 
as  I  would  now  call  them,  constitutional  units — has  been 
adopted  by  several  zoologists  under  modified  forms.  So  far 
as  I  am  aware,  the  alleged  general  law  of  organic  symmetry 
has  not  called  forth  any  manifestations  of  dissent.  The 
suggested  theory  of  vertebrate  structure  appears  to  have 
become  current ;  and  from  the  investigations  of  the  late 
Prof.  Cope,  has  received  verification.  The  conclusions 
drawn  in  Part  YI  on  "  The  Laws  of  Multiplication,"  have 
not,  I  believe,  been  controverted.  And  though  only  some 
works  on  botany  have  given  currency  to  the  doctrine  set 
forth  in  Appendix  C,  "  On  Circulation  and  the  Formation 
of  Wood  in  Plants,"  yet  I  have  met  with  no  attempt  to  dis- 
prove it.  The  only  views  contested  by  certain  of  the  gen- 
tlemen above  named,  are  those  concerning  the  origin  of 
the  two  great  phsenogamic  types  of  plants,  and  the  origin 
of  the  annulose  type  of  animals.  I  have  not,  however, — 
perhaps  because  of  natural  bias — found  myself  compelled 


viii  PREFACE  TO   THE  REVISED   EDITION. 

to  surrender  these  views.  My  reasons  for  adhering  to  them 
will  be  found  in  notes  to  the  ends  of  Chapters  III  and  IV 
in  Part  IV,  and  in  Appendix  D9. 

On  now  finally  leaving  biological  studies,  it  remains  only 
to  say  that  I  am  glad  I  have  survived  long  enough  to  give 
this  work  its  finished  form. 

BRIGHTON, 

October,  1899. 


PREFACE  TO  VOL.   II. 


THE  proof  sheets  of  this  volume,  like  those  of  the  last 
volume,  have  been  looked  through  by  Dr.  Hooker  and 
Prof.  Huxley ;  and  I  have,  as  before,  to  thank  them  for 
their  valuable  criticisms,  and  for  the  trouble  they  have 
taken  in  checking  the  numerous  statements  of  fact  on 
which  the  arguments  proceed.  The  consciousness  that 
their  many  duties  render  time  extremely  precious  to  them, 
makes  me  feel  how  heavy  is  my  obligation. 

Part  IV.,  with  which  this  volume  commences,  contains 
numerous  figures.  Nearly  one  half  of  them  are  repeti- 
tions, mostly  altered  in  scale  and  simplified  in  execution,  of 
figures,  or  parts  of  figures,  contained  in  the  works  of  vari- 
ous Botanists  and  Zoologists.  Among  the  authors  whom  I 
have  laid  under  contribution,  I  may  name  Berkeley,  Car- 
penter, Cuvier,  Green,  Harvey,  Hooker,  Huxley,  Milne- 
Edwards,  Ralfs,  Smith.  The  remaining  figures,  numbering 
150,  are  from  original  sketches  and  diagrams. 

The  successive  instalments  which  compose  this  volume, 
were  issued  to  the  Subscribers  at  the  following  dates : — No. 
13  (pp.  1—80)  in  January,  1865 ;  No.  14  (pp.  81—160)  in 
June,  1865  ;  No.  15  (pp.  161—240)  in  December,  1865  ;  No. 
16  (pp.  241—320)  in  June,  1866  ;  No.  17  (pp.  321—400)  in 
November,  1866  ;  and  No.  18  (pp.  401—566)  in  March,  1867. 

LONDON,  March  23rd,  1867. 


CONTENTS   OF  VOL.  II. 


PART  IV.— MORPHOLOGICAL   DEVELOPMENT. 

CHAP.  PAGE 

I. — THE   PROBLEMS   OF   MORPHOLOGY       .  .  .  ...  3 

II. — THE   MORPHOLOGICAL   COMPOSITION   OF   PLANTS          .1  .         17 

III. — THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS — Continued  .      37 

IV. — THE   MORPHOLOGICAL   COMPOSITION   OF   ANIMALS       ...         85 

V. — THE  MORPHOLOGICAL   COMPOSITION  OF  ANIMALS — Continued    111 
VI.— MORPHOLOGICAL  DIFFERENTIATION  IN  PLANTS       .        .        .128 

VII. — THE   GENERAL   SHAPES   OF   PLANTS 134 

VIII.— THE  SHAPES  OF  BRANCHES      .        .        .        .    '    .  .  .145 

IX. — THE  SHAPES  OF  LEAVES  .      . .        .       .      ^ .       .  .  .    152 

X.— THE  SHAPES  OF  FLOWERS       .;....  .  .161 

XL— THE  SHAPES  OF  VEGETAL  CELLS     .        .        .        .  .  .    175 

XII.— CHANGES  OF  SHAPE  OTHERWISE  CAUSED         .        .  .  .178 

XIII. — MORPHOLOGICAL  DIFFERENTIATION  IN  ANIMALS      .  .  .    183 

XIV.— THE   GENERAL   SHAPES    OF    ANIMALS  .  ...  .186 

XV. — THE  SHAPES  OF  VERTEBRATE  SKELETONS       ....    209 

XVI. — THE  SHAPES  OF  ANIMAL-CELLS 228 

XVII.— SUMMARY  OF  MORPHOLOGICAL  DEVELOPMENT  .  .    231 


PART   V.— PHYSIOLOGICAL   DEVELOPMENT. 

I.— THE   PROBLEMS   OF    PHYSIOLOGY 239 

II. — DIFFERENTIATIONS   BETWEEN   THE   OUTER   AND   INNER  TIS- 
SUES OF  PLANTS 244 

III. — DIFFERENTIATIONS  AMONG  THE  OUTER  TISSUES  OF  PLANTS   .    251 
IV. — DIFFERENTIATIONS  AMONG  THE  INNER  TISSUES  OF  PLANTS    .    272 

xi 


xli  CONTENTS. 

CHAP.  PAGE 

V. — PHYSIOLOGICAL  INTEGRATION  IN  PLANTS 292 

VI.— DIFFERENTIATIONS  BETWEEN  THE  OUTER  AND  INNER  TISSUES 

OF   ANIMALS 299 

VII.— DIFFERENTIATIONS  AMONG  THE  OUTER  TISSUES  OF  ANIMALS  .    309 

VIII. — DIFFERENTIATIONS  AMONG  THE  INNER  TISSUES  OF  ANIMALS  .    323 

IX.— PHYSIOLOGICAL  INTEGRATION  IN  ANIMALS      .        .        .        .373 

X. — SUMMARY  OF  PHYSIOLOGICAL  DEVELOPMENT   ....    384 

XA.— THE   INTEGRATION   OF   THE   ORGANIC   WORLD    ....      396 

PAET   VL— LAWS   OF   MULTIPLICATION. 

I.— THE  FACTORS .........       ,        .411 

II.— A  PRIORI  PRINCIPLE        .  .  ....  .,...-       .417 

III.— OBVERSE  A  PRIORI  PRINCIPLE     .        .        .       .       ,  .    424 

IV.— DIFFICULTIES  OF  INDUCTIVE  VERIFICATION    .        .  ,     ,  .432 

V. — ANTAGONISM  BETWEEN  GROWTH  AND  ASEXUAL  GENESIS  .    439 

VI.— ANTAGONISM  BETWEEN  GROWTH  AND  SEXUAL  GENESIS  .  .    448 

VII. — THE      ANTAGONISM     BETWEEN     DEVELOPMENT     AND     GENESIS, 

ASEXUAL   AND   SEXUAL .461 

VIII.— ANTAGONISM  BETWEEN  EXPENDITURE  AND  GENESIS       .        .  467 

IX. — COINCIDENCE  BETWEEN  HIGH  NUTRITION  AND  GENESIS  .        .  475 

X.— SPECIALITIES  OF  THESE  RELATIONS         .        .       ...  486 

XI. — INTERPRETATION  AND  QUALIFICATION     .        .        .       .        .  497 

"XII.— MULTIPLICATION  OF  THE  HUMAN  RACE 506 

XIII.— HUMAN  POPULATION  IN  THE  FUTURE 522 

APPENDICES. 

A.— SUBSTITUTION  OF  AXIAL  FOR  FOLIAR  ORGANS  IN  PLANTS   .       .    541 
B.— A  CRITICISM   ON  PROF.  OWEN'S   THEORY   OF   THE  VERTEBRATE 

SKELETON 548 

C.— ON  CIRCULATION  AND  THE   FORMATION  OF  WOOD  IN  PLANTS  .        .  567 

D.—ON   THE   ORIGIN   OF   THE   VERTEBRATE   TYPE 599 

D8.— THE   ANNULOSE  TYPE 602 

E.— THE  SHAPES  AND  ARRANGEMENTS  OF  FLOWERS    .        .        .        .608 
F.— PHYSIOLOGICAL  (OR  CONSTITUTIONAL)  UNITS         .        .        .        .612 

G.— THE   INHERITANCE   OF   FUNCTIONALLY-CAUSED   MODIFICATIONS        .      618 


PART  IT. 

MORPHOLOGICAL   DEVELOPMENT, 


47 


CHAPTER  I. 

THE  PROBLEMS  OF  MORPHOLOGY. 

§  175.  THE  division  of  Morphology  from  Physiology,  is 
one  which  may  be  tolerably-well  preserved  so  long  as  we  do 
not  carry  our  inquiries  beyond  the  empirical  generalizations 
of  their  respective  phenomena;  but  it  is  one  which  becomes 
in  great  measure  nominal,  when  the  phenomena  are  to  be 
rationally  interpreted.  It  would  be  possible,  after  analyzing 
our  Solar  System,  to  set  down  certain  general  truths  respect- 
ing the  sizes  and  distances  of  its  primary  and  secondary 
members,  omitting  all  mention  of  their  motions ;  and  it  would 
be  possible  to  set  down  certain  other  general  truths  respect- 
ing their  motions,  without  specifying  their  dimensions  or 
positions,  further  than  as  greater  or  less,  nearer  or  more  re- 
mote. But  on  seeking  to  account  for  these  general  truths, 
arrived  at  by  induction,  we  find  ourselves  obliged  to  consider 
simultaneously  the  relative  sizes  and  places  of  the  masses, 
and  the  relative  amounts  and  directions  of  their  motions. 
Similarly  with  organisms.  Though  we  may  frame  sundry 
comprehensive  propositions  respecting  the  arrangements  of 
their  organs,  considered  as  so  many  inert  parts;  and  though 
we  may  establish  several  wide  conclusions  respecting  the  sepa- 
rate and  combined  actions  of  their  organs,  without  knowing 
anything  definite  respecting  the  forms  and  positions  of  these 
organs;  yet  we  cannot  reach  such  a  rationale  of  the  facts  as 


4  MORPHOLOGICAL  DEVELOPMENT. 

the  hypothesis  of  Evolution  aims  at,  without  contemplating 
structures  and  functions  in  their  mutual  relations.  Every- 
where structures  in  great  measure  determine  functions;  and 
everywhere  functions  are  incessantly  modifying  structures. 
In  Nature  the  two  are  inseparable  co-operators;  and  Science 
can  give  no  true  interpretation  of  Nature  without  keeping 
their  co-operation  constantly  in  view.  An  account  of  organic 
evolution,  in  its  more  special  aspects,  must  be  essentially  an 
account  of  the  inter-actions  of  structures  and  functions,  as 
perpetually  altered  by  changes  of  conditions. 

Hence,  when  treating  apart  Morphological  Development 
and  Physiological  Development,  all  we  can  do  is  to  direct  our 
attention  mainly  to  the  one  or  to  the  other,  as  the  case  may 
be.  In  dealing  with  the  facts  of  structure,  we  must  consider 
the  facts  of  function  only  in  such  general  way  as  is  needful 
to  explain  the  facts  of  structure;  and  conversely  when  deal- 
ing with  the  facts  of  function. 

§  176.  The  problems  of  Morphology  fall  into  two  distinct 
classes,  answering  respectively  to  the  two  leading  aspects  of 
Evolution.  In  things  which  evolve  there  go  on  two  processes 
— increase  of  mass  and  increase  of  structure.  Increase  of 
mass  is  primary,  and  in  simple  evolution  takes  place  almost 
alone.  Increase  of  structure  is  secondary,  accompanying  or 
following  increase  of  mass  with  more  or  less  regularity,  wher- 
ever evolution  rises  above  that  form  which  small  inorganic 
bodies,  such  as  crystals,  present  to  us.  As  the  fundamental 
antagonism  between  Dissolution  and  Evolution  consists  in  this, 
that  while  the  one  is  an  integration  of  motion  and  dis- 
integration of  matter,  the  other  is  an  integration  of  matter 
and  disintegration  of  motion ;  and  as  this  integration  of  mat- 
ter accompanying  disintegration  of  motion,  is  a  necessary 
antecedent  to  the  differentiation  of  the  matter  so  integrated ; 
it  follows  that  questions  concerning  the  mode  in  which  the 
parts  are  united  into  a  whole,  must  be  dealt  with  before 


THE  PROBLEMS  OF  MORPHOLOGY.         5 

questions  concerning  the  mode  in  which  these  parts  become 
modified.* 

This  is  not  obviously  a  morphological  question.  But  an 
illustration  or  two  will  make  it  manifest  that  fundamental 
differences  may  be  produced  between  aggregates  by  differences 
in  the  degrees  of  composition  of  the  increments :  the  ultimate 
units  of  the  increments  being  the  same.  Thus  an  accumula- 
tion of  things  of  a  given  kind  may  be  made  by  adding  one  at 
a  time.  Or  the  things  may  be  tied  up  into  bundles  of  ten, 
and  the  tens  placed  together.  Or  the  tens  may  be  united 
into  hundreds,  and  a  pile  of  hundreds  formed.  Such  unlike- 
nesses  in  the  structures  of  masses  are  habitually  seen  in  our 
mercantile  transactions.  Articles  which  the  consumer  re- 
cognizes as  single,  the  retailer  keeps  wrapped  up  in  dozens, 
the  wholesaler  sends  in  gross,  and  the  manufacturer  supplies 
in  packages  of  a  hundred  gross.  That  is,  they  severally  in- 
crease their  stocks  by  units  of  simple,  of  compound,  and  of 
doubly-compound  kinds.  Similarly  result  those  differences  of 
morphological  composition  which  we  have  first  to  consider. 
An  organism  consists  of  units.  These  units  may  be  aggre- 
gated into  a  mass  by  the  addition  of  unit  to  unit.  Or  they 
may  be  united  into  groups,  and  the  groups  joined  together. 
Or  these  groups  of  groups  may  be  so  combined  as  to  form  a 
doubly-compound  aggregate.  Hence  there  arises  respecting 
each  organic  form  the  question — is  its  composition  of  the 
first,  second,  third,  or  fourth  order? — does  it  exhibit  units  of 
a  singly-compounded  kind  only,  or  are  these  consolidated 
into  units  of  a  doubly-compounded  kind,  or  a  triply-com- 
pounded kind?  And  if  it  displays  double  or  triple  composi- 

*  It  seems  needful  here  to  say,  that  allusion  is  made  in  this  paragraph  to  a 
proposition  respecting  the  ultimate  natures  of  Evolution  and  Dissolution, 
which  is  contained  in  an  essay  on  The  Classification  of  tJie  Sciences,  pub- 
lished in  March,  1864.  When  the  opportunity  comes,  I  hope  to  make  the 
definition  there  arrived  at,  the  basis  of  a  re-organization  of  the  second  part  of 
First  Principles:  giving  to  that  work  a  higher  development,  and  a  greater 
cohesion,  than  it  at  present  possesses.  [The  intention  here  indicated  was 
duly  carried  out  in  1867.] 


6  MORPHOLOGICAL  DEVELOPMENT. 

tion,  the  homologies  of  its  different  parts  become  problems. 
Under  the  disguises  induced  by  the  consolidation  of  primary, 
secondary,  and  tertiary  units,  it  has  to  be  ascertained  which 
answer  to  which,  in  their  degrees  of  composition. 

Such  questions  are  more  intricate  than  they  at  first  ap- 
pear ;  since,  besides  the  obscurities  caused  by  progressive  inte- 
gration, and  those  due  to  accompanying  modifications  of  form, 
further  obscurities  result  from  the  variable  growths  of  units 
of  the  different  orders.  Just  as  an  army  may  be  augmented 
by  recruiting  each  company,  without  increasing  the  number 
of  companies;  or  may  be  augmented  by  making  up  the  full 
complement  of  companies  in  each  regiment,  while  the  num- 
ber of  regiments  remains  the  same;  or  may  be  augmented 
by  putting  more  regiments  into  each  division,  other  things 
being  unchanged;  or  may  be  augmented  by  adding  to  the 
number  of  its  divisions  without  altering  the  components  of 
each  division;  or  may  be  augmented  by  two  or  three  of  these 
processes  at  once;  so,  in  organisms,  increase  of  mass  may 
result  from  additions  of  units  of  the  first  order,  or  those  of  the 
second  order,  or  those  of  still  higher  orders ;  or  it  may  be  due 
to  simultaneous  additions  to  units  of  several  orders.  And 
this  last  mode  of  integration  being  the  general  mode,  puts 
difficulties  in  the  way  of  analysis.  Just  as  the  structure  of 
an  army  would  be  made  less  easy  to  understand  if  companies 
often  outgrew  regiments,  or  regiments  became  larger  than 
brigades;  so  these  questions  of  morphological  composition 
are  complicated  by  the  indeterminate  sizes  of  the  units  of 
each  kind:  relatively-simple  units  frequently  becoming  more 
bulky  than  relatively-compound  units. 

§  177.  The  morphological  problems  of  the  second  class 
are  those  having  for  their  subject-matter  the  changes  of  shape 
which  accompany  changes  of  aggregation.  The  most  general 
questions  respecting  the  structure  of  an  organism,  having  been 
answered  when  it  is  ascertained  of  what  units  it  is  composed 
as  a  whole,  and  in  its  several  parts;  there  come  the  more 


THE  PROBLEMS  OF  MORPHOLOGY.         7 

special  questions  concerning  its  form — form  in  the  ordinary 
sense.  After  the  contrasts  caused  by  variations  in  the  process 
of  integration,  we  have  to  consider  the  contrasts  caused  by 
variations  in  the  process  of  differentiation.  To  speak  speci- 
fically— the  shape  of  the  organism  as  a  whole,  irrespective 
of  its  composition,  has  to  be  accounted  for.  Reasons  have 
to  be  found  for  the  unlikeness  between  its  general  outlines 
and  the  general  outlines  of  allied  organisms.  And  there 
have  to  be  answered  kindred  inquiries  respecting  the  propor- 
tions of  its  component  parts: — Why,  among  such  of  these  as 
are  homologous  with  one  another,  have  there  arisen  the 
differences  that  exist?  And  how  have  there  been  produced 
the  contrasts  between  them  and  the  homologous  parts  of 
organisms  of  the  same  type  ? 

Very  numerous  are  the  heterogeneities  of  form  presenting 
themselves  for  interpretation  under  these  heads.  The  ulti- 
mate morphological  units  combined  in  any  group,  may  be  dif- 
ferentiated individually,  or  collectively,  or  both :  each  of  them 
may  undergo  changes  of  shape;  or  some  of  them  may  be 
changed  and  others  not;  or  the  group  may  be  rendered  mul- 
tiform by  the  greater  growth  of  some  of  its  units  than  of 
others.  Similarly  with  the  compound  units  arising  by  union 
of  these  simple  units.  Aggregates  of  the  second  order  may 
be  made  relatively  complex  in  form,  by  inequalities  in  the 
rates  of  multiplication  of  their  component  units  in  diverse 
directions;  and  among  a  number  of  such  aggregates,  numer- 
ous unlikenesses  may  be  constituted  by  differences  in  their 
degrees  of  growth,  and  by  differences  in  their  modes  of  growth. 
Manifestly,  at  each  higher  stage  of  composition  the  possible 
sources  of  divergence  are  multiplied  still  further. 

That  facts  of  this  order  can  be  accounted  for  in  detail  is 
not  to  be  expected — the  data  are  wanting.  All  that  we  may 
hope  to  do  is  to  ascertain  their  general  laws.  How  this  is  to 
be  attempted  we  will  now  consider. 

§  178.  The  task  before  us  is  to  trace  throughout  these 


8  MORPHOLOGICAL  DEVELOPMENT. 

phenomena  the  process  of  evolution;  and  to  show  how,  as 
displayed  in  them,  it  conforms  to  those  first  principles  which 
evolution  in  general  conforms  to.  Two  sets  of  factors  have 
to  be  taken  into  account.  Let  us  look  at  them. 

The  factors  of  the  first  class  are  those  which  tend  directly 
to  change  an  organic  aggregate,  in  common  with  every  other 
aggregate,  from  that  more  simple  form  which  is  not  in  equi- 
librium with  incident  forces,  to  that  more  complex  form  which 
is  in  equilibrium  with  them.  We  have  to  mark  how,  in  corre- 
spondence with  the  universal  law  that  the  uniform  lapses  into 
the  multiform,  and  the  less  multiform  into  the  more  multi- 
form, the  parts  of  each  organism  are  ever  becoming  further 
differentiated;  and  we  have  to  trace  the  varying  relations  to 
incident  forces  by  which  further  differentiations  are  entailed. 
We  have  to  observe,  too,  how  each  primary  modification  of 
structure,  induced  by  an  altered  distribution  of  forces,  becomes 
a  parent  of  secondary  modifications — how,  through  the  neces- 
sary multiplication  of  effects,  change  of  form  in  one  part 
brings  about  changes  of  form  in  other  parts.  And  then  we 
have  also  to  note  the  metamorphoses  constantly  being  induced 
by  the  process  of  segregation — by  the  gradual  union  of  like 
parts  exposed  to  like  forces,  and  the  gradual  separation  of  like 
parts  exposed  to  unlike  forces.  The  factors  of  the  second 

class  which  we  have  to  keep  in  view  throughout  our  interpret- 
ations, are  the  formative  tendencies  of  organisms  themselves 
— the  proclivities  inherited  by  them  from  antecedent  organ- 
isms, and  which  past  processes  of  evolution  have  bequeathed. 
We  have  seen  it  to  be  inferable  from  various  orders  of  facts 
(§§  65,  84,  97-97<7),  that  organisms  are  built  up  of  certain 
highly-complex  molecules,  which  we  distinguished  as  physio- 
logical units  [or  constitutional  units  as  they  might  otherwise 
be  called] — each  kind  of  organism  being  built  up  of  units  pe- 
culiar to  itself.  We  recognized  in  these  units,  powers  of  ar- 
ranging themselves  into  the  forms  of  the  organisms  to  which 
they  belong;  analogous  to  the  powers  which  the  molecules  of 
inorganic  substances  have  of  aggregating  into  specific  crystal- 


THE  PROBLEMS  OF  MORPHOLOGY.         9 

line  forms.  We  have  consequently  to  regard  this  proclivity  of 
the  physiological  units,  as  producing,  during  the  development 
of  any  organism,  a  combination  of  internal  forces  that  expend 
themselves  in  working  out  a  structure  in  equilibrium  with 
the  forces  to  which  ancestral  organisms  were  exposed;  but 
not  in  equilibrium  with  the  forces  to  which  the  existing  organ- 
ism is  exposed,  if  the  environment  has  been  changed.  Hence 
the  problem  in  all  cases  is,  to  ascertain  the  resultant  of  inter- 
nal organizing  forces,  tending  to  reproduce  the  ancestral  form, 
and  external  modifying  forces,  tending  to  cause  deviations 
from  that  form.  Moreover,  we  have  to  take  into  account, 

not  only  the  characters  of  immediately-preceding  ancestors, 
but  also  those  of  their  ancestors,  and  ancestors  of  all  degrees 
of  remoteness.  Setting  out  with  rudimentary  types,  we  have 
to  consider  how,  in  each  successive  stage  of  evolution,  the 
structures  acquired  during  previous  stages  have  been  ob- 
scured by  further  integrations  and  further  differentiations; 
or,  conversely,  how  the  lineaments  of  primitive  organisms 
have  all  along  continued  to  manifest  themselves  under  the 
superposed  modifications. 

§179.  Two  ways  of  carrying  on  the  inquiry  suggest  them- 
selves. We  may  go  through  the  several  great  groups  of 
organisms,  with  the  view  of  reaching,  by  comparison  of  parts, 
certain  general  truths  respecting  the  homologies,  the  forms, 
and  the  relations  of  their  parts;  and  then,  having  dealt  with 
the  phenomena  inductively,  may  retrace  our  steps  with  the 
view  of  deductively  interpreting  the  general  truths  reached. 
Or,  instead  of  thus  separating  the  two  investigations,  we 
may  carry  them  on  hand  in  hand — first  establishing  each 
general  truth  empirically,  and  then  proceeding  to  the  ra- 
tionale of  it.  This  last  method  will,  I  think,  conduce  to 
both  brevity  and  clearness.  Let  us  now  thus  deal  with  the 
first  class  of  morphological  problems. 

[NOTE. — In  preparation  for  treating  of  morphological  de- 


10  MORPHOLOGICAL  DEVELOPMENT. 

velopment,  sundry  other  general  considerations  should  have 
been  included  in  the  foregoing  chapter  when  originally  pub- 
lished. This  seems  the  most  appropriate  place  for  now  nam- 
ing them.  Some  were  implicitly  contained  in  the  first  vol- 
ume, but  it  will  be  well  definitely  to  state  these,  as  well  as 
the  others  not  yet  implied. 

Interpretation  of  the  forms  of  organisms  and  the  forms 
of  their  parts,  must  depend  mainly  on  the  conclusions  pre- 
viously drawn  respecting  their  phylogeny;  and  the  drawing 
of  such  conclusions  must  be  guided  by  recognition  of  the 
various  factors  of  Evolution,  as  well  as  by  recognition  of 
certain  extremely  general  results  of  Evolution  and  certain 
concomitants  of  Evolution. 

A  primary  one  among  these  is  that  no  existing  species  can 
exhibit  more  than  approximately  the  ancestral  structure  of 
any  other  existing  species.  As  all  ancestors  have  disappeared, 
so,  in  a  greater  or  less  degree,  the  traits,  specific,  generic,  or 
ordinal,  which  distinguished  the  earlier  of  them  have  disap- 
peared. Setting  out  with  the  familiar  symbol,  a  tree,  let  us 
regard  its  peripheral  twigs  as  representing  extant  species; 
let  us  assume  that  the  interior  of  the  tree  is  filled  up  with 
some  supporting  substance,  leaving  only  the  ends  of  the 
living  twigs  projecting;  and  let  us  suppose  the  trunk,  main 
branches,  secondary  branches,  tertiary  branches,  &c.,  have 
decayed  away.  Then  if  we  take  these  decayed  parts  to  stand 
for  the  divergent  and  re-divergent  lines  of  evolution  which 
are  represented  by  fossils  in  the  Earth's  crust,  it  will  be 
manifest,  first,  that  no  one  of  the  living  superficial  twigs  (or 
species)  exhibits  the  ancestral  organization  whence  any  other 
of  the  living  superficial  twigs  (or  species)  has  been  developed; 
it  will  be  manifest,  second,  that  the  generic  structure  in- 
herited by  any  existing  species  must  be  a  structure  out  of 
which  came  sundry  allied  species — the  fork,  as  it  were,  at 
which  adjacent  twigs  diverged;  and  third,  that  the  ancestor 
of  an  order  must,  in  like  manner,  be  sought  at  some  point 
deeper  down  in  the  symbolic  tree — a  place  of  divergence  of 


THE  PROBLEMS  OF  MORPHOLOGY.        H 

the  sub-branches  representing  allied  genera.  Similarly  with 
the  ancestral  types  of  classes,  still  deeper  down  in  the  tree  or 
further  back  in  time.  So  that  phylogeny  becomes  more  and 
more  speculative  as  its  questions  become  more  and  more 
radical.  And  the  difficulty  is  made  greater  by  the  deficiency 
of  palasontological  evidence. 

One  obvious  corollary  is  that  an  ancestral  type  from  which 
sundry  allied  types  now  existing  diverged,  was,  speaking 
generally,  simpler  than  these;  since  the  divergent  types  be- 
came different  by  the  superposing  of  modifications,  adding 
to  their  complexities.  There  is  a  further  reason  for  inferring 
that  the  least  specialized  member  of  any  group  is  more  like 
the  remote  ancestor  than  any  of  the  others;  for  every  adap- 
tation stands  in  the  way  of  subsequent  re-adaptations:  it 
presents  a  greater  amount  of  structure  to  be  undone.  To  get 
some  idea  of  the  ancestral  type  where  no  extant  member  of 
the  group  is  manifestly  simpler  than  the  rest,  the  method 
must  be  to  take  all  its  extant  members  and,  after  letting 
their  differences  mutually  cancel,  observe  what  remains  com- 
mon to  them  all. 

But  there  are  difficulties  standing  in  the  way  of  phylogeny, 
and  consequently  of  morphology,  much  greater  than  these. 
Returning  to  our  symbolic  tree,  it  is  clear  that  it  would  be 
far  from  easy  to  say  of  any  one  twig  which  extinct  sub- 
branch,  branch,  and  main  branch  it  belonged  to,  even  sup- 
posing that  the  growths  of  all  parts  had  been  uniformly  out- 
wards. Immensely  more  perplexing,  then,  must  be  the 
affiliation  if  various  of  the  branches,  sub-branches,  &c.,  have 
sent  out  backward-growing  shoots  which  have  come  to  the 
surface  only  after  prolonged  retrograde  courses,  and  if  other 
branches  have  sent  shoots  into  regions  occupied  by  alien 
branches — shoots  bearing  twigs  which  come  to  the  surface 
along  with  those  to  which  they  are  but  remotely  allied.  The 
problems  of  origin  and  of  structure  which  organisms  present, 
are  met  by  Both  of  the  difficulties  thus  symbolized. 

One  of  them  arises  from  the  prevalence  of  retrograde 


12        MORPHOLOGICAL  DEVELOPMENT. 

metamorphoses.  Throughout  the  animal  world  these  are 
variously  displayed  by  parasites,  multitudinous  in  their 
kinds;  for  most  of  them  belong  to  types  much  higher  in 
organization.  Changed  habits  and  consequent  changed  struc- 
tures have  so  transferred  them  that  only  by  study  of  their 
embryonic  stages  can  their  kinships  be  made  out.  And  these 
retrograde  metamorphoses,  conspicuous  among  parasites,  have, 
in  the  course  of  evolution,  affected  some  members  of  all 
groups;  for  in  all  groups  the  struggle  for  existence  has  com- 
pelled some  to  adopt  careers  less  trying  but  less  profitable. 

Not  only  by  forcing  on  many  kinds  of  organisms  simpler 
ways  of  living,  and  consequent  degeneracy,  has  the  universal 
competition  caused  obscuring  transformations.  It  has  done 
this  also  by  tempting  many  other  kinds  of  organisms 
to  adopt  ways  of  life  not  simpler  than  before  but  merely 
different.  Pressure  continually  prompts  every  type  to  in- 
trude on  other  types'  spheres  of  activity;  and  so  causes  it  to 
assume  certain  structural  characters  of  the  types  whose 
spheres  it  invades,  masking  its  previous  characters.  Modifi- 
cations hence  arising  have,  in  the  great  mass  of  cases,  been 
superposed  one  on  another  time  after  time.  The  aquatic 
animal  becomes  through  several  transitions  a  land-animal, 
and  then  the  land-animal  through  other  transitions  becomes 
now  an  aerial  animal  like  the  bat  and  now  an  aquatic  animal 
like  the  whale.  Certain  kinds  of  birds  furnish  extreme 
illustrations.  There  was  the  change  from  the  fish  to  the 
water-breathing  amphibian  and  then  to  the  air-breathing 
amphibian;  thence  to  the  reptile  living  on  the  Earth's  sur- 
face; thence  to  the  flying  reptile  and  the  bird;  then  came 
the  diving  birds,  joining  with  their  aerial  life  a  life  passed 
partly  in  the  water;  and  finally  came  a  type  like  the 
penguin,  in  which  the  power  of  flight  has  been  lost  and  the 
water  has  again  become  the  almost  exclusive  medium,  except 
for  breathing.  Of  course  the  mouldings  and  re-mouldings  of 
structure  resulting  from  these  successive  unlike  modes  of 
life,  in  many  cases  put  great  difficulties  in  the  way  of  ascer- 


THE  PROBLEMS  OF  MORPHOLOGY.        13 

taining  which  are  the  original  corresponding  parts.  Some 
parts  have  become  abnormally  large;  others  have  dwindled 
or  disappeared;  and  the  relative  positions  of  parts  have  often 
been  greatly  changed.  A  bat's  wing  and  a  bird's  wing  are 
analogous  organs,  but  their  frameworks  are  but  partially 
homologous.  While  in  the  bird  the  terminal  parts  of  the 
fore-limb  do  little  towards  supporting  the  wing,  in  the  bat 
the  wing  is  mainly  supported  by  enormously-developed  termi- 
nal parts. 

The  effects  of  the  struggle  to  survive,  which  here  prompts 
a  simpler  life  with  resulting  degeneracy  and  there  a  different 
life  with  resulting  new  developments,  are  far  from  being  the 
only  causes  of  morphological  obscurations.  Fulfilment  of 
certain  highly  general  requirements  gives  certain  common 
traits  to  plants  of  widely  divergent  classes ;  and  fulfilment  of 
certain  other  highly  general  requirements  gives  certain 
common  traits  to  animals  of  widely  divergent  classes.  It  was 
remarked  in  the  first  volume  (§  54/)  that  the  cardinal  distinc- 
tion between  the  characters  of  plants  and  animals  arises  from 
the  fact  that  while  the  chief  food  of  plants  is  universally 
present  the  food  of  animals  is  scattered.  Here  it  has  to  be 
added  that  to  utilize  the  universally  distributed  food  the 
ordinary  plant  needs  the  aid  of  light,  and  has  to  acquire 
structures  enabling  it  to  get  that  aid;  while  the  ordinary 
animal,  to  utilize  the  scattered  food,  must  acquire  the  struc- 
tures needful  for  locomotion.  Let  us  contemplate  separately 
the  traits  hence  resulting  in  the  vegetal  world  and  the  traits 
hence  resulting  in  the  animal  world. 

The  familiar  plantain  meets  the  requirement  by  growing 
stiff  leaves  enabling  it  to  press  down  the  competing  grasses 
around  which  would  else  shade  it;  but  the  great  majority  of 
ordinary  plants  meet  the  requirement  by  raising  themselves 
into  the  air.  Hence  the  need  for  a  stem,  and  hence  the  fact 
that  plants  of  widely  unlike  natures  similarly  form  stems 
which,  in  achieving  strength  enough  to  support  the  foliage 
and  resist  the  wind,  acquire  certain  adaptive  structures  hav- 


14:  MORPHOLOGICAL  DEVELOPMENT. 

ing  a  general  similarity.  Here  from  the  edge  of  a  pool  is 
a  reed,  and  here  from  the  adjacent  copse  is  a  hemlock:  the 
one  having  grown  tall  in  escaping  the  shade  of  its  com- 
panions and  the  other  in  escaping  the  shade  of  the  surround- 
ing brushwood.  On  being  cut  across  each  discloses  a  tube, 
and  each  exhibits  septa  dividing  this  tube  into  chambers. 
In  either  case  by  the  tubular  structure  is  gained  the  greatest 
strength  with  the  least  material;  but  there  is  no  morpho- 
logical kinship  between  the  tubes  nor  between  the  septa. 
Still  more  marked  is  the  simulation  of  homology  by  analogy 
in  another  plant  which  the  adjacent  ditch  may  furnish — the 
common  Horsetail.  In  this,  again,  we  see  an  elongated  ver- 
tical-growing part,  raising  the  foliage  into  the  air;  and,  as 
before,  this  is  tubular  and  divided  by  septa.  A  type  utterly 
alien  from  the  other  two  has,  by  survival  of  the  fittest,  been 
similarly  moulded  to  meet  mechanical  needs. 

Passing  now  to  the  obscurations  in  the  animal  world 
caused  by  alterations  favouring  locomotion,  we  note  first  that 
the  locomotive  power  is  at  the  outset  very  slight.  Among 
many  orders  of  Protozoa,  as  also  among  many  low  types  of 
Metazoa,  vibratile  cilia  are  the  most  general  agents  of  loco- 
motion—  necessarily  feeble  locomotion.  Eegarded  in  the 
mass,  the  Ccelenterata,  when  not  stationary  like  the  Hydra  or 
higher  types  in  the  hydroid  stage,  usually  possess  only  such 
small  self-mobility  as  the  slow  rhythmical  contractions  of 
their  umbrella-disks  effect,  or  else  such  as  is  effected  by  bands 
of  cilia  or  of  vibratile  plates,  as  in  the  Beroe.  Even  among 
these  low  tpes  of  Metazoa,  however,  in  which  ordinarily  the 
radial  structure  is  conspicuous,  or  but  slightly  obscured  by 
an  ovoid  form  as  in  the  Ctenophora,  we  find,  in  the  Cestus 
veneris,  extreme  obscuration  caused  by  an  elongation  which 
facilitates  movement  through  the  water;  alike  by  the  actions 
of  its  vibratile  plates  and  by  its  undulations,  which  simulate 
those  of  sundry  higher  animals. 

And  here  we  come  upon  the  essential  fact  to  be  recognized. 
Elongation  favours  locomotion  in  various  ways  that  are 


THE  PROBLEMS  OF  MORPHOLOGY.        15 

severally  taken  advantage  of  by  different  types  of  creatures. 
(1)  To  a  given  mass  of  moving  matter  the  resistance  of  the 
medium  decreases  along  with  decrease  in  the  area  of  its 
transverse  section,  and  this  implies  increase  of  length :  a  given 
force  will  move  the  lengthened  mass  along  with  greater 
facility.  (2)  Eeaching  a  certain  point  the  elongated  form 
enables  an  animal  to  progress  by  undulations,  as  in  the 
water  fish  do,  and  even  some  ccelenterates  and  turbellarians 
do,  and  as  on  land  snakes  do:  lateral  resistances  serving  in 
either  case  as  fulcra.  (3)  Lengthening  of  the  body  serves 
otherwise  to  aid  locomotion  in  the  creeping  or  burrowing 
worm,  which,  utilizing  the  statical  resistance  of  its  hinder 
part  thrusts  onwards  its  fore  part,  and  then,  holding  fast  its 
fore  part  by  the  aid  of  minute  setae,  draws  the  hinder  part 
after  it.  But  elongation,  doubly  advantageous  at  first,  while 
the  body  is  itself  the  chief  instrument  of  locomotion,  gradually 
loses  its  advantageousness  as  special  instruments  of  locomo- 
tion are  developed.  (4)  This  we  see  in  that  locomotive 
action  effected  by  limbs,  which,  many  and  small  in  the  lower 
Arthropoda  and  becoming  few  and  larger  in  the  higher,  at 
length  give  great  activity  to  a  shortened  and  consolidated 
body:  a  stage  reached  only  through  stages  of  decreasing 
elongation  accompanying  increase  of  limb-power.  (5)  In 
the  Vertebrata  locomotion  by  undulations  comes,  along  cer- 
tain lines  of  evolution,  to  be  replaced  by  that  limb-locomo- 
tion which  accompanies  the  rise  from  water-life  to  land-life: 
the  evolution  of  Amphibians  exhibiting  the  transition.  (6) 
Further,  we  see  among  mammals  that  as  limbs  become  effi- 
cient the  elongated  body  ceases  to  be  itself  instrumental  in 
locomotion,  but  that  still  some  elongation  remains  a  charac- 
teristic. (7)  Finally,  where  limb-locomotion  reaches  its  high- 
est degree,  as  in  birds,  elongation  disappears. 

These  classes  of  familiar  facts  I  have  recalled  to  show 
that,  in  the  course  of  evolution,  achievement  by  plants  of  the 
all-essential  elevation  into  the  air  and  by  animals  of  the 
all-essential  power  of  movement  have  developed  this  trait 


16  MORPHOLOGICAL  DEVELOPMENT. 

of  elongation  in  various  types;  and  that  in  each  kingdom 
acquisition  of  the  common  trait  has  had  a  tendency  now  to 
obscure  morphological  equivalence,  and  now  to  give  the  ap- 
pearance of  kinship  where  there  is  none.  A  further  pur- 
pose has  been  to  prepare  the  way  for  a  question  hereafter 
to  be  discussed — whether,  in  the  various  types  of  either  king- 
dom, the  elongation  is  effected  in  the  same  ways  or  in  dif- 
ferent ways.  We  shall  have  to  ask  whether  the  vertically 
growing  part  is  always,  like  that  of  Lessonia,  a  simple  in- 
dividual, or  whether,  as  possibly  in  Phasnogams,  it  is  a  united 
series  of  individuals;  and  similarly  whether  the  elongated 
body  is  always  single,  like  that  of  a  mollusc,  or  whether,  as 
possibly  in  annulose  animals,  it  is  a  series  of  united  in- 
dividuals.] 


CHAPTER  II. 

THE  MORPHOLOGICAL   COMPOSITION   OF   PLANTS. 

§  180.  EVOLUTION  implies  insensible  modifications  and 
gradual  transitions,  which  render  definition  difficult — which 
make  it  impossible  to  separate  absolutely  the  phases  of 
organization  from  one  another.  And  this  indefiniteness  of 
distinction,  to  be  expected  a  priori,  we  are  compelled  to 
recognize  a  posteriori,  the  moment  we  begin  to  group  morpho- 
logical phenomena  into  general  propositions.  Thus,  on 
inquiring  what  is  the  morphological  unit,  whether  of  plants 
or  of  animals,  we  find  that  the  facts  refuse  to  be  included  in 
any  rigid  formula.  The  doctrine  that  all  organisms  are  built 
up  of  cells,  or  that  cells  are  the  elements  out  of  which  every 
tissue  is  developed,  is  but  approximately  true.  There  are 
living  forms  of  which  cellular  structure  cannot  be  asserted; 
and  in  living  forms  that  are  for  the  most  part  cellular,  there 
are  nevertheless  certain  portions  which  are  not  produced  by 
the  metamorphosis  of  cells.  Supposing  that  clay  were  the 
only  material  available  for  building,  the  proposition  that  all 
houses  are  built  of  bricks,  would  bear  about  the  same  relation 
to  the  truth,  as  does  the  proposition  that  all  organisms  are 
composed  of  cells.  This  generalization  respecting  houses 
would  be  open  to  two  criticisms: — first,  that  certain  houses 
of  a  primitive  kind  are  formed,  not  of  bricks,  but  out  of 
unmoulded  clay;  and  second,  that  though  other  houses  con- 
sist mainly  of  bricks,  yet  their  chimney-pots,  drain-pipes,  and 
48  17 


18  MORPHOLOGICAL  DEVELOPMENT. 

ridge-tiles,  do  not  result  from  combination  or  metamorphosis 
of  bricks,  but  are  made  directly  out  of  the  original  clay. 
And  of  like  natures  are  the  criticisms  which  must  be  passed 
on  the  generalization,  that  cells  are  the  morphological  units 
of  organisms.  To  continue  the  simile,  the  truth  turns  out  to 
be,  that  the  primitive  clay  or  protoplasm  out  of  which 
organisms  are  built,  may  be  moulded  either  directly,  or 
with  various  degrees  of  indirectness,  into  organic  structures. 
The  physiological  units  which  we  are  obliged  to  assume  as 
the  components  of  this  protoplasm,  must,  as  we  have  seen, 
be  the  possessors  of  those  proclivities  which  result  in 
the  structural  arrangements  of  the  organism.  The  assump- 
tion of  such  structural  arrangements  may  go  on,  and  in 
many  cases  does  go  on,  by  the  shortest  route;  without  the 
passage  through  what  we  call  metamorphoses.  But  where 
such  structural  arrangements  are  reached  by  a  circuitous 
route,  the  first  stage  is  the  formation  of  these  small  aggre- 
gates which,  under  the  name  of  cells,  are  currently  regarded 
as  morphological  units. 

The  rationale  of  these  truths  appears  to  be  furnished  by 
the  hypothesis  of  evolution.  We  set  out  with  molecules 
some  degrees  higher  in  complexity  than  those  molecules  of 
nitrogenous  colloidal  substance  into  which  organic  matter  is 
resolvable;  and  we  regard  these  very  much  more  complex 
molecules  as  having  the  implied  greater  instability,  greater 
sensitiveness  to  surrounding  influences,  and  consequent 
greater  mobility  of  form.  Such  being  the  primitive  physio- 
logical units,  organic  evolution  must  begin  with  the  formation 
of  a  minute  aggregate  of  them — an  aggregate  showing  vitality 
by  a  higher  degree  of  that  readiness  to  change  its  form  of 
aggregation  which  colloidal  matter  in  general  displays;  and 
by  its  ability  to  unite  the  nitrogenous  molecules  it  meets 
with,  into  complex  molecules  like  those  of  which  it  is  com- 
posed. Obviously,  the  earliest  forms  must  have  been  minute ; 
since,  in  the  absence  of  any  but  diffused  organic  matter,  no 
form  but  a  minute  one  could  find  nutriment.  Obviously,  too, 


THE  MORPHOLOGICAL  COMPOSITION  OP   PLANTS.     19 

it  must  have  been  structureless;  since,  as  differentiations  are 
producible  only  by  the  unlike  actions  of  incident  forces,  there 
could  have  been  no  differentiations  before  such  forces  had  had 
time  to  work.  Hence,  distinctions  of  parts  like  those  required 
to  constitute  a  cell  were  necessarily  absent  at  first.  And  we 
need  not  therefore  be  surprised  to  find,  as  we  do  find,  specks 
of  protoplasm  manifesting  life,  and  yet  showing  no  signs  of 
organization.  A  further  stage  of  evolution  is 

reached  when  the  imperfectly  integrated  molecules  forming 
one  of  these  minute  aggregates,  become  more  coherent;  at 
the  same  time  as  they  pass  into  a  state  of  heterogeneity, 
gradually  increasing  in  its  defmiteness.  That  is  to  say,  we 
may  look  for  the  assumption  by  them,  of  some  distinctions  of 
parts,  such  as  we  find  in  cells  and  in  what  are  called  uni- 
cellular organisms.  They  cannot  retain  their  primordial 
uniformity ;  and  while  in  a  few  cases  they  may  depart  from  it 
but  slightly,  they  will,  in  the  great  majority  of  cases,  acquire 
a  decided  multiformity:  there  will  result  the  comparatively 
integrated  and  comparatively  differentiated  Protophyta  and 
Protozoa.  The  production  of  minute  aggregates 

of  physiological  units  being  the  first  step,  and  the  passage 
of  such  minute  aggregates  into  more  consolidated  and  more 
complex  forms  being  the  second  step,  it  must  naturally 
happen  that  all  higher  organic  types,  subsequently  arising  by 
further  integrations  and  differentiations,  will  everywhere  bear 
the  impress  of  this  earliest  phase  of  evolution.  From  the 
law  of  heredity,  considered  as  extending  to  the  entire  suc- 
cession of  living  things  during  the  Earth's  past  history,  it 
follows  that  since  the  formation  of  these  small,  simple  organ- 
isms must  have  preceded  the  formation  of  larger  and  more 
complex  organisms,  the  larger  and  more  complex  organisms 
must  inherit  their  essential  characters.  We  may  anticipate 
that  the  multiplication  and  combination  of  these  minute 
aggregates  or  cells,  will  be  conspicuous  in  the  early  develop- 
mental stages  of  plants  and  animals;  and  that  throughout 
all  subsequent  stages,  cell-production  and  cell-differentiation 


20  MORPHOLOGICAL  DEVELOPMENT. 

will  be  dominant  characteristics.  The  physiological  units 
peculiar  to  each  higher  species  will,  speaking  generally, 
pass  through  this  form  of  aggregation  on  their  way  towards 
the  final  arrangement  they  are  to  assume;  because  those 
primordial  physiological  units  from  which  they  are  remotely 
descended,  aggregated  into  this  form.  And  yet,  just  as  in 
other  cases  we  found  reasons  for  inferring  (§131)  that  the 
traits  of  ancestral  organization  may,  under  certain  conditions, 
be  partially  or  wholly  obliterated,  and  the  ultimate  structure 
assumed  without  passing  through  them;  so,  here,  it  is  to  be 
inferred  that  the  process  of  cell- formation  may,  in  some  cases, 
be  passed  over.  Thus  the  hypothesis  of  evolution 

prepares  us  for  those  two  radical  modifications  of  the  cell- 
doctrine  which  the  facts  oblige  us  to  make.  It  leads  us  to 
expect  that  as  structureless  portions  of  protoplasm  must  have 
preceded  cells  in  the  process  of  general  evolution;  so,  in  the 
special  evolution  of  each  higher  organism,  there  will  be  an 
habitual  production  of  cells  out  of  structureless  blastema. 
And  it  leads  us  to  expect  that  though,  generally,  the  physio- 
logical units  composing  a  structureless  blastema,  will  display 
their  inherited  proclivities  by  cell-development  and  meta- 
morphosis; there  will  nevertheless  occur  cases  in  which  the 
tissue  to  be  formed,  is  formed  by  direct  transformation  of  the 
blastema.* 

*  Let  me  here  refer  those  who  are  interested  in  this  question,  to  Prof. 
Huxley's  criticism  on  the  cell-doctrine,  published  in  the  Medico-  Chirurgical 
Review  in  1853. 

A  critic  who  thinks  the  above  statements  are  "  rather  misleading "  ad- 
mits that  the  lowest  types  of  organisms  yield  them  support,  saying  that 
"there  are  certainly  masses  of  protoplasm  containing  many  nuclei,  but  no 
trace  of  cellular  structure,  in  both  animals  and  plants.  Such  non-cellular 
masses  may  exist  during  development  and  later  become  separated  up  into 
cells,  but  there  arc  certain  low  organisms  in  which  such  masses  exist  in  the 
adult  state.  They  arc  called  by  some  botanists  non  cellular,  by  others 
multi-nucleate  colls.  Clearly  the  difference  lies  in  the  criteria  of  a  cell. 
There  are  also  some  Protozoa,  and  the  Bflc'eria,  in  which  no  nucleus  has 
certainly  been  demonstrated.  But  it  is  usual  to  consider  the  bodies  of  such 
organisms  as  cells  nevertheless,  and  it  is  supposed  that  such  cells  represent 


THE  MORPHOLOGICAL  COMPOSITION   OF   PLANTS.     21 

Interpreting  the  facts  in  this  manner,  we  may  recognize 
that  large  amount  of  truth  which  the  cell-doctrine  contains, 
without  committing  ourselves  to  the  errors  involved  by  a 
sweeping  assertion  of  it.  We  are  enabled  to  understand  how 
it  happens  that  organic  structures  are  usually  cellular  in  their 
composition,  at  the  same  time  that  they  are  not  universally 
so.  We  are  shown  that  while  we  may  properly  continue  to 
regard  the  cell  as  the  morphological  unit,  we  must  constantly 
bear  in  mind  that  it  is  such  only  in  a  qualified  sense. 

§  181.  These  aggregates  of  the  lowest  order,  each  formed 
of  physiological  units  united  into  a  group  that  is  structurally 
single  and  cannot  be  divided  without  destruction  of  its 
individuality,  may,  as  above  implied,  exist  as  independent 
organisms.  The  assumption  to  which  we  are  committed  by 
the  hypothesis  of  evolution,  that  such  so-called  uni-cellular 
plants  were  at  first  the  only  kinds  of  plants,  is  in  harmony 
with  the  fact  that  habitats  not  occupied  by  plants  of  higher 
orders,  commonly  contain  these  protophytes  in  great  abund- 
ance and  great  variety.  The  various  species  of  Pleurococ- 
cacece,  of  Desmidiacece,  and  Diatomacece,  supply  examples  of 
morphological  units  living  and  propagating  separately,  under 
numerous  modifications  of  form  and  structure.  Figures  1,  2, 
and  3,  represent  a  few  of  the  commonest  types. 

Mostly,  simple  plants  are  too  small  to  be  individually 

a  stage  of  development  in  which  the  nucleus  has  not  yet  been  evolved,  though 
the  chemical  substance  '  nuclein '  has  been  formed  in  some  of  them  " 

Perhaps  it  will  be  most  correct  to  say  that,  excluding  the  minute,  non- 
nucleated  organisms,  all  the  higher  organisms — Metazoa  and  Metaphyta — are 
composed  throughout  of  cells,  or  of  tissues  originally  cellular,  or  of  materials 
which  have  in  the  course  of  development  been  derived  from  cells.  It  must, 
however,  be  borne  in  mind  that,  according  to  sundry  leading  biologists,  cells 
in  the  strict  sense  are  not  the  immediate  products  either  of  the  primitive 
fissions  or  of  subsequent  fissions  ;  but  that  the  multiplying  so-called  cells  are 
nucleated  masses  of  protoplasm  which  remain  connected  by  strands  of  proto- 
plasm, and  which  acquire  limiting  membranes  by  a  secondary  process.  So 
that,  in  the  view  of  Mr.  Adam  Sedgwick  and  others,  the  substance  of  an 
organism  is  in  fact  a  continuous  mass  of  vacuolated  protoplasm. 


22 


MORPHOLOGICAL  DEVELOPMENT. 


visible  without  the  microscope.      But,  in  some  cases,  these 
vegetal  aggregates  of  the  first  order  grow  to   appreciable 


sizes.  In  the  mycelium  of  some  fungi,  we  have  single  cells 
developed  into  long  branched  filaments,  or  ramified  tubules, 
that  are  of  considerable  lengths.  An  analogous  structure  char- 
acterizes certain  tribes  of  Algae,  of  which  C odium  adhcerens, 
Fig.  4,  may  serve  as  an  example.  In  Botrydium,  another 
alga,  Fig.  5,  we  have  a  structure  which  is  described  as  simu- 
lating a  higher  plant,  with  root,  stem,  bud,  and  fruit,  all 
produced  by  the  branching  of  a  single  cell.  And  among 


fungi  the  genus  Mucor,  Fig.  6,  furnishes  an  example  of 
allied  kind.*  Here,  though  the  size  attained  is  much  greater 
than  that  of  many  organisms  which  are  morphologically 
compound,  we  are  compelled  to  consider  the  morphological 
composition  as  simple;  since  the  whole  can  no  more  be 
separated  into  minor  wholes,  than  can  the  branched  vascular 

*  In  further  illustration,  Mr.  Tanslcy  names  the  fact  that  in  the  genus 
Caulrrpa  we  have  extremely  complicated  forms  often  of  considerable  size 
produced  in  the  same  way.  The  various  snocics  simulate  very  perfectly  the 
members  of  different  crrouns  amonrr  the  higher  plants,  such  as  Horse-tails, 
Mosses,  Cactuses,  Conifers  and  the  like. 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     23 

system  of  an  animal.  In  these  cases  we  have  considerable 
bulk  attained,  not  by  a  number  of  aggregates  of  the  first 
order  being  united  into  an  aggregate  of  the  second  order,  but 
by  the  continuous  growth  of  an  aggregate  of  the  first  order. 

§  182.  The  transition  to  higher  forms  begins  in  a  very 
unobtrusive  manner.  Among  these  aggregates  of  the  first 
order,  an  approach  towards  that  union  by  which  aggregates 
of  the  second  order  are  produced,  is  indicated  by  mere  juxta- 
position. Protophytes  multiply  rapidly;  and  their  rapid 
multiplication  sometimes  causes  crowding.  When,  instead 
of  floating  free  in  the  water,  they  form  a  thin  film  on  a  moist 
surface,  or  are  imbedded  in  a  common  matrix  of  mucilage; 
the  mechanical  obstacles  to  dispersion  result  in  a  kind  of 
feeble  integration,  vaguely  shadowing  forth  a  combined 
group.  Somewhat  more  definite  combination  is  shown  us  by 
such  plants  as  Palmella  botryoides.  Here  the  members  of  a 
family  of  cells,  arising  by  the  spontaneous  fission  of  a  parent- 
cell,  remain  united  by  slender  threads  of  that  jelly-like  sub- 
stance which  envelops  their  surfaces.  In  some  Diatomacece 
several  individuals,  instead  of  completely  separating,  hold 
together  by  their  angles;  and  in  other  Diatomacece,  as  the 
Bacillaria,  a  variable  number  of  units  cohere  so  slightly,  that 
they  are  continually  moving  in  relation  to  one  another. 

This  formation  of  aggregates  of  the  second  order,  faintly 
indicated  in  feeble  and  variable  unions  like  the  above,  may 
be  traced  through  phases  of  increasing  permanence  and  de- 
finiteness,  as  well  as  increasing  extent.  In  the  yeast-plant, 
Fig.  7,  we  have  cells  which  may  exist  singly,  or  joined  into 
groups  of  several;  and  which  have  their  shapes  scarcely  at 
all  modified  by  their  connexion.  Among  the  Desmidiacece,  it 
happens  in  many  cases  that  the  two  individuals  produced  by 
division  of  a  parent-individual,  part  as  soon  as  they  are  fully 
formed;  but  in  other  cases,  instead  of  parting  they  compose 
a  group  of  two.  Allied  kinds  show  us  how,  by  subsequent 
fissions  of  the  adherent  individuals  and  their  progeny,  there 


24 


MORPHOLOGICAL  DEVELOPMENT. 


result  longer  groups ;  and  in  some  species,  a  continuous  thread 
of  them  is  thus  produced.  Figs.  8,  9,  11,  exhibit  these 
several  stages.  Fig.  10  represents  a  Scenedesmus  in  which 


the  individuation  of  the  group  is  manifest.  Instead  of  linear 
aggregation,  many  protophytes  illustrate  central  aggregation ; 
as  shown  in  Figs.  12,  13,  14,  15.  Other  instances  are  fur- 
nished by  such  forms  as  the  Gonium  pectorale,  Fig.  16  (a 
being  the  front  view,  and  b  the  edge  view),  and  the  Sarcina 
ventriculi,  Fig.  17.  Further,  we  have  that  spherical  mode 
of  aggregation  of  which  the  Volvox  globator  furnishes  a 
familiar  instance. 

Thus  far,  however,  the  individuality  of  the  secondary  ag- 
gregate is  feebly  pronounced:  not  simply  in  the  sense  that 
it  is  small;  but  also  in  the  sense  that  the  individualities  of 
the  primary  aggregates  are  very  little  subordinated.  But  on 
seeking  further,  we  find  transitions  towards  forms  in  which 
the  compound  individuality  is  more  dominant,  while  the 
simple  individualities  are  more  obscured.  Obscuration 

of  one  kind  accompanies  mere  increase  of  size  in  the  second- 
ary aggregate.  In  proportion  to  the  greater  number  of  the 
morphological  units  held  together  in  one  mass,  becomes  their 
relative  insignificance  as  individuals.  We  see  this  in  the 
irregularly-spreading  lichens  that  form  patches  on  rocks; 
and  in  such  creeping  fungi  as  grow  in  films  or  laminae  on 
decaying  wood  and  the  bark  of  trees.  In  these  cases,  how- 
ever, the  integration  of  the  component  cells  is  of  an  almost 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     25 

mechanical  kind.  The  aggregate  of  them  is  scarcely  more 
individuated  than  a  lump  of  inorganic  matter :  as  witness  the 
way  in  which  the  lichen  extends  its  curved  edges  in  this  or 
that  direction,  as  the  surface  favours;  or  the  way  in  which 
the  fungus  grows  round  and  imbeds  the  shoots  and  leaves 
that  lie  in  its  way,  just  as  so  much  plastic  clay  might  do. 
Though  here,  in  the  augmentation  of  mass,  we  see  a  progress 
towards  the  evolution  of  a  higher  type,  we  have  as  yet  none 
of  that  definiteness  required  to  constitute  a  compound  unit, 
or  true  aggregate  of  the  second  order.  Another  kind 

of  obscuration  of  the  morphological  units,  is  brought  about 
by  their  more  complete  coalescence  into  the  form  of  some 
structure  made  by  their  union.  This  is  well  exemplified 
among  the  Confervoidece  and  Conjugates.  In  Fig.  18,  there 


are  represented  the  stages  of  a  growing  Mougeotia  genuflexa, 
in  which  this  merging  of  the  simple  individualities  into  the 
compound  individuality,  is  shown  in  the  history  of  a  single 
plant;  and  in  Figs.  19,  20,  21;  22,  23,  are  represented  a 
series  of  species  from  this  group,  and  that  of  Cladophora*  in 
which  we  see  a  progressing  integration.  While,  in  the  lower 
types,  the  primitive  spheroidal  forms  of  the  cells  are  scarcely 
altered,  in  the  higher  types  the  cells  are  so  fused  together 
as  to  constitute  cylinders  divided  by  septa.  Here,  however, 

*  It  may  be  objected  that  in  Cladophora  the  separate  compartments  of 
the  thallus  severally  contain  many  nuclei,  making  it  doubtful  whether  they 
descend  from  uni-nucleate  cells.  If,  however,  they  do  not  they  simply  illus- 
trate another  form  of  integration. 


MORPHOLOGICAL  DEVELOPMENT. 


the  indefiniteness  is  still  great.  There  are  no  specific  limits 
to  the  length  of  any  thread  thus  produced,  and  there  is  none 
of  that  differentiation  of  parts  required  to  give  a  decided  in- 
dividuality to  the  whole. 

To  constitute  something  like  a  true  aggpegate  of  the  second 
order,  capable  of  serving  as  a  compound  unit  that  may  be 
combined  with  others  like  itself  into  still  higher  aggregates, 
there  must  exist  both  mass  and  definiteness. 

§  183.  An  approach  towards  plants  which  unite  these 
characters,  may  be  traced  in  such  forms  as  Bangia  ciliaris, 
Fig.  24.  The  multiplication  of  cells  here  takes  place,  not  in 
a  longitudinal  direction  only,  but  also  in 
a  transverse  direction;  and  the  transverse 
multiplication  being  greater  towards  the 
middle  of  the  frond,  there  results  a  differ- 
ence between  the  middle  and  the  two  ex- 
tremities— a  character  which,  in  a  feeble 
way,  unites  all  the  parts  into  a  whole. 
Even  this  slight  individuation  is,  however, 
very  indefinitely  marked;  since,  as  shown 
by  the  figures,  the  lateral  multiplication 
of  cells  does  not  go  on  in  a  precise  manner. 
From  some  such  type  as  this  there  ap- 
pear to  arise,  through  slight  differences  in 
the  modes  of  growth,  two  closely-allied 
groups  9f  plants,  having  individualities 
somewhat  more  pronounced.  If,  while 
the  cells  multiply  longitudinally,  their  lateral  multiplication 
goes  on  in  one  direction  only,  there  results  a  flat  surface,  as 
in  the  genus  Ulva  (Sea-lettuce)  or  in  the  upper  part  of  the 
thallus  of  Enteromorpha  Lima,  Fig.  25 ;  or  where  the  lateral 
multiplication  is  less  uniform  in  its  rate,  in  types  like 
Fig.  26.  But  where  the  lateral  multiplication  occurs  in  two 
directions  transverse  to  one  another,  a  hollow  frond  may  be 
produced — sometimes  irregularly  spheroidal,  and  sometimes 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     27 

irregularly  tubular;  as  in  Enteromorpha  intesiinalis,  Fig.  27. 
And  often,  as  in  Enteromorpha  compressa,  Fig.  28,  and  other 


2S 


species,  this  tubular  frond  becomes  branched.  Figs.  29  and 
30  are  magnified  portions  of  such  fronds,  showing  the 
simple  cellular  aggregation  which  allies  them  with  the  pre- 
ceding forms. 

In  the  common  Fuci  of  our  coasts,  other  and  somewhat 
higher  stages  of  this  integration  are  displayed.  We  have 
fronds  preserving  something  like  constant  breadths  and 
dividing  dichotomously  with  approximate  regularity.  Though 
the  subdivisions  so  produced  are  not  to  be  regarded  as 
separate  fronds,  but  only  as  extensions  of  one  frond,  they 
foreshadow  a  higher  degree  of  composition;  and  by  the  com- 
paratively methodic  way  in  which  they  are  united,  give  to 
the  aggregate  a  more  definite,  as  well  as  a  more  complex,  in- 
dividuality. Many  of  the  higher  lichens  exhibit  an 
analogous  advance.  While  in  the  lowest  lichens,  the  different 
parts  of  the  thallus  are  held  together  only  by  being  all 
attached  to  the  supporting  surface,  in  the  higher  lichens  the 
thallus  is  so  far  integrated  that  it  can  support  itself  by 
attachment  to  such  surface  at  one  point  only.  And  then,  in 
still  more  developed  kinds,  we  find  the  thallus  assuming  a 


28         MORPHOLOGICAL  DEVELOPMENT. 

dichotomously-branched  form,  and  so  gaining  a  more  specific 
character  as  well  as  greater  size. 

Where,  as  in  types  like  these,  the  morphological  units 
show  an  inherent  tendency  to  arrange  themselves  in  a  manner 
which  is  so  far  constant  as  to  give  characteristic  proportions, 
we  may  say  that  there  is  a  recognizable  compound  individual- 
ity. Considering  the  Thallophytes  which  grow  in  this  way 
apart  from  their  kinships,  and  wholly  with  reference  to  their 
morphological  composition,  we  might  not  inaptly  describe 
them  as  pseudo-foliar. 

§  184.  Another  mode  in  which  aggregation  is  so  carried 
on  as  to  produce  a  compound  individuality  of  considerable 
definiteness,  is  variously  displayed  among  other  families  of 
Alga.  When  the  cells,  instead  of  multiplying  longitudinally 
alone,  and  instead  of  all  multiplying  laterally  as  well  as 
longitudinally,  multiply  laterally  only  at  particular  places, 
they  produce  branched  structures. 

Indications  of  this  mode  of  aggregation  occur  among  the 
Confervoidece,  as  shown  in  Figs.  22,  23.  Though,  in  some  of 
the  more  developed  Algce  which  exhibit  the  ramified  arrange- 
ment in  a  higher  degree,  the  component  cells  are,  like  those 
of  the  lower  Algce,  united  together  end  to  end,  in  such  way 
as  but  little  to  obscure  their  separate  forms,  as  in  Cladophora 
Hutcliinsice,  Fig.  31 ;  they  nevertheless  evince  greater  sub- 
ordination to  the  whole  of  which  they  are  parts,  by  arranging 
themselves  more  methodically.  Still  further  pronounced 
becomes  the  compound  individuality  when,  while  the  com- 
ponent cells  of  the  branches  unite  completely  into  jointed 
cylinders,  the  component  cells  of  the  stem  form  an  axis  dis- 
tinguished by  its  relative  thickness  and  complexity.  Such 
types  of  structures  are  indicated  by  Figs.  32,  33 — figures 
representing  small  portions  of  plants  which  are  quite  tree- 
like in  their  entire  outlines.  On  examining  Figs.  34,  35,  36, 
which  show  the  structures  of  the  stems  in  these  types,  it 
will  be  seen,  too,  that  the  component  cells  in  becoming  more 


THE  MORPHOLOGICAL  COMPOSITION  OP  PLANTS.     29 

coherent,  have  undergone  changes  of  form  which  obscure 
their  individualities  more  than  before.      Not  only  are  they 


much  elongated,  but  they  are  so  compressed  as  to  be  pris- 
matic rather  than  cylindrical.  This  structure,  besides  dis- 
playing integration  of  the  morphological  units  carried  on  in 
two  directions  instead  of  one;  and  besides  displaying  this 
higher  integration  in  the  greater  merging  of  the  individuali- 
ties of  the  morphological  units  in  the  general  individuality; 
also  displays  it  in  the  more  pronounced  subordination  of  the 
branches  and  branchlets  to  the  main  stem.  This  differentia- 
tion and  consolidation  of  the  stem,  brings  all  the  secondary 
growths  into  more  marked  dependence;  and  so  renders  the 
individuality  of  the  aggregate  more  decided. 

We  might  not  inappropriately  call  this  type  of  structure 
pseud-axial.  It  simulates  that  of  the  higher  plants  in  cer- 
tain superficial  characters.  We  see  in  it  a  primary  axis  along 
which  development  may  continue  indefinitely,  and  from 
which  there  bud  out,  laterally,  secondary  axes  of  like  nature, 
bearing  like  tertiary  axes;  and  this  is  a  mode  of  growth 
with  which  PhaBnogams  make  us  familiar. 

§  185.  Some  of  the  larger  Algce  supply  examples  of  an 
integration  still  more  advanced;  not  simply  inasmuch  as 
they  unite  much  greater  numbers  of  morphological  units 


30        MORPHOLOGICAL  DEVELOPMENT. 

into  continuous  masses,  but  also  inasmuch  as  they  combine 
the  pseudo-foliar  structure  with  the  pseud-axial  structure. 
Our  own  shores  furnish  an  instance  of  this  in  the  common 
Laminaria;  and  certain  gigantic  Laminariacece  of  the  Ant- 
arctic seas,  furnish  yet  better  instances.  In  Necrocystis  the 
germ  develops  a  very  long  slender  stem,  which  eventually 
expands  into  a  large  bladder-like  or  cylindrical  air-vessel; 
and  the  surface  of  this  bears  numerous  leaf-shaped  expan- 
sions. Another  kind,  Lessonia  fuscescens,  Fig.  37,  shows  us  a 
massive  stem  growing  up  through  water 
many  feet  deep — a  stem  which,  bifurcating 
as  it  approaches  the  surface,  flattens  out  the 
ends  of  its  subdivisions  into  fronds  like 
ribands.  These,  however,  are  not  true  foliar 
appendages,  since  they  are  merely  expanded 
continuations  of  the  stem.  In  Egregia 
branches  of  the  thallus  not  only  take  the 
form  of  leaves,  but  these  are  differentiated 
into  several  categories  in  accordance  with  a 
division  of  labour.  In  any  of  these  Lamin- 
ariacece the  whole  plant,  great  as  may  be  its 
size,  and  made  up  though  it  seems  to  be  of 
many  groups  of  morphological  units,  united 
into  a  compound  group  by  their  marked  subordination  to  a 
connecting  mass,  is  nevertheless  a  single  thallus,  which  is 
added  to  by  intercalary  growth  at  the  "  transition-place,"  at 
the  junction  of  the  stem-like  and  leaf-like  portions.  The 
aggregate  is  still  an  aggregate  of  the  second  order. 

But  among  certain  of  the  highest  Algce,  we  do  find  some- 
thing more  than  this  union  of  the  pseud-axial  with  the 
pseudo-foliar  structure.  In  addition  to  pseud-axes  of  com- 
parative complexity;  and  in  addition  to  pseudo-folia  that 
are  like  leaves,  not  only  in  their  general  shapes  but  in  hav- 
ing mid-ribs  and  even  veins;  there  are  the  beginnings  of 
a  higher  stage  of  integration.  Figs.  38,  39,  and  40,  show 
some  of  the  steps.  In  Rhodymenia  palmata,  Fig.  38,  the 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     31 

parent-frond  is  comparatively  irregular  in  form,  and  without 
a  mid-rib;  and  along  with  this  very  imperfect  integration, 


we  see  that  the  secondary  fronds  growing  from  the  edges  are 
distributed  very  much  at  random,  and  are  by  no  means 
specific  in  their  shapes.  A  considerable  advance  is  displayed 
by  Pliylloplwra  rubens,  Fig.  39.  Here  the  frond,  primary, 
secondary,  or  tertiary,  betrays  some  approach  towards  regu- 
larity in  both  form  and  size;  by  which,  as  also  by  its 
partially-developed  mid-rib,  there  is  established  a  more 
marked  individuality;  and  at  the  same  time,  the  growth  of 
the  secondary  fronds  no  longer  occurs  anywhere  on  the  edge, 
in  the  same  plane  as  the  parent-frond,  but  from  the  surface 
at  specific  places.  Delesseria  sanguined,  Fig.  40,  illustrates  a 
much  more  definite  arrangement  of  the  same  kind.  The 
fronds  of  this  plant,  quite  regularly  shaped,  have  their  parts 
decidedly  subordinated  to  the  whole;  and  from  their  mid- 
ribs grow  other  fronds  Avhich  are  just  like  them.  Each  of 
? these  fronds  is  an  organized  group  of  those  morphological 
units  which  we  distinguish  as  aggregates  of  the  first  order. 
And  in  this  case,  two  or  more  such  aggregates  of  the  second 


32  MORPHOLOGICAL  DEVELOPMENT. 

order,  well  individuated  by  their  forms  and  structures,  are 
united  together;  and  the  plant  composed  of  them  is  thus 
rendered,  in  so  far,  an  aggregate  of  the  third  order. 

Just  noting  that  in  certain  of  the  most-developed  Algce,  as 
the  Sargassum,  or  common  gulf-weed,  this  tertiary  degree  of 
composition  is  far  more  completely  displayed,  so  as  to  pro- 
duce among  Thallophytes  a  type  of  structure  closely  simulat- 
ing that  of  the  higher  plants,  let  us  now  pass  to  the  considera- 
tion of  these  higher  plants. 

§  186.  Having  the  surface  of  the  soil  for  a  support  and  the 
air  for  a  medium,  terrestrial  plants  are  mechanically  circum- 
stanced in  a  manner  widely  different  from  that  in  which 
aquatic  plants  are  circumstanced.  Instead  of  being  buoyed 
up  by  a  surrounding  fluid  of  specific  gravity  equal  to  their 
own,  they  have  to  erect  themselves  into  a  rare  fluid  which 
yields  no  appreciable  support.  Further,  they  are  dis- 
similarly conditioned  in  having  two  sources  of  nutriment  in 
place  of  one.  Unlike  the  Algce,  which  derive  all  the  mate- 
rials for  their  tissues  from  the  water  bathing  their  entire 
surfaces,  and  use  their  roots  only  for  attachment,  most  of  the 
plants  which  cover  the  Earth's  surface,  absorb  part  of  their 
food  through  their  imbedded  roots  and  part  through  their 
exposed  leaves.  These  two  marked  unlikenesses  in  the  rela- 
tions to  surrounding  conditions,  profoundly  affect  the  respec- 
tive modes  of  growth.  We  must  duly  bear  them  in  mind 
while  studying  the  further  advance  of  composition. 

The  class  of  plants  to  which  we  now  turn — that  of  the 
Archegoniatce — is  nearly  related  by  its  lower  members  to  the 
classes  above  dealt  with :  so  much  so,  that  some  of  the  inferior 
liverworts  are  quite  licheniform,  and  are  often  mistaken  for 
lichens.  Passing  over  these,  let  us  recommence  our  synthesis 
with  such  members  of  the  class  as  repeat  those  indications  of 
progress  towards  a  higher  composition,  which  we  have  just  6b-* 
served  among  the  more-developed  Algce.  The  Jungerman- 
niacece  furnish  us  with  a  series  of  types,  clearly  indicating  the 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     33 

transition  from  an  aggregate  of  the  second  order  to  an  aggre- 
gate of  the  third  order.  Figs.  41,  and  42,  indicate  the  struc- 
ture among  the  lowest  of  this  group.  Here  there  is  but  an  in- 
complete development  of  the  second  order  of  aggregate.  The 


frond  grows  as  irregularly  as  the  thallus  of  a  lichen :  it  is  in- 
definite in  size  and  outline,  spreading  hither  or  thither  as 
the  conditions  favour.  Moreover,  it  lacks  the  differentiations 
required  to  subordinate  its  parts  to  the  whole :  it  is  uniformly 
cellular,  having  neither  mid-rib  nor  veins;  and  it  puts  out 
rootlets  indifferently  from  all  parts  of  its  under-surface.  In 
Fig.  43,  Pellia  epiphylla,  we  have  an  advance  on  this  type. 
There  is  here,  as  shown  in  the  transverse  section,  Fig.  44,  a 
thickening  of  the  frond  along  its  central  portion,  producing 
something  like  an  approach  towards  a  mid-rib;  and  from 
this  the  rootlets  are  chiefly  given  off.  The  outline,  too,  is 
much  less  irregular;  whence  results  greater  distinctness  of 
the  individuality.  A  further  step  is  displayed  in  Metzgeria 
furcata,  Fig.  45.  The  frond  of  this  plant,  comparatively  well 
integrated  by  the  distribution  of  its  substance  around  a 
decided  mid-rib,  and  by  its  comparatively-definite  outlines, 
produces  secondary  fronds.  There  is  what  is  called  prolifer- 
ous growth ;  and  occasionally,  as  shown  in  Fig.  46,  represent- 
ing an  enlarged  portion,  the  growth  is  doubly-proliferous.  In 
these  cases,  however,  the  tertiary  aggregate,  so  far  as  it  is 
formed,  is  but  very  feebly  integrated;  and  its  integration  is 
but  temporary.  For  not  only  do  these  younger  fronds  that 
bud  out  from  the  mid-ribs  of  older  fronds,  develop  rootlets  of 
their  own ;  but  as  soon  as  they  are  well  grown  and  adequately 
rooted,  they  dissolve  their  connexions  with  the  parent-fronds, 
49 


34 


MORPHOLOGICAL  DEVELOPMENT. 


and  become  quite  independent.  From  these  transi- 

tional forms  we  pass,  in  the  higher  Jungermanniacece,  to 


forms  composed  of  many  fronds  that  are  permanently  united 
by  a  continuous  stem.  A  more-developed  aggregate  of  the 
third  order  is  thus  produced.  But  though,  along  with  in- 
creased definiteness  in  the  secondary  aggregates,  there  is  here 
an  integration  of  them  so  extensive  and  so  regular,  that  they 
are  visibly  subordinated  to  the  whole  they  form;  yet  the 
subordination  is  really  very  incomplete.  In  some  instances, 
as  in  Radula  complanata,  Fig.  47,  the  leaflets  develop  roots 
from  their  under  surfaces,  just  as  the  primitive  frond  does; 
and  in  the  majority  of  the  group,  as  in  J.  capitata,  Fig.  48, 
roots  are  given  off  all  along  the  connecting  stem,  at  the  spots 
where  the  leaflets  or  frondlets  join  it:  the  result  being  that 
though  the  connected  frondlets  form  a  physical  whole,  they 
do  not  form,  in  any  decided  manner,  a  physiological  whole; 
since  successive  portions,  of  the  united  series,  carry  on  their 
functions  independently  of  the  rest.  Finally,  the 

most  developed  members  of  the  group,  whether  lineally  de- 
scended from  the  less  developed  or  from  an  early  type  com- 
mon to  the  two,  present  us  with  tertiary  aggregates  which  are 
physiologically  as  well  as  physically  integrated.*  Not  lying 

*  The  great  mass  of  early  ancestral  types— plant  and  animal— consisting  of 
soft  tissues,  have  left  no  remains  whatever,  and  we  have  no  reason  to  suppose 
that  those  which  left  remains  fell  within  the  direct  ancestral  lines  of  any 
existing  forms.  Contrariwise,  we  have  reason  to  suppose  that  they  fell  with- 
in lines  of  evolution  out  of  which  the  lines  ending  in  existing  forms  diverged. 
We  must  therefore  infer  that  the  difficulties  of  affiliation  which  arise  if  we 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     35 

prone  like  the  kinds  thus  far  described,  but  growing  erect, 
the  stem  and  attached  leaflets  become  dependent  upon  a 
single  root  or  group  of  roots;  and  being  so  prevented  from 
carrying  on  their  functions  separately,  are  made  members  of 
a  compound  individual:  there  arises  a  definitely-established 
aggregate  of  the  third  degree  of  composition. 

The  facts  as  arranged  in  the  above  order  are  suggestive. 
Minute  aggregates,  or  cells,  the  grouping  of  which  we  traced 
in  §  182,  showed  us  analogous  phases  of  indefinite  union, 
which  appeared  to  lead  the  way  towards  definite  union.  We 
see  here  among  compound  aggregates,  as  we  saw  there 
among  simple  aggregates,  the  establishment  of  a  specific 
form,  and  a  size  that  falls  within  moderate  limits  of  varia- 
tion. This  passage  from  less  definite  extension  to  more 
definite  extension,  seems  in  the  one  case,  as  the  other,  to  be 
accompanied  by  the  result,  that  growth  exceeding  a  certain 
rate,  ends  in  the  formation  of  a  new  aggregate,  rather  than 
an  enlargement  of  the  old.  And  on  the  higher  stage,  as  on 
the  lower,  this  process,  irregularly  carried  out  in  the  simpler 
types,  produces  in  them  unions  that  are  but  temporary ;  while 
in  the  more-developed  types,  it  proceeds  in  a  systematic  way, 
and  ends  in  the  production  of  a  permanent  aggregate  that  is 
doubly  compound. 

contemplate  divergent  types  now  existing,  would  not  arise  if  we  had  before 
us  all  the  early  intermediate  types.  The  Mammalia  differ  in  sundry  respects 
from  all  other  kinds  of  Vertebrata— Fishes,  Reptiles,  Birds;  and  if  the 
absence  of  hair,  mammae,  and  two  occipital  condyles,  in  these  other  verte- 
brates were  taken  to  imply  a  fundamental  distinction,  it  might,  in  the  absence 
of  any  known  fossil  links,  be  inferred  that  the  Mammalia  belonged  to  a  sepa- 
rate phylum.  But  these  differences  are  not  held  to  negative  the  assumed 
relationship.  Similarly  among  plants.  We  must  not  reject  an  hypothesis 
respecting  a  certain  supposed  type,  because  the  existing  types  it  must  have 
been  akin  to  present  traits  which  it  could  not  have  had.  We  are  justified  in 
assuming,  within  limits,  a  hypothetical  type,  unlike  existing  types  in  traits  of 
some  importance.  Hence  results  the  answer  to  a  criticism  passed  on  the 
above  argument,  that  it  implies  relations  between  the  undeveloped  and  devel- 
oped forms  of  the  Jungermanniaccce  such  as  the  facts  do  not  show  us.  Thia 
objection  is  met  on  remembering  that  the  types  in  which  the  supposed  transi- 
tion took  place  disappeared  myriads  of  years  ago. 


36        MORPHOLOGICAL  DEVELOPMENT. 

Must  we  then  conclude  that  as  cells,  or  morphological 
units,  are  integrated  into  a  unit  of  a  higher  order,  which  we 
call  a  thallus  or  frond;  so,  by  the  integration  of  fronds,  there 
is  evolved  a  structure  such  as  the  above-delineated  species 
possess?  Whether  this  is  the  interpretation  to  be  given  of 
these  plants,  we  shall  best  see  when  considering  whether  it  is 
the  interpretation  to  be  given  of  plants  which  rank  above 
them.  Thus  far  we  have  dealt  only  with  the  Cryptogamia. 
.We  have  now  to  deal  with  the  Phanerogamia  or  Phaenogamia. 


CHAPTER  III. 

THE    MORPHOLOGICAL    COMPOSITION    OF    PLANTS, 
CONTINUED. 

§  187.  THAT  advanced  composition  arrived  at  in  the 
'Arcliegoniatce,  is  carried  still  further  in  the  Flowering  Plants. 
In  these  most-elevated  vegetal  forms,  aggregation  of  the  third 
order  is  always  distinctly  displayed;  and  aggregates  of  the 
fourth,  fifth,  sixth,  &c.,  orders  are  very  common. 

Our  inquiry  into  the  morphology  of  these  flowering  plants, 
may  be  advantageously  commenced  by  studying  the  develop- 
ment of  simple  leaves  into  compound  leaves.  It  is  easy  to 
trace  the  transition,  as  well  as  the  conditions  under  which  it 
occurs;  and  tracing  it  will  prepare  us  for  understanding 
how,  and  when,  metamorphoses  still  greater  in  degree  take 
place. 

§  188.  If  we  examine  a  branch  of  the  common  bramble, 
when  in  flower  or  afterwards,  we  shall  not  unfrequently  find 
a  simple  or  undivided  leaf,  at  the  insertion  of  one  of  the 
lateral  flower-bearing  axes,  composing  the  terminal  cluster 
of  flowers.  Sometimes  this  lea«f  is  partially  lobed ;  sometimes 
cleft  into  three  small  leaflets.  Lower  down  on  the  shoot,  if 
it  be  a  lateral  one,  occur  larger  leaves,  composed  of  three 
leaflets;  and  in  some  of  these,  two  of  the  leaflets  may  be 
lobed  more  or  less  deeply.  On  the  main  stem  the  leaves, 
usually  still  larger,  will  be  found  to  have  five  leaflets.  Sup- 
posing the  plant  to  be  a  well-grown  one,  it  will  furnish  all 

87 

8  n  R  i\  % 


38 


MORPHOLOGICAL  DEVELOPMENT. 


gradations  between  the  simple,  very  small  leaf,  and  the  large 
composite  leaf,  containing  sometimes  even  seven  leaflets. 
Figs.  50  to  64,  represent  leading  stages  of  the  transition. 


What  determines  this  transition?  Observation  shows  that 
the  quintuple  leaves  occur  where  the  materials  for  growth 
are  supplied  in  greatest  abundance;  that  the  leaves  become 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     39 

less  and  less  compound,  in  proportion  to  their  remoteness 
from  the  main  currents  of  sap;  and  that  where  an  entire 
absence  of  divisions  or  lobes  is  observed,  it  is  on  leaves  within 
the  flower-bunch :  at  the  place,  that  is,  where  the  forces  which 
cause  growth  are  nearly  equilibrated  by  the  forces  which 
oppose  growth;  and  where,  as  a  consequence,  gamogenesis  is 
about  to  be  set  in  (§78).  Additional  evidence  that  the  degree 
of  nutrition  determines  the  degree  of  composition  of  the  leaf, 
is  furnished  by  the  relative  sizes  of  the  leaves.  Not  only,  on 
the  average,  is  the  quintuple  leaf  much  larger  in  its  total  area 
than  the  triple  leaf;  but  the  component  leaflets  of  the  one, 
are  usually  much  larger  than  those  of  the  other.  The  like 
contrasts  are  still  more  marked  between  triple  leaves  and 
simple  leaves.  This  connection  of  decreasing  size  with  de- 
creasing composition,  is  conspicuous  in  the  series  of  figures: 
the  differences  shown  being  not  nearly  so  great  as  may  be 
frequently  observed.  Confirmation  may  be  drawn  from  the 
fact  that  when  the  leading  shoot  is  broken  or  arrested  in  its 
growth,  the  shoots  it  gives  off  (provided  they  are  given  off 
after  the  injury),  and  into  which  its  checked  currents  of  sap 
are  thrown,  produce  leaves  of  five  leaflets  where  ordinarily 
leaves  of  three  leaflets  occur.  Of  course  incidental  circum- 
stances, as  variations  in  the  amounts  of  sunshine,  or  of  rain, 
or  of  matter  supplied  to  the  roots,  are  ever  producing  changes 
in  the  state  of  the  plant  as  a  whole;  and  by  thus  affecting 
the  nutrition  of  its  leaf-buds  at  the  times  of  their  formation, 
cause  irregularities  in  the  relations  of  size  and  composition 
above  described.  But  taking  these  causes  into  account,  it  is 
abundantly  manifest  that  a  leaf-bud  of  the  bramble  will 
develop  into  a  simple  leaf  or  into  a  leaf  compounded  in 
different  degrees,  according  to  the  quantity  of  assimilable 
matter  brought  to  it  at  the  time  when  the  rudiments  of  its 
structure  are  being  fixed.  And  on  studying  the  habits  of 
other  plants — on  observing  how  annuals  that  have  compound 
leaves  usually  bear  simple  leaves  at  the  outset,  when  the 
assimilating  surface  is  but  small ;  and  how,  when  compound- 


40 


MORPHOLOGICAL  DEVELOPMENT. 


leaved  plants  in  full  growth  bear  simple  leaves  in  the  midst 
of  compound  ones,  the  relative  smallness  of  such  simple 
leaves  shows  that  the  buds  from  which  they  arose  were  ill- 
supplied  with  sap;  it  will  cease  to  be  doubted  that  a  foliar 
organ  may  be  metamorphosed  into  a  group  of  foliar  organs, 
if  furnished,  at  the  right  time,  with  a  quantity  of  matter 
greater  than  can  be  readily  organized  round  a  single  centre  of 
growth.  An  examination  of  the  transitions  through  which  a 
compound  leaf  passes  into  a  doubly-compound  leaf,  as  seen 
in  the  various  intermediate  forms  of  leaflets  in  Fig.  65,  will 
further  enforce  this  conclusion. 


65 


Here  we  may  advantageously  note,  too,  how  in  such  cases 
the  leaf-stalk  undergoes  concomitant  changes  of  structure. 
In  the  bramble-leaves  above  described,  it  becomes  compound 
simultaneously  with  the  leaf — the  veins  become  mid-ribs  while 
the  mid-ribs  become  petioles.  Moreover,  the  secondary  stalks, 
and  still  more  the  main  stalks,  bear  thorns  similar  in  their 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     41 

shapes,  and  approaching  in  their  sizes,  to  those  on  the  stem; 
besides  simulating  the  stem  in  colour  and  texture.  In  the 
petioles  of  large  compound  leaves,  like  those  of  the  com- 
mon Heracleum,  we  see  still  more  distinctly  both  internal 
and  external  approximations  in  character  to  axes.  Nor  are 
there  wanting  plants  whose  large,  though  simple,  leaves,  are 
held  out  far  from  the  stems  by  foot-stalks  that  are,  near  the 
ends,  sometimes  so  like  axes  that  the  transverse  sections  of 
the  two  are  indistinguishable;  as  instance  the  Calla  palustris. 

One  other  fact  respecting  the  modifications  which  leaves 
undergo,  should  be  set  down.  Not  only  may  leaf-stalks 
assume  to  a  great  degree  the  characters  of  stems,  when  they 
have  to  discharge  the  functions  of  stems,  by  supporting  many 
leaves  or  very  large  leaves;  but  they  may  assume  the  charac- 
ters of  leaves,  when  they  have  to  undertake  the  functions 
of  leaves.  The  Australian  Acacias  furnish  a  remarkable 
illustration  of  this.  Acacias  elsewhere  found  bear  pinnate 
leaves;  but  the  majority  of  those  found  in  Australia  bear 
what  appear  to  be  simple  leaves.  It  turns  out,  however,  that 
these  are  merely  leaf-stalks  flattened  out  into  foliar  shapes: 
the  lamina  of  the  leaves  being  undeveloped.  And  the  proof 
is  that  in  young  plants,  showing  their  kinships  by  their 
embryonic  characters,  these  leaf-like  petioles  bear  true  leaflets 
at  their  ends.  A  metamorphosis  of  like  kind  occurs  in  Oxalis 
bupleurifolia,  Fig.  66.  The  fact  most  deserving  of  notice, 
however,  is  that  these  leaf- 
stalks, in  usurping  the  gen-  66 
eral  aspects  and  functions 
of  leaf-blades,  have,  to  some 
extent,  also  usurped  their 
structures :  though  their  venation  is  not  like  that  of  the  leaf- 
blades  they  replace,  yet  they  have  veins,  and  in  some  cases 
mid-ribs. 

Reduced  to  their  most  general  expression,  the  truths  above 
shadowed  forth  are  these: — That  group  of  morphological 
units,  or  cells,  which  v/e  see  integrated  into  the  compound 


4:2  MORPHOLOGICAL  DEVELOPMENT. 

unit  called  a  leaf,  has,  in  each  higher  plant,  a  typical  form, 
due  to  the  special  arrangement  of  these  cells  around  a  mid- 
rib and  veins.  If  the  multiplication  of  morphological  units, 
at  the  time  when  the  leaf-bud  is  taking  on  its  main  outlines, 
exceeds  a  certain  limit,  these  units  begin  to  arrange  them- 
selves round  secondary  centres,  or  lines  of  growth,  in  such 
ways  as  to  repeat,  in  part  or  wholly,  the  typical  form:  the 
larger  veins  become  transformed  into  imperfect  mid-ribs  of 
partially  independent  leaves;  or  into  complete  mid-ribs  of 
quite  separate  leaves.  And  as  there  goes  on  this  transition 
from  a  single  aggregate  of  cells  to  a  group  of  such  aggregates, 
there  simultaneously  arises,  by  similarly  insensible  steps,  a 
distinct  structure  which  supports  the  several  aggregates  thus 
produced,  and  unites  them  into  a  compound  aggregate.  These 
phenomena  should  be  carefully  studied;  since  they  give  us  a 
key  to  more  involved  phenomena.* 

§  189.  Thus  far  we  have  dealt  with  leaves  ordinarily  so 
called:  briefly  indicating  the  homologies  between  the  parts 
of  the  simple  and  the  compound.  Let  us  now  turn  to  the 
homologies  among  foliar  organs  in  general.  These  have  been 

*  There  is  much  force  in  the  criticism  passed  on  the  above  paragraph,  and 
by  implication  on  some  preceding  paragraphs,  that  though  in  plants  which 
tend  to  produce  compound  leaves  the  production  is  largely  dependent  on  the 
supply  of  nutriment,  yet  the  unqualified  statement  of  this  relation  as  a  gen- 
eral one,  is  negatived  by  the  existence  of  plants  which  bear  only  simple 
leaves,  however  much  high  nutrition  causes  growth.  But  mostly  valid  though 
this  objection  is,  it  is  probably  not  universally  valid.  I  am  led  to  say  this  by 
what  occasionally  occurs  in  flowers.  The  flowering  stem  of  the  Hyacinth  is 
single;  but  I  have  seen  a  cultivated  Hyacinth  in  which  one  of  the  flowers 
had  developed  into  a  lateral  spike.  Still  more  striking  evidence  was  once 
supplied  to  me  by  Agrimony.  All  samples  of  this  plant  previously  seen  had 
single  flowering  spikes,  but  some  years  ago  I  met  with  one,  extremely  luxuri- 
ant, in  which  some  flowers  of  the  primitive  spike  were  replaced  by  lateral 
spikes ;  and  I  am  not  sure  that  some  of  these,  again,  did  not  bear  lateral 
spikes.  Now  if  in  plants  which,  in  probably  millions  of  cases,  have  their 
flowering  stems  single,  excessive  nutrition  changes  certain  of  their  flowers  into 
new  spikes,  it  is  a  reasonable  supposition  that  in  like  manner  plants  which 
are  thought  invariably  to  bear  only  single  leave?,  will,  under  kindred  condi- 
tions, bear  compound  leaves. 


THE   MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     43 

made  familiar  to  readers  of  natural  history  by  popularized 
outlines  of  The  Metamorphosis  of  Plants — a  title,  by  the 
way,  which  is  far  too  extensive;  since  the  phenomena  treated 
of  under  it,  form  but  a  small  portion  of  those  it  properly 
includes. 

Passing  over  certain  vague  anticipations  which  have  been 
quoted  from  ancient  writers,  and  noting  only  that  some 
clearer  recognitions  were  reached  by  Joachim  Jung,  a  Ham- 
burg professor,  in  the  middle  of  the  17th  century;  we  come 
to  the  Theoria  Generationis,  which  Wolff  published  in  1759, 
and  in  which  he  gives  definite  forms  to  the  conceptions  that 
have  since  become  current.  Specifying  the  views  of  Wolff, 
Dr.  Masters  writes : — "  After  speaking  of  the  homologous 
nature  of  the  leaves,  the  sepals  and  petals,  an  homology 
consequent  on  their  similarity  of  structure  and  identity  of 
origin,  he  goes  on  to  state  that  the  '  pericarp  is  manifestly 
composed  of  several  leaves,  as  in  the  calyx,  with  this  differ- 
ence only,  that  the  leaves  which  are  merely  placed  in  close 
contact  in  the  calyx,  are  here  united  together '  ;  a  view  which 
he  corroborates  by  referring  to  the  manner  in  which  many 
capsules  open  and  separate  '  into  their  leaves/  The  seeds, 
too,  he  looks  upon  as  consisting  of  leaves  in  close  combina- 
tion. His  reasons  for  considering  the  petals  and  stamens  as 
homologous  with  leaves,  are  based  upon  the  same  facts  as 
those  which  led  Linnaeus,  and,  many  years  afterwards,  Goethe, 
to  the  same  conclusion.  '  In  a  word,'  says  Wolff,  '  we  see 
nothing  in  the  whole  plant,  whose  parts  at  first  sight  differ 
so  remarkably  from  each  other,  but  leaves  and  stem,  to  which 
latter  the  root  is  referrible/  "  It  'appears  that  Wolff,  too, 
enunciated  the  now-accepted  interpretation  of  compound 
fruits:  basing  it  on  the  same  evidence  as  that  since  assigned. 
In  the  essay  of  Goethe,  published  thirty  years  after,  these 
relations  among  the  parts  of  flowering  plants  were  traced  out 
in  greater  detail,  but  not  in  so  radical  a  way;  for  Goethe  did 
not,  as  did  Wolff,  verify  his  hypothesis  by  dissecting  buds  in 
their  early  stages  of  development.  Goethe  appears  to  have 


44  MORPHOLOGICAL  DEVELOPMENT. 

arrived  at  his  conclusions  independently.  But  that  they 
were  original  with  him,  and  that  he  gave  a  more  variously- 
illustrated  exposition  of  them  than  had  been  given  by  Wolff, 
does  not  entitle  him  to  anything  beyond  a  secondary  place, 
among  those  who  have  established  this  important  generaliza- 
tion. 

Were  it  not  that  these  pages  may  be  read  by  some  to 
whom  Biology,  in  all  its  divisions,  is  a  new  subject  of  study,  it 
would  be  needless  to  name  the  evidence  on  which  this  now- 
familiar  generalization  rests.  For  the  information  of  such 
it  will  suffice  to  say,  that  the  fundamental  kinship  existing 
among  all  the  foliar  organs  of  a  flowering  plant,  is  shown  by 
the  transitional  forms  which  may  be  traced  between  them, 
and  by  the  occasional  assumption  of  one  another's  forms. 
"  Floral  leaves,  or  bracts,  are  frequently  only  to  be  distin- 
guished from  ordinary  leaves  by  their  position  at  the  base  of 
the  flower;  at  other  times  the  bracts  gradually  assume  more 
and  more  of  the  appearance  of  the  sepals."  The  sepals,  or 
divisions  of  the  calyx,  are  not  unlike  undeveloped  leaves: 
sometimes  assuming  quite  the  structure  of  leaves.  In  other 
cases,  they  acquire  partially  or  wholly  the  colours  of  the 
petals — as,  indeed,  the  bracts  and  uppermost  stem-leaves 
occasionally  do.  Similarly,  the  petals  show  their  alliances  to 
the  foliar  organs  lower  down  on  the  axis,  and  to  those  higher 
up  on  the  axis.  On  the  one  hand,  they  may  develop  into 
ordinary  leaves  that  are  green  and  veined;  and,  on  the  other 
hand,  as  so  commonly  seen  in  double  flowers,  they  may  bear 
anthers  on  their  edges.  All  varieties  of  gradation  into 
neighbouring  foliar  organs  may  be  witnessed  in  stamens. 
Flattened  and  tinted  in  various  degrees,  they  pass  insensibly 
into  petals,  and  through  them  prove  their  homology  with 
leaves;  into  which,  indeed,  they  are  transformed  in  flowers 
that  become  wholly  foliaceous.  The  style,  too,  is  occasionally 
changed  into  petals  or  into  green  leaflets;  and  even  the 
ovules  are  now  and  then  seen  to  take  on  leaf-like  forms. 
Thus  we  have  clear  evidence  that  in  Phaenogams,  all  the 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     45 

appendages  of  the  axis  are  homologues :  they  are  all  modified 
leaves. 

Wolff  established,  and  Goethe  further  illustrated,  another 
general  law  of  structure  in  flowering  plants.  Each  leaf 
commonly  contains  in  its  axil  a  bud,  similar  in  structure  to 
the  terminal  bud.  This  axillary  bud  may  remain  unde- 
veloped; or  it  may  develop  into  a  lateral  shoot  like  the 
main  shoot;  or  it  may  develop  into  a  flower.  If  a  shoot 
bearing  lateral  flowers  be  examined,  it  will  be  found  that  the 
internode,  or  space  which  separates  each  leaf  with  its  axillary 
flower  from  the  leaf  and  axillary  flower  above  it,  becomes 
gradually  less  towards  the  upper  end  of  the  shoot.  In  some 
plants,  as  in  the  fox-glove,  the  internodes  constitute  a 
regularly-diminishing  series.  In  other  plants,  the  series  they 
form  suddenly  begins  to  diminish  so  rapidly,  as  to  bring  the 
flowers  into  a  short  spike :  instance  the  common  orchis.  And 
again,  by  still  more  sudden  dwarfing  of  the  internodes,  the 
flowers  are  brought  into  a  cluster;  as  they  are  in  the  cow- 
slip. On  contemplating  a  clover  flower,  in  which  this 
clustering  has  been  carried  so  far  as  to  produce  a  com- 
pact head;  and  on  considering  what  must  happen  if,  by  a 
further  arrest  of  axial  development,  the  foot-stalks  of  the 
florets  disappear;  it  will  be  seen  that  there  must  result  a 
crowd  of  flowers,  seated  close  together  on  the  end  of  the  axis. 
And  if,  at  the  same  time,  the  internodes  of  the  upper  stem- 
leaves  also  remain  undeveloped,  these  stem-leaves  will  be 
grouped  into  a  common  involucre:  we  shall  have  a  composite 
flower,  such  as  the  thistle.  Hence,  to  modifications  in  the 
developments  of  foliar  organs,  have  to  be  added  modifications 
in  the  developments  of  axial  organs.  Comparisons  disclose 
the  gradations  through  which  axes,  like  their  appendages, 
pass  into  all  varieties  of  size,  proportion,  and  structure.  And 
we  learn  that  the  occurrence  of  these  two  kinds  of  metamor- 
phosis, in  all  conceivable  degrees  and  combinations,  furnishes 
us  with  a  proximate  interpretation  of  morphological  com- 
position in  Phaenogams. 


46  MORPHOLOGICAL  DEVELOPMENT. 

I  say  a  proximate  interpretation,  because  there  remain  to 
be  solved  certain  deeper  problems;  one  of  which  at  once 
presents  itself  to  be  dealt  with  under  the  present  head. 
Leaves,  petals,  stamens,  &c.,  being  shown  to  be  homologous 
foliar  organs;  and  the  part  to  which  they  are  attached, 
proving  to  be  an  indefinitely-extended  axis  of  growth,  or 
axial  organ;  we  are  met  by  the  questions, — What  is  a  foliar 
organ  ?  and  What  is  an  axial  organ  ?  The  morphological  com- 
position of  a  Phaenogam  is  undetermined,  so  long  as  we  can- 
not say  to  what  lower  structures  leaves  and  shoots  are  homo- 
logous; and  how  this  integration  of  them  originates.  To 
these  questions  let  us  now  address  ourselves. 

§  190-1.  Already,  in  §  78,  reference  has  been  made  to  the 
occasional  development  of  foliar  organs  into  axial  organs: 
the  special  case  there  described  being  that  of  a  fox-glove,  in 
which  some  of  the  sepals  were  replaced  by  flower-buds. 
The  observation  of  these  and  some  analogous  monstrosities, 
raising  the  suspicion  that  the  distinction  between  foliar 
organs  and  axial  organs  is  not  absolute,  led  me  to  examine 
into  the  matter;  and  the  result  has  been  the  deepening  of 
this  suspicion  into  a  conviction.  Part  of  the  evidence  is  given 
in  Appendix  A. 

Some  time  after  having  reached  this  conviction,  I  found  on 
looking  into  the  literature  of  the  subject,  that  analogous  ir- 
regularities had  suggested  to  other  observers,  beliefs  similarly 
at  variance  with  the  current  morphological  creed.  Diffi- 
culties in  satisfactorily  defining  these  two  elements,  have 
served  to  shake  this  creed  in  some  minds.  To  others,  the 
strange  leaf-like  developments  which  axes  undergo  in  certain 
plants,  have  afforded  reasons  for  doubting  the  constancy  of 
this  distinction  which  vegetal  morphologists  usually  draw. 
And  those  not  otherwise  rendered  sceptical,  have  been  made 
to  hesitate  by  such  cases  as  that  of  the  Nepaul-barley,  in 
which  the  glume,  a  foliar  organ,  becomes  developed  into  an 
axis  and  bears  flowers.  In  his  essay — "Vegetable  Morph- 


THE  MORPHOLOGICAL  COMPOSITION  OP   PLANTS.     47 

ology :  its  History  and  Present  Condition,"  *  whence  I  have 
already  quoted,  Dr.  Masters  indicates  sundry  of  the  grounds 
for  thinking  that  there  is  no  impassable  demarcation  between 
leaf  and  stem.  Among  other  difficulties  which  meet  us  if  we 
assume  that  the  distinction  is  absolute,  one  is  implied  by  this 
question : — "  What  shall  we  say  to  cases  such  as  those 
afforded  by  the  leaves  of  Guarea  and  Tricliilia,  where  the 
leaves  after  a  time  assume  the  condition  of  branches  and  de- 
velop young  leaflets  from  their  free  extremities,  a  process  less 
perfectly  seen  in  some  of  the  pinnate-leaved  kinds  of  Herberts 
or  Mdhonia,  to  be  found  in  almost  every  shrubbery  ?  " 

A  class  of  facts  on  which  it  will  be  desirable  for  us  here  to 
dwell  a  moment,  before  proceeding  to  deal  with  the  matter 
deductively,  is  presented  by  the  Cactaceaz.  In  this  remark- 
able group  of  plants,  deviating  in  such  varied  ways  from  the 
ordinary  phsenogamic  type,  we  find  many  highly  instructive 
modifications  of  form  and  structure.  By  contemplating  the 
changes  here  displayed  within  the  limits  of  a  single  order, 
we  shall  greatly  widen  our  conception  of  the  possibilities  of 
metamorphosis  in  the  vegetal  kingdom,  taken  as  a  whole. 
Two  different,  but  similarly-significant,  truths  are  illustrated. 
First,  we  are  shown  how,  of  these  two  components  of  a 
flowering  plant,  commonly  regarded  as  primordially  distin- 
guished, one  may  assume,  throughout  numerous  species,  the 
functions,  and  to  a  great  degree  the  appearance,  of  the  other. 
Second,  we  are  shown  how,  in  the  same  individual,  there 
may  occur  a  re-metamorphosis:  the  usurped  function  and 
appearance  being  maintained  in  one  part  of  the  plant,  while 
in  another  part  there  is  a  return  to  the  ordinary  appearance 
and  function.  We  will  consider  these  two  truths  sepa- 
rately. Some  of  the  Euphorbiacece,  which  simulate 
Cactuses,  show  us  the  stages  through  which  such  abnormal 
structures  are  arrived  at.  In  Euphorbia  splendens,  the  lateral 
axes  are  considerably  swollen  at  their  distal  ends,  so  as  often 
to  be  club-shaped:  still,  however,  being  covered  with  bark 

*  See  British  and  Foreign  Medico- Chirurgical  Review  for  January,  1862. 


48        MORPHOLOGICAL  DEVELOPMENT. 

of  the  ordinary  colour,  and  still  bearing  leaves.  But  in 
kindred  plants,  as  Euphorbia  neriifolia,  this  swelling  of  the 
lateral  axes  is  carried  to  a  far  greater  extent;  and,  at  the 
same  time,  a  green  colour  and  a  fleshy  consistence  have  been 
acquired:  the  typical  relations  nevertheless  being  still  shown 
by  the  few  leaves  that  grow  out  of  these  soft  and  swollen 
axes.  In  the  Cactacece,  which  are  thus  resembled  by  plants 
not  otherwise  allied  to  them,  we  have  indications  of  a 
parallel  transformation.  Some  kinds,  not  commonly  brought 
to  England,  bear  leaves;  but  in  the  species  most  familiar  to 
us,  the  leaves  are  undeveloped  and  the  axes  assume  their 
functions.  Passing  over  the  many  varieties  of  form  and 
combination  which  these  green  succulent  growths  display,  we 
have  to  note  that  in  some  genera,  as  in  Phyllocactus,  they 
become  flattened  out  into  foliaceous  shapes,  having  mid-ribs 
and  something  approaching  to  veins.  So  that  here,  and  in 
the  genus  Epiphyllum,  which  has  this  character  still  more 
marked,  the  plant  appears  to  be  composed  of  fleshy  leaves 
growing  one  upon  another.  And  then,  in  Rhipsalis,  the 
same  parts  are  so  leaf-like,  that  an  uncritical  observer 
would  regard  them  as  leaves.  These  which  are  axial  organs 
in  their  homologies,  have  become  foliar  organs  in  their 
analogies.  When,  instead  of  comparing  these 

strangely-modified  axes  in  different  genera  of  Cactuses,  we 
compare  them  in  the  same  individual,  we  meet  with  transfor- 
mations no  less  striking.  Where  a  tree-like  form  is  pro- 
duced by  the  growth  of  these  foliaceous  shoots,  one  on  another ; 
and  where,  as  a  consequence,  the  first-formed  of  them  become 
the  main  stem  that  acts  as  support  to  secondary  and  tertiary 
stems;  they  lose  their  green,  succulent  character,  acquire 
bark,  and  become  woody.  In  resuming  the  functions  of  axes 
they  resume  the  structures  of  axes,  from  which  they  had  devi- 
ated. In  Fig.  71  are  shown  some  of  the  leaf-like  axes  of 
Rhipsalis  rhombea  in  their  young  state ;  while  Fig.  72  repre- 
sents the  oldest  portion  of  the  same  plant,  in  which  the  foli- 
aceous characters  are  quite  obliterated,  and  there  has  resulted 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     49 

an  ordinary  stem-structure.  One  further  fact  is  to 

be  noted.  At  the  same  time  that  their  leaf-like  appearances 
are  lost,  the  axes  also  lose 
their  separate  individuali- 
ties. As  they  become  stem- 
like,  they  also  become  inte- 
grated; and  they  do  this  so 
effectually  that  their  origi- 
nal points  of  junction,  at 
first  so  strongly  marked,  are  effaced,  and  a  consolidated  trunk 
is  produced. 

Joined  with  the  facts  previously  specified,  these  facts 
help  us  to  conceive  how,  in  the  evolution  of  flowering  plants 
in  general,  the  morphological  components  that  were  once 
distinct,  may  become  extremely  disguised.  We  may  ration- 
ally expect  that  during  so  long  a  course  of  modification, 
much  greater  changes  of  form,  and  much  more  decided  fusions 
of  parts,  have  taken  place.  Seeing  how,  in  an  individual 
plant,  the  single  leaves  pass  into  compound  leaves,  by  the 
development  of  their  veins  into  mid-ribs  while  their  petioles 
begin  to  simulate  axes;  and  seeing  that  leaves  ordinarily 
exhibiting  definitely-limited  developments,  occasionally  pro- 
duce other  leaves  from  their  edges;  we  are  led  to  suspect  the 
possibility  of  still  greater  changes  in  foliar  organs.  When, 
further,  we  find  that  within  the  limits  of  one  natural  order, 
petioles  usurp  the  functions  and  appearances  of  leaves,  at  the 
same  time  that  in  other  orders,  as  in  Ruscus,  lateral  axes  so 
simulate  leaves  that  their  axial  nature  would  by  most  not  be 
suspected,  did  they  not  bear  flowers  on  their  mid-ribs  or 
edges;  and  when,  among  Cactuses,  we  perceive  that  such 
metamorphoses  and  re-metamorphoses  take  place  with  great 
facility;  our  suspicion  that  the  morphological  elements  of 
Phffinogams  admit  of  profound  transformations,  is  deepened. 
And  then,  on  discovering  how  frequent  are  the  monstrosities 
which  do  not  seem  satisfactorily  explicable  without  admitting 
the  development  of  foliar  organs  into  axial  organs ;  we  become 
50 


50        MORPHOLOGICAL  DEVELOPMENT. 

ready  to  entertain  the  hypothesis  that  during  the  evolution 
of  the  phasnogamic  type,  the  distinction  between  leaves  and 
axes  has  arisen  by  degrees. 

With  our  preconceptions  loosened  by  such  facts,  and 
carrying  with  us  the  general  idea  which  such  facts  suggest, 
let  us  now  consider  in  what  way  the  typical  structure  of  a 
flowering  plant  may  be  interpreted. 

§  192.  To  proceed  methodically,  we  must  seek  a  clue  to 
the  structures  of  Phanerogams,  in  the  structures  of  those 
inferior  plants  that  approach  to  them — Archegoniatce.  The 
various  divisions  of  this  class  present,  along  with  sundry 
characters  which  ally  them  with  Thallophytes,  other  charac- 
ters by  which  the  phaenogamic  structure  is  shadowed  forth. 
While  some  of  the  inferior  Hepaticce  or  Liverworts,  severally 
consist  of  little  more  than  a  thallus-like  frond,  among  the 
higher  members  of  this  group,  and  still  more  among  the 
Mosses  and  Ferns,  we  find  a  distinctly  marked  stem.*  Some 
Archegoniates  (or  rather  Ehizoids)  have  foliar  expansions 
that  are  indefinite  in  their  forms;  and  some  have  quite 
definitely-shaped  leaves.  Eoots  are  possessed  by  all  the 
more  developed  genera  of  the  class;  but  there  are  other 
genera,  as  Sphagnum,  which  have  no  roots.  Here  the 
fronds  are  formed  of  only  a  single  layer  of  cells;  and 
there  a  double  layer  gives  them  a  higher  character — a  differ- 
*  Schleiden,  who  chooses  to  regard  as  an  axis  that  which  Mr.  Berkeley, 
with  more  obvious  truth,  calls  a  mid-rib,  says : — "  The  flat  stem  of  the  Liver- 
worts presents  many  varieties,  consisting  frequently  of  one  simple  layer  of 
thin-walled  cells,  or  it  exhibits  in  its  axis  the  elements  of  the  ordinary  stem." 
This  passage  exemplifies  the  wholly  gratuitous  hypotheses  which  men  will 
sometimes  espouse,  to  escape  hypotheses  they  dislike.  Schleiden,  with  the 
positiveness  characteristic  of  him,  asserts  the  primordial  distinction  between 
axial  organs  and  foliar  organs.  In  the  higher  Archegoniates  he  sees  an 
undeniable  stem.  In  the  lower  Archegoniates,  clearly  allied  to  them  by 
their  fructification,  there  is  no  structure  having  the  remotest  resemblance 
to  a  stem.  But  to  save  his  hypothesis,  Schleiden  calls  that  "  a  flat  stem," 
which  is  obviously  a  structure  in  which  stem  and  leaf  are  not  differ- 
entiated. He  is  the  more  to  be  blamed  for  this  unphilosophical  assumption, 
Bince  he  is  merciless  in  his  strictures  on  the  unphilosophical  assumptions  of 
other  botanists. 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     51 

ence  exhibited  between  closely  allied  genera  of  one  group,  the 
Mosses.  Equally  varied  are  the  developments  of  the  foliar- 
organs  in  their  detailed  structures:  now  being  without  mid- 
ribs or  veins;  now  having  mid-ribs  but  no  veins;  now  having 
both  mid-ribs  and  veins.  Nor  must  we  omit  the  similarly-sig- 
nificant circumstance,  that  whereas  in  the  lower  Archegoniates 
the  reproductive  elements  are  immersed  here  and  there  in  the 
thallus-like  frond,  they  are,  in  the  higher  orders,  seated  in 
well-specialized  and  quite  distinct  fructifying  organs,  having 
analogies  with  the  flowers  of  Phsenogams.  Thus,  many  facts 
imply  that  if  the  Phasnogamic  type  is  to  be  analyzed  at  all, 
we  must  look  among  the  Archegoniates  for  its  morphological 
components,  and  the  manner  of  their  integration. 

Already  we  have  seen  among  the  lower  Cryptogamia,  how, 
as  they  became  integrated  and  definitely  limited,  aggregates 
acquire  the  habit  of  budding  out  other  aggregates,  on  reach- 
ing certain  stages  of  growth.  Cells  produce  other  cells 
endogenously  or  exogenously;  and  fronds  give  origin  to 
other  fronds  from  their  edges  or  surfaces.  We  have  seen,  too, 
that  the  new  aggregates  so  produced,  whether  of  the  first 
order  or  the  second  order,  may  either  separate  or  remain 
connected.  Fissiparously-multiplying  cells  in  some  cases 
part  company,  while  in  other  cases  they  unite  into  threads  or 
laminae  or  masses;  and  fronds  originating  proliferously  from 
other  fronds,  sometimes  when  mature  disconnect  themselves 
from  their  parents,  and  sometimes  continue  attached  to  them. 
Whether  they  do  or  do  not  part,  is  clearly  determined  by 
their  nutrition.  If  the  conditions  are  such  that  they  can 
severally  thrive  better  by  separating  after  a  certain  develop- 
ment is  reached,  it  will  become  their  habit  then  to  separate; 
since  natural  selection  will  favour  the  propagation  of  those 
which  separate  most  nearly  at  that  time.  If,  conversely,  it 
profits  the  species  for  the  cells  or  fronds  to  continue  longer 
attached,  which  it  can  only  do  if  their  growths  and  subse- 
quent powers  of  multiplication  are  thereby  increased,  it  must 
happen,  through  the  continual  survival  of  the  fittest,  that 


52         MORPHOLOGICAL  DEVELOPMENT. 

longer  attachment  will  become  an  established  characteristic; 
and,  by  persistence  in  this  process,  permanent  attachment 
will  result  when  permanent  attachment  is  advantageous. 
That  disunion  is  really  a  consequence  of  relative  innutrition, 
and  union  a  consequence  of  relative  nutrition,  is  clear  d 
posteriori.  On  the  one  hand,  the  separation  of  the  new  indi- 
viduals, whether  in  germs  or  as  developed  aggregates,  is  a 
dissolving  away  of  the  connecting  substance ;  and  this  implies 
that  the  connecting  substance  has  ceased  to  perform  its 
function  as  a  channel  of  nutriment.  On  the  other  hand, 
where,  as  we  see  among  Phanogams,  there  is  about  to  take 
place  a  separation  of  new  individuals  in  the  shape  of  germs, 
at  the  point  where  the  nutrition  is  the  lowest,  a  sudden 
increase  of  nutrition  will  cause  the  impending  separation  to 
be  arrested;  and  the  fructifying  elements,  reverting  towards 
the  ordinary  form,  thereupon  develop  in  connexion  with  the 
parent.  Turning  to  the  Archegoniates,  we  find  among 

them  many  indications  of  this  transition  from  discontinuous 
development  to  continuous  development.  Thus  the  Liverworts 
give  origin  to  new  plants  by  cells  which  they  throw  off  from 
their  surfaces;  as,  indeed,  we  have  seen  that  much  higher 
plants  do.  "According  to  Bischoff,"  says  Schleiden,  "both 
the  cells  of  the  stem  (Jungermannia  [now  Lophocolea]  biden- 
tata)  and  those  of  the  leaves  (J.  exsecta)  separate  themselves 
as  propagative  cells  from  the  plant,  and  isolated  cells  shoot 
out  and  develop  while  still  connected  with  the  parent  plant 
into  small  cellular  bodies  (Metzgeria  furcata),  which  separate 
from  the  plant,  and  grow  into  new  plants,  as  in  Mnium  andro- 
gynum  among  the  Mosses."  Now  in  the  way  above  explained, 
these  propagative  cells  and  proliferous  buds,  may  continue 
developing  in  connexion  with  the  parent  to  various  degrees 
before  separating;  or  the  buds  which  are  about  to  become 
fructifying  organs  may  similarly,  under  increased  nutrition, 
develop  into  young  fronds.  As  Sir  W.  Hooker  says  of  the 
male  fructification  in  Metzgeria  furcata, — "  It  has  the  appear- 
ance of  being  a  young  shoot  or  innovation  (for  in  colour 


THE   MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     53 


and  texture  I  can  perceive  no  difference)  rolled  up  into 
a  spherical  figure."  On  finding  in  this  same  plant,  that 
sometimes  the  proliferously-produced  frond  buds  out  from 
itself  another  frond  before  separating  from  the  parent, 
as  shown  in  Fig.  46,  it  becomes  clear 
that  this  long-continued  connexion  may 
readily  pass  into  permanent  connexion. 
And  when  we  see  how,  even  among  Phae- 
nogams,  buds  may  either  detach  them- 
selves as  bulbils,  or  remain  attached  and 
become  shoots;  we  can  scarcely  doubt 
that  among  inferior  plants,  less  definite  in 
their  modes  of  organization,  such  transi- 
tions must  continually  occur. 

Let  us  suppose,  then,  that  Fig.  73  is 
the  frond  of  some  primitive  Archegoniate, 
similar  in  general  characters  to  Pellia 
epiphylla,  Fig.  43 ;  bearing,  like  it,  the 
fructifying  buds  on  its  upper  surface, 
and  having  a  slightly-marked  mid-rib  and 
rootlets.  And  suppose  that,  as  shown, 
a  secondary  frond  is  proliferously  pro- 
duced from  the  mid-rib,  and  continues 
attached  to  it.  Evidently  the  ordinary  dis- 
continuous development,  can  thus  become 
a  continuous  development,  only  on  con- 
dition that  there  is  an  adequate  supply, 
to  the  secondary  frond,  of  such  materials 
as  are  furnished  by  the  rootlets:  the  re- 
maining materials  being  obtainable  by 
itself  from  the  air.  Hence,  that  portion  of 
the  mid-rib  lying  between  the  secondary 
frond  and  the  chief  rootlets,  having  its  function  increased, 
will  increase  in  bulk.  An  additional  consequence  will  be  a 
greater  concentration  of  the  rootlets — there  will  be  extra 
growth  of  those  which  are  most  serviceably  placed.  Observe, 


54:  MORPHOLOGICAL  DEVELOPMENT. 

next,  that  the  structure  so  arising  is  likely  to  be  main- 
tained. Such  a  variation  implying,  as  it  does,  circumstances 
especially  favourable  to  the  growth  of  the  plant,  will  give 
to  the  plant  extra  chances  of  leaving  descendants;  since 
the  area  of  frond  supported  by  a  given  area  of  the  soil, 
being  greater  than  in  other  individuals,  there  may  be  a 
greater  production  of  spores.  And  then,  among  the  more 
numerous  descendants  thus  secured  by  it,  the  variation  will 
give  advantages  to  those  in  which  it  recurs.  Such  a  mode 
of  growth  having,  in  this  manner,  become  established,  let 
us  ask  what  is  next  likely  to  result.  If  it  becomes  the 
habit  of  the  primary  frond  to  bear  a  secondary  frond  from  its 
mid-rib,  this  secondary  frond,  composed  of  physiological 
units  of  the  same  kind,  will  inherit  the  habit ;  and  supposing 
that  the  supply  of  mineral  matters  obtained  by  the  rootlets 
suffices  for  the  full  development  of  the  secondary  frond,  there 
is  a  likelihood  that  the  growth  from  it  of  a  tertiary  frond, 
will  become  an  habitual  characteristic  of  the  variety.  Along 
with  the  establishment  of  such  a  tertiary  frond,  as  shown  in 
Fig.  74,  there  must  arise  a  further  development  of  mid-rib 
in  the  primary  frond,  as  well  as  in  the  secondary  frond — a 
development  which  must  bring  with  it  a  greater  integration 
of  the  two;  while,  simultaneously,  extra  growth  will  take 
place  in  such  of  the  rootlets  as  are  most  directly  connected 
with  this  main  channel  of  circulation.  Without  further  ex- 
planation it  will  be  seen,  on  inspecting  Figs.  75  and  76, 
that  there  may  in  this  manner  result  an  integrated  series  of 
fronds,  placed  alternately  on  opposite  sides  of  a  connecting 
vascular  structure.  That  this  connecting  vascular  structure 
will,  as  shown  in  the  figures,  become  more  distinct  from  the 
foliar  surfaces  as  these  multiply,  is  no  unwarranted  assump- 
tion ;  for  we  have  seen  in  compound-leaved  plants,  how,  under 
analogous  conditions,  mid-ribs  become  developed  into  sepa- 
rate supporting  parts,  which  acquire  some  of  the  characters 
of  axes  while  assuming  their  functions.  And  now 

mark  how  clearly  the  structure  thus  built  up  by  integration 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     55 

of  proliferously-growing  fronds,  corresponds  with  the  struc- 
ture of  the  more  developed  Jungermanniacece.  Each  of  the 
fronds  successively  produced,  repeating  the  characters  of  its 
parent,  will  bear  roots;  and  will  bear  them  in  homologous 
places,  as  shown.  Further,  the  united  mid-ribs  having  but 
very  little  rigidity,  will  be  unable  to  maintain  an  erect  posi- 
tion. Hence  there  will  result  the  recumbent,  continuously- 
rooted  stem,  which  these  types  exhibit :  an  embryo  phaenogam 
having  the  weakness  of  an  embryo.* 

A  natural  concomitant  of  the  mode  of  growth  here  de- 
scribed, is  that  the  stem,  while  it  increases  longitudinally, 
increases  scarcely  at  all  transversely:  hence  the  old  name 
Acrogens.  Clearly  the  transverse  development  of  a  stem  is 
the  correlative,  partly  of  its  function  as  a  channel  of  circula- 
tion, and  partly  of  its  function  as  a  mechanical  support. 
That  an  axis  may  lift  its  attached  leaves  into  the  air,  implies 
thickness  and  solidity  proportionate  to  the  mass  of  such 
leaves;  and  an  increase  of  its  sap-vessels,  also  proportionate 
to  the  mass  of  such  leaves,  is  necessitated  when  the  roots  are 
all  at  one  end  and  the  leaves  at  the  other.  But  in  the 
generality  of  Acrogens,  these  conditions,  under  which  arises 
the  necessity  for  transverse  growth  of  the  axis,  are  absent 
wholly  or  in  great  part.  The  stem  habitually  creeps  below 
the  surface,  or  lies  prone  upon  the  surface;  and  where  it 
grows  in  a  vertical  or  inclined  direction,  does  this  by  attach- 
ing itself  to  a  vertical  or  inclined  object.  Moreover,  throwing 
out  rootlets,  as  it  mostly  does,  at  intervals  throughout  its 
length,  it  is  not  called  upon  in  any  considerable  degree,  to 
transfer  nutritive  materials  from  one  of  its  ends  to  the  other. 

•To  this  interpretation  it  is  objected  that  "the  more  developed  Junger- 
manniacece "  do  not  appear  to  have  arisen  from  the  lower  forms  of  Junger- 
manniacece — that  is  to  say,  from  such  lower  forms  as  are  now  existing.  It 
may,  however,  be  contended  that  this  fact  does  not  exclude  the  interpretation 
given ;  since  the  higher  forms  may  well  have  been  evolved,  not  from  any  of 
the  lower  forms  we  now  know,  but  from  lower  forms  which  have  become 
extinct.  This,  indeed,  is  the  implication  of  the  evolutionary  process  as 
pointed  out  in  the  note  to  Chap.  I.  If  then  we  assume  some  early  type  of 
intermediate  structure,  the  explanation  may  not  improbably  hold. 


56        MORPHOLOGICAL  DEVELOPMENT. 

Hence  this  peculiarity  which  gives  their  name  to  the  Acrogens, 
now  called  Archegoniates,  is  a  natural  accompaniment  of  the 
low  degree  of  specialization  reached  in  them.  And  that  it  is 
an  incidental  and  not  a  necessary  peculiarity,  is  demonstrated 
by  two  converse  facts.  On  the  one  hand,  in  those  higher 
Acrogens  which,  like  the  tree-ferns,  lift  large  masses  of 
foliage  into  the  air,  there  is  just  as  decided  a  transverse  ex- 
pansion of  the  axis  as  in  dicotyledonous  trees.  On  the  other 
hand,  in  those  Dicotyledons  which,  like  the  common  Dodder, 
gain  support  and  nutriment  from  the  surfaces  over  which 
they  creep,  there  is  no  more  lateral  expansion  of  the  axis 
than  is  habitual  among  Acrogens  or  Archegoniates.  Con- 
cluding, as  we  are  thus  fully  justified  in  doing,  that  the 
lateral  expansion  accompanying  longitudinal  extension,  which 
is  a  general  characteristic  of  Phanerogams  as  distinguished 
from  Archegoniates,  is  nothing  more  than  a  concomitant  of 
their  usually-vertical  growth ;  *  let  us  now  go  on  to  consider 
how  vertical  growth  originates,  and  what  are  the  structural 
changes  it  involves. 

§  193.  Plants  depend  for  their  prosperity  mainly  on  air 
and  light :  they  dwindle  where  they  are  smothered,  and  thrive 
where  they  can  expand  their  leaves  into  free  space  and  sun- 
shine. Those  kinds  which  assume  prone  positions,  conse- 
quently labour  under  disadvantages  in  being  habitually  inter- 
fered with  by  one  another — they  are  mutually  shaded  and 

*  I  am  indebted  to  Dr.  Hooker  for  pointing  out  further  facts  supporting 
this  view.  In  his  Flora  Antarctica,  he  describes  the  genus  Lessonia  (sec 
Fig.  37),  and  especially  L.  ovata,  as  having  a  mode  of  growth  simulating  that 
of  the  dicotyledonous  trees,  not  only  in  general  form  but  in  internal  struc- 
ture. The  tall  vertical  stem  thickens  as  it  grows,  by  the  periodical  addition 
of  layers  to  its  periphery.  That  even  Thallophytes  should  thus,  under 
certain  conditions,  present  a  transversely-increasing  axis,  shows  that  there 
is  nothing  absolutely  characteristic  of  Phanerogams  in  their  habit  of 
stem-thickening.  Mr.  Tansley  gives  me  further  verification  by  the  state- 
ment that  "it  is  also  now  certain  that  members  of  the  Equisetinece  and 
Lycopodinece,  as  well  as  some  Ferns  which  flourished  in  Carboniferous  times, 
had  secondary  thickening  in  their  stems  quite  comparable  to  that  of  modern 
Dicotyledonous  trees." 


THE  MORPHOLOGICAL  COMPOSITION   OF  PLANTS.     57 


mutually  injured.  Such  of  them,  however,  as  happen,  by 
variations  in  mode  of  growth,  to  rise  higher  than  others, 
are  more  likely  to  flourish  and  leave  offspring  than  others. 
That  is  to  say,  natural  selection  will  favour  the  more  upright- 
growing  forms.  Individuals  with  structures  which  lift  them 
above  the  rest,  are  the  fittest  for  the  conditions;  and  by  the 
continual  survival  of  the  fittest,  such  structures  must  become 
established.  There  are  two  essentially-different  ways  in 
which  the  integrated  series  of  fronds  above  described,  may  be 
modified  so  as  to  acquire  the  stiffness  needful  for  maintaining 
perpendicularity.  We  will  consider  them  separately. 

A  thin  layer  of  substance  gains  greatly  in  power  of  resist- 
ing a  transverse  strain,  if  it  is  bent  round  so  as  to  form  a 
tube :  witness  the  difference  between  the  pliability  of  a  sheet 
of  paper  when  outspread,  and  the  rigidity  of  the  same  sheet 
of  paper  when  rolled  up.  Engineers  constantly  recognize 
this  truth,  in  devising  appliances  by  which  the  greatest 
strength  shall  be  obtained  at  the  smallest  cost  of  material; 
and  among  organisms,  we  see  that  natural  selection  habit- 
ually establishes  structures  conforming  to  the  same  principle, 
wherever  lightness  and  stiffness  are  to  be  combined.  The 
cylindrical  bones  of  mammals  and  birds,  and  the  hollow 
shafts  of  feathers,  are  examples.  The  lower  plants,  too,  fur- 
nish cases  where  the  strength  ' 
needful  for  maintaining  an 
upright  position,  is  acquired 
by  this  rolling  up  of  a  flat 
thallus  or  frond.  In  Fig.  77 
we  have  an  Alga  which  ap- 
proaches towards  a  tubular 
distribution  of  substance ; 
and  which  has  a  consequent 
rigidity.  Sundry  common 
forms  of  lichen,  having  the 
thallus  folded  into  a  branched  tube,  still  more  decidedly  dis- 
play the  connexion  between  this  structural  arrangement 


77 


58  MORPHOLOGICAL  DEVELOPMENT. 

and  this  mechanical  advantage.  And  from  the  particular 
class  of  plants  we  are  here  dealing  with — the  Archegoniates — 
a  type  is  shown  in  Fig.  78,  Riella  helicophylla,  similarly  char- 
acterized by  a  thin  frond  that  is  made  stiff  enough  to  stand, 
by  an  incurving  which,  though  it  does  not  produce  a  hollow 
cylinder,  produces  a  kindred  form.  If,  then,  as  we  have 
seen,  natural  selection  or  survival  of  the  fittest  will  favour 
such  among  these  recumbent  Archegoniates  as  are  enabled, 
by  variations  in  their  structures,  to  maintain  raised  postures; 
it  will  favour  the  formation  of  fronds  that  curve  round  upon 
themselves,  and  curve  round  upon  the  fronds  growing  out 
of  them.  What,  now,  will  be  the  result  should  such  a 
modification  take  place  in  the  group  of  proliferous  fronds 
represented  in  Fig.  76?  Clearly,  the  result  will  be  a 
structure  like  that  shown  in  Fig.  79.  And  if  this  inrolling 
becomes  more  complete,  a  form  like  Jungermannia  cordifolia, 

represented  in  Fig.  80,  will 
be  produced. 

When  the  successive 
fronds  are  thus  folded  round 
so  completely  that  their 
opposite  edges  meet,  these 
opposite  edges  will  be  apt  to 
unite:  not  that  they  will 
grow  together  after  being 
formed,  but  that  they  will 
79  80  develop  in  connexion;  or, 

in  botanical  language,  will  become  "  adnate."  That  foliar 
surfaces  which,  in  their  embryonic  state,  are  in  close  contact, 
often  join  into  one,  is  a  familiar  fact.  It  is  habitually  so 
with  sepals  or  divisions  of  the  calyx.  In  all  campanulate 
flowers  it  is  so  with  petals.  And  in  some  tribes  of  plants 
it  is  so  with  stamens.  We  are  therefore  well  warranted  in 
inferring  that,  under  the  conditions  above  described,  the  suc- 
cessive fronds  or  leaflets  will,  by  union  of  their  remote  edges, 
first  at  their  points  of  origin  and  afterwards  higher  up, 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     59 

form  sheaths  inserted  one  within  another,  and  including  the 
axis.  This  incurving  of  the  successive  fronds, 

ending  in  the  formation  of  sheaths,  may  be  accompanied  by 
different  sets  of  modifications.  Supposing  Fig.  81  to  be  a 
transverse  section  of  such  type  (a  being  the  mid-rib,  and 
6  the  expansion  of  an  older  frond ;  while  c  is  a  younger  frond 
proliferously  developed  within  it),  there  may  begin  two  di- 
vergent kinds  of  changes,  leading  to  two  contrasted  struc- 
tures. If,  while  frond  continues  to  grow  out  of  frond,  the 
series  of  united  mid-ribs  continues  to  be  the  channel  of  circu- 
lation between  the  uppermost  fronds  and  the  roots — if,  as  a 
consequence,  the  compound  mid-rib,  or  rudimentary  axis,  con- 
tinues to  increase  in  size  laterally;  there  will  arise  the  series 
of  transitional  forms  represented  by  the  transverse  sections 
82,  83,  84,  85;  ending  in  the  production  of  a  solid  axis, 


everywhere  wrapped  round  by  the  foliar  surface  of  the 
frond,  as  an  outer  layer  or  sheath.  But  if,  on  the  other 
hand,  circumstances  favour  a  form  of  plant  which  maintains 
its  uprightness  at  the  smallest  cost  of  substance — if  the 
vascular  bundles  of  each  succeeding  mid-rib,  instead  of  re- 
maining concentrated,  become  distributed  all  round  the  tube 


60 


MORPHOLOGICAL  DEVELOPMENT. 


formed  by  the  infolded  frond;  then  the  structure  eventually 
reached,  through  the  transitional  forms  86,  87,  88,  89,  will 
be  a  hollow  cylinder.*  And  now  observe  how  the 

two  structures  thus  produced,  correspond  with  two  kinds  of 
Monocotyledons.  Fig.  90  represents  a  species  of  Dendrobium, 
in  which  we  see  clearly  how  each  leaf  is  but  a  continuation 
of  the  external  layer  of  a  solid  axis — a  sheath  such  as  would 
result  from  the  infolded  edges  of  a  frond  becoming  adnate; 
and  on  examining  how  the  sheath  of  each  leaf  includes  the 
one  above  it,  and  how  the  successive  sheaths  include  the  axis, 
it  will  be  manifest  that  the  relations  of  parts  are  just  such 
as  exist  in  the  united  series  of  fronds  shown  in  Fig.  79 — the 
successive  nodes  answering  to  the  successive  points  of  origin 
of  the  fronds.  Conversely,  the  stem  of  a  grass,  Fig.  91,  dis- 


90 


plays  just  such  relations  of  parts,  as  would  result  from  the  de- 
velopment of  the  type  shown  in  Fig.  79,  if  instead  of  the  mid- 
ribs thickening  into  a  solid  axis,  the  matter  composing  them 
became  evenly  distributed  round  the  foliar  surfaces,  at  the 
same  time  that  the  incurved  edges  of  the  foliar  surfaces 
united.  The  arrangements  of  the  tubular  axis  and  its  ap- 
pendages, thus  resulting,  are  still  more  instructive  than  those 
*  Sec  note  at  the  end  of  the  chapter. 


THE  MORPHOLOGICAL  COMPOSITION  OP  PLANTS.     61 

of  the  solid  axis.  For  while,  even  more  clearly  than  in  the 
Dendrobium,  we  see  at  the  point  b,  a  continuity  of  structure 
between  the  substance  of  the  axis  below  the  node,  and  the 
substance  of  the  sheath  above  the  node:  we  see  that  this 
sheath,  instead  of  having  its  edges  united  as  in  Dendrobium, 
has  them  simply  overlapping,  so  as  to  form  an  incomplete 
hollow  cylinder  which  may  be  taken  off  and  unrolled; 
and  we  see  that  were  the  overlapping  edges  of  this  sheath 
united  all  the  way  from  the  node  a  to  the  node  &,  it  would 
constitute  a  tubular  axis,  like  that  which  precedes  it  or  like 
that  which  it  includes.  And  then,  giving  an  unexpected 
collusiveness  to  the  argument,  it  turns  out  that  in  one 
family  of  grasses,  the  overlapping  edges  of  the  sheaths  do 
unite:  thus  furnishing  us  with  a  demonstration  that  tubular 
structures  are  produced  by  the  incurving  and  joining  of 
foliar  surfaces;  and  that  so,  hollow  axes  may  be  interpreted 
as  above,  without  making  any  assumption  unwarranted  by 
fact.  One  further  correspondence  between  the  type 

thus  ideally  constructed,  and  the  monocotyledonous  type, 
must  be  noted.  If,  as  already  pointed  out,  the  transverse 
growth  of  an  axis  arises  when  the  axis  comes  to  be  a  channel 
of  circulation  between  all  the  roots  at  one  of  its  extremities 
and  all  the  leaves  at  the  other;  and  if  this  lateral  bulging 
must  increase  as  fast  as  the  quantity  of  foliage  to  be  brought 
in  communication  with  the  roots  increases — especially  if  such 
foliage  has  at  the  same  time  to  be  raised  high  above  the 
earth's  surface;  what  must  happen  to  a  plant  constructed  in 
the  manner  just  described  ?  The  elder  fronds  or  foliar  organs, 
ensheathing  the  younger  ones,  as  well  as  the  incipient 
axis  serving  as  a  bond  of  union,  are  at  first  of  such  circum- 
ference only  as  suffices  to  inclose  these  undeveloped  parts. 
What,  then,  will  take  place  when  the  inclosed  parts  grow — 
when  the  axis  thickens  while  it  elongates  ?  Evidently  the 
earliest-formed  sheaths,  not  being  large  enough  for  the 
swelling  axis,  must  burst;  and  evidently  each  of  the  later- 
formed  sheaths  must,  in  its  tiirn,  do  the  like.  There  must 


62  MORPHOLOGICAL  DEVELOPMENT. 

result  a  gradual  exfoliation  of  the  successive  sheaths,  like 
that  indicated  as  beginning  in  the  above  figure  of  Dendro- 
bium;  which,  at  a,  shows  the  bud  of  the  undeveloped  parts 
just  visible  above  the  enwrapping  sheaths,  while  at  &,  and  c, 
it  shows  the  older  sheaths  in  process  of  being  split  open. 
That  is  to  say,  there  must  result  the  mode  of  growth  which 
helped  to  give  the  name  Endogens  to  this  class. 

The  other  way  in  which  an  integrated  series  of  fronds 
may  acquire  the  rigidity  needful  for  maintaining  an  erect 
position,  has  next  to  be  considered.  If  the  successive  fronds 
do  not  acquire  such  habit  of  curling  as  may  be  taken  ad- 
vantage of  by  natural  selection,  so  as  to  produce  the  requisite 
stiffness;  then,  the  only  way  in  which  the  requisite  stiffness 
appears  producible,  is  by  the  thickening  and  hardening  of 
the  fused  series  of  mid-ribs.  The  incipient  axis  will  not,  in 
this  case,  be  inclosed  by  the  rolled-up  fronds;  but  will  con- 
tinue exposed.  Survival  of  the  fittest  will  favour  the  genesis 
of  a  type,  in  which  those  portions  of  the  successive  mid-ribs 
that  enter  into  the  continuous  bond,  become  more  bulky  than 
the  disengaged  portions  of  the  mid-ribs:  the  individuals 
which  thrive  and  have  the  best  chances  of  leaving  offspring, 
being,  by  the  hypothesis,  individuals  having  axes  stiff 
enough  to  raise  their  foliage  above  that  of  their  fellows. 
At  the  same  time,  under  the  same  influences,  there  will  tend 
to  result  an  elongation  of  those  portions  of  the  mid-ribs, 
which  become  parts  of  the  incipient  axis;  seeing  that  it  will 
profit  the  plant  to  have  its  leaves  so  far  removed  from  one 
another,  as  to  prevent  mutual  interferences.  Hence,  from  the 
recumbent  type  there  will  evolve,  by  indirect  equilibration 
(§  167),  such  modifications  as  are  shown  in  Figs.  92,  93,  94; 
the  first  of  which  is  a  slight  advance  on  the  ideal  type 
represented  in  Fig.  76,  arising  in  the  way  described;  and 
the  others  of  which  are  actual  plants — Haplomitrium  Hoolceri, 
and  Plagiochila  decipiens.  Thus  the  higher  Archegoniates 
show  us  how,  along  with  an  assumption  of  the  upright  atti- 
tude, there  does  go  on,  as  we  see  there  must  go  on,  a  separa- 


THE  MORPHOLOGICAL  COMPOSITION  OP  PLANTS.     63 

tion  of  the  leaf-producing  parts  from  the  root-producing 
parts;  a  greater  development  of  that  connecting  portion  of 
the  successive  fronds,  by  which  they  are  kept  in  communica- 


tion with  the  roots,  and  raised  above  the  ground;  and  a  con- 
sequent increased  differentiation  of  such  connecting  portion 
from  the  parts  attached  to  it.  And  this  lateral  bulging  of 
the  axis,  directly  or  indirectly  consequent  on  its  functions  as 
a  support  and  a  channel,  being  here  unrestrained  by  the 
early-formed  fronds  folded  round  it,  goes  on  without  the 
bursting  of  these.  Hence  arises  a  leading  character  of  what 
is  called  exogenous  growth — a  growth  which  is,  however,  still 
habitually  accompanied  by  exfoliation,  in  flasks,  of  the  outer- 
most layers,  continually  being  cracked  and  split  by  the  accu- 
mulation of  layers  within  them.  And  now  if  we  ex- 
amine plants  of  the  exogenous  type,  we  find  among  them  many 
displaying  the  stages  of  this  metamorphosis.  In  Fig.  95,  is 
shown  a  form  in  which  the  continuity  of  the  axis  with  the 
mid-rib  of  the  leaf,  is  manifest — a  continuity  that  is  con- 
spicuous in  the  common  thistle.  Here  the  foliar  expansion, 
running  some  distance  down  the  axis,  makes  the  included 
portion  of  the  axis  a  part  of  its  mid-rib;  just  as  in  the  ideal 
types  above  drawn.  By  the  greater  growth  of  the  internodes, 


64  MORPHOLOGICAL  DEVELOPMENT. 

which  are  very  variable,  not  only  in  different  plants  but  in 
the  same  plant,  there  results  a  modification  like  that  de- 
lineated in  Fig.  96.  And  then,  in  such  forms  as  Fig.  97,  there 
is  shown  the  arrangement  that  arises  when,  by  more  rapid 
development  of  the  proximal  end  of  the  mid-rib,  the  distal 


part  of  the  foliar  surface  is  separated  from  the  part  which 
embraces  the  axis:  the  wings  of  the  mid-rib  still  serving, 
however,  to  connect  the  two  portions  of  the  foliar  surface. 
Such  a  separation  is,  as  pointed  out  in  §  188,  an  habitual 
occurrence;  and  in  some  compound  leaves,  an  actual  tearing 
of  the  inter-venous  tissue  is  caused  by  extra  growth  of  the 
mid-rib.  Modifications  like  this,  and  the  further  one  in  Fig. 
98,  we  may  expect  to  be  established  by  survival  of  the  fittest, 
among  those  plants  which  produce  considerable  masses  of 
leaves;  since  the  development  of  mid-ribs  into  footstalks,  by 
throwing  the  leaves  further  away  from  the  axes,  will  diminish 
the  shading  of  the  leaves,  one  by  another.  And  then,  among 
plants  of  bushy  growth,  in  which  the  assimilating  surfaces 
become  still  more  liable  to  intercept  one  another's  light, 
natural  selection  will  continue  to  give  an  advantage  to  those 
which  carry  their  assimilating  surfaces  at  the  ends  of  the 
petioles,  and  do  not  develop  assimilating  surfaces  close  to  the 
axis,  where  they  are  most  shaded.  Whence  will  result  a  dis- 
appearance of  the  stipules  and  the  foliar  fringes  of  the  mid- 
ribs; ending  in  the  production  of  the  ordinary  stalked  leaf, 
Fig.  99,  which  is  characteristic  of  trees.  Meanwhile,  the  axis 
thickens  in  proportion  to  the  number  of  leaves  it  has  to 
carry,  and  to  put  in  communication  with  the  roots;  and  so 


THE  MOKPHOLOGICAL  COMPOSITION  OF  PLANTS.     65 

there  comes  to  be  a  more  marked  contrast  between  it  and  the 
petioles,  severally  carrying  a  leaf  each.* 

§  194.  When,  in  the  course  of  the  process  above  sketched 
out,  there  has  arisen  such  community  of  nutrition  among  the 
fronds  thus  integrated  into  a  series,  that  the  younger  ones 
are  aided  by  materials  which  the  older  ones  have  elaborated; 
the  younger  fronds  will  begin  to  show,  at  earlier  and  earlier 
periods  of  development,  the  structures  about  to  originate 
from  them.  Abundant  nutrition  will  abbreviate  the  intervals 
between  the  successive  prolifications ;  so  that  eventually, 
while  each  frond  is  yet  imperfectly  formed,  the  rudiment  of 
the  next  will  begin  to  show  itself.  All  embryology  justifies 
this  inference.  The  analogies  it  furnishes  lead  us  to  expect 
that  when  this  serial  arrangement  becomes  organic,  the 
growing  part  of  the  series  will  show  the  general  relations  of 
the  forthcoming  parts,  while  they  are  very  small  and  un- 
specialized.  What  will  in  such  case  be  the  appearances  they 
assume?  We  shall  have  no  difficulty  in  perceiving  what  it 
will  be,  if  we  take  a  form  like  that  shown  in  Fig.  92,  and 
dwarf  its  several  parts  at  the  same  time  that  we  generalize 
them.  Figs.  100,  101,  102,  and  103,  will  show  the  result; 


t03 

and  in  Fig.  104,  which  is  the  bud  of  a  dicotyledon,  we  see 
how  clear  is  the  morphological  correspondence:  a  being  the 
rudiment  of  a  foliar  organ  beginning  to  take  shape;  6  being 
the  almost  formless  rudiment  of  the  next  foliar  organ;  and 

*  Since  this  paragraph  was  put  in  type  [this  refers  to  the  first  edition], 
I  have  observed  that  in  some  varieties  of  Cineraria,  as  probably  in  other 
plants,  a  single  individual  furnishes  all  these  forms  of  leaves — all  gradations 
between  unstipulated  leaves  on  long  petioles,  and  leaves  that  embrace  the 
axis.  It  may  be  added  that  the  distribution  of  these  various  forms  is  quite 
in  harmony  with  the  rationale  above  given. 
51 


66 


MORPHOLOGICAL  DEVELOPMENT. 


c  being  the  quite-undifferentiated  part  whence  the  rudiments 
of  subsequent  foliar  organs  are  to  arise. 

And  now  we  are  prepared  for  entering  on  a  still-remaining 
question  respecting  the  structure  of  Phsenogams — what  is  the 
origin  of  axillary  buds?  As  the  synthesis  at  present  stands, 
it  does  not  account  for  these;  but  on  looking  a  little  more 
closely  into  the  matter,  we  shall  find  that  the  axillary  buds 
are  interpretable  in  the  same  manner  as  the  terminal  buds. 
So  to  interpret  them,  however,  we  must  return  to  that  pro- 
cess of  proliferous  growth  with  which  we  set  out,  for  the  pur- 
pose of  observing  some  facts  not  before  named.  Delesseria 
hypoglossum,  Fig.  105,  represents  a  seaweed  of  the  same  genus 
as  one  outlined  in  Fig.  40;  but  of  a  species  in  which  pro- 
liferous growth  is  carried  much  further.  Here,  not  only  does 
the  primary  frond  bud  out  many  secondary  fronds  from  its 
mid-rib;  but  most  of  the  secondary  fronds  similarly  bud  out 
several  tertiary  fronds;  and  even  by  some  of  the  tertiary 


ses 


fronds,  this  prolification  is  repeated.  Besides  being  shown 
that  the  budding  out  of  several  fronds  from  one  frond,  may 
become  habitual;  we  are  also  shown  that  it  may  become  a 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     6? 

habit  inherited  by  the  fronds  so  produced,  and  also  by  the 
fronds  they  produce:  the  manifestation  of  the  tendency 
being  probably  limited  only  by  failure  of  nutrition.  That 
under  fit  conditions  an  analogous  mode  of  growth  will  occur 
in  fronds  of  the  acrogenic  type,  like  those  we  set  out  with, 
is  shown  by  the  case  of  Metzgeria  furcata,  Figs.  45,  46,  in 
which  such  compound  prolification  is  partially  displayed. 
Let  us  suppose,  then,  that  the  frond  a,  Fig.  106,  produces 
not  only  a  single  secondary  frond  &,  but  also  another  such 
secondary  frond  &'.  Let  us  suppose,  further,  that  the  frond 
b  is  in  like  manner  doubly  proliferous:  producing  both  c 
and  c'.  Lastly,  let  us  suppose  that  in  the  second  frond  &' 
which  a  produces,  as  well  as  in  the  second  frond  c'  which  & 
produces,  the  doubly-proliferous  habit  is  manifested.  If, 
now,  this  habit  grows  organic — if  it  becomes,  as  it  naturally 
will  become,  the  characteristic  of  a  plant  of  luxuriant  growth, 
the  unfolding  parts  of  which  can  be  fed  by  the  unfolded 
parts;  it  will  happen  with  each  lateral  series,  as  with  the 
main  series,  that  its  successive  components  will  begin  to 
show  themselves  at  earlier  and  earlier  stages  of  development. 
And  in  the  same  way  that,  by  dwarfing  and  generalizing 
the  original  series,  we  arrive  at  a  structure  like  that  of  the 
terminal  bud;  by  dwarfing  and  generalizing  a  lateral  series, 
as  shown  in  Figs.  107 — 110,  we  arrive  at  a  structure  answer- 
ing in  nature  and  position  to  the  axillary  bud. 


Facts  confirming  these  interpretations  are  afforded  by  the 
structure  and  distribution  of  buds.  The  phasnogamic  axis  in 
its  primordial  form,  being  an  integrated  series  of  folia;  and 
the  development  of  that  part  by  which  these  folia  are 
held  together  at  considerable  distances  from  one  another, 
taking  place  afterwards;  it  is  inferable  from  the  general 


G8  MORPHOLOGICAL  DEVELOPMENT. 

principles  of  embryology,  that  in  its  rudimentary  stages,  the 
phaanogainic  shoot  will  have  its  foliar  parts  more  clearly 
marked  out  than  its  axial  parts.  This  we  see  in  every  bud. 
Every  bud  consists  of  the  rudiments  of  leaves  packed  to- 
gether without  any  appreciable  internodal  spaces;  and  the 
internodal  spaces  begin  to  increase  with  rapidity,  only  when 
the  foliar  organs  have  been  considerably  developed.  More- 
over, where  nutrition  falls  short,  and  arrest  of  development 
takes  place — that  is,  where  a  flower  is  formed — the  inter- 
nodes  remain  undeveloped:  the  unfolding  ceases  before 
the  later-acquired  characters  of  the  phaBnogamic  shoot 
are  assumed.  Lastly,  as  the  hypothesis  leads  us  to  expect, 
axillary  buds  make  their  appearances  later  than  the  foliar 
organs  which  they  accompany;  and  where,  as  at  the  ends  of 
shoots,  these  foliar  organs  show  failure  of  chlorophyll,  the 
axillary  buds  are  not  produced  at  all.  That  these  are  in- 
ferable traits  of  structure,  will  be  manifest  on  inspecting 
Figs.  106 — 110;  and  on  observing,  first,  that  the  doubly- 
proliferous  tendency  of  which  the  axillary  bud  is  a  result, 
implies  abundant  nutrition;  and  on  observing,  next,  that  the 
original  place  of  secondary  prolification,  is  such  that  the  foliar 
surface  on  which  it  occurs,  must  grow  to  some  extent  before 
the  bud  appears. 

On  thus  looking  at  the  matter — on  contemplating  afresh 
the  ideal  type  shown  in  Fig.  106,  and  noting  how,  by  the 
conditions  of  the  case,  the  secondary  prolifications  must  cease 
before  that  primary  prolification  which  produces  the  main 
axis;  we  are  enabled  to  reconcile  all  the  phenomena  of  axil- 
lary gemmation.  We  see  harmony  among  the  several  facts — 
first,  that  the  axillary  bud  becomes  a  lateral,  leaf-bearing 
axis  if  there  is  abundant  material  for  growth;  second,  that 
its  development  is  arrested,  or  it  becomes  a  flower-bearing 
axis,  if  the  supply  of  sap  is  but  moderate;  third,  that  it  is 
absent  when  the  nutrition  is  failing.  We  are  no  longer 
committed  to  the  gratuitous  assumption  that,  in  the  phsno- 
gamic  type,  there  must  exist  an  axillary  bud  to  each  foliar 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     69 

organ;  but  we  are  led  to  conclude,  a  priori,  that  which  we 
find,  a  posteriori,  that  axillary  buds  are  as  normally  absent 
in  flowers  as  they  are  normally  present  lower  down  the  axis. 
And  then,  to  complete  the  argument,  we  are  prepared  for  the 
corollary  that  axillary  prolification  may  naturally  arise  even 
at  the  ends  of  axes,  should  the  failing  nutrition  which 
causes  the  dwarfing  of  the  foliar  organs  to  form  a  flower,  be 
suddenly  changed  into  such  high  nutrition  as  to  transform 
the  components  of  the  flower  into  appendages  that  are  green, 
if  not  otherwise  leaf -like — a  condition  under  which  only,  this 
phenomenon  is  proved  to  occur. 

§  195.  One  more  question  presents  itself,  when  we  con- 
trast the  early  stages  of  development  in  the  two  classes  of 
Phsenogams;  and  a  further  answer,  supplied  by  the  hypo- 
thesis, gives  to  the  hypothesis  a  further  probability.  It  is 
characteristic  of  a  monocotyledon,  to  have  a  single  seed-leaf 
or  cotyledon;  and  it  is  characteristic  of  a  dicotyledon,  to 
have  at  least  two  cotyledons,  if  not  more  than  two.  That  is 
to  say,  the  monocotyledonous  mode  of  germination  every- 
where coexists  with  the  endogenous  mode  of  growth;  and 
along  with  the  exogenous  mode  of  growth,  there  always  goes 
either  a  dicotyledonous  or  polycotyledonous  germination. 
Why  is  this?  Such  correlations  cannot  be  accidental — 
cannot  be  meaningless.  A  true  theory  of  the  phanogamic 
types  in  their  origin  and  divergence,  should  account  for  the 
connexion  of  these  traits.  Let  us  see  whether  the  foregoing 
theory  does  this. 

The  higher  plants,  like  the  higher  animals,  bequeath  to 
their  offspring  more  or  less  of  nutriment  and  structure. 
Superior  organisms  of  either  kingdom  do  not,  as  do  all 
inferior  organisms,  cast  off  their  progeny  in  the  shape  of 
minute  portions  of  protoplasm,  unorganized  and  without 
stocks  of  material  for  them  to  organize;  but  they  either 
deposit  along  with  the  germs  they  cast  off,  certain  quantities 
of  albuminoid  substance  to  be  appropriated  by  them  while  they 


70  MORPHOLOGICAL  DEVELOPMENT. 

develop  themselves,  or  else  they  continue  to  supply  such 
substance  while  the  germs  partially  develop  themselves  before 
their  detachment.  Among  plants  this  constitutes  one  dis- 
tinction between  seeds  and  spores.  Every  seed  contains  a 
store  of  food  to  serve  the  young  plant  during  the  first  stages 
of  its  independent  life;  and  usually,  too,  before  the  seed  is 
detached,  the  young  plant  is  so  far  advanced  in  structure, 
that  it  bears  to  the  attached  stock  of  nutriment  much  the 
same  relation  that  the  young  fish  bears  to  the  appended  yelk- 
bag  at  the  time  of  leaving  the  egg.  Sometimes,  indeed,  the 
development  of  chlorophyll  gives  the  seed-leaves  a  bright 
green,  while  the  seed  is  still  contained  in  the  parent- 
pod.  This  early  organization  of  the  phasnogam 
must  be  supposed  rudely  to  indicate  the  type  out  of  which 
the  phagnogamic  type  arose.  On  the  foregoing  hypothesis, 
the  seed-leaves  therefore  represent  the  primordial  fronds; 
which,  indeed,  they  simulate  in  their  simple,  cellular,  un- 
veined  structures.  And  the  question  here  to  be  asked  is — 
do  the  different  relations  of  the  parts  in  young  monocotyledons 
and  dicotyledons  correspond  with  the  different  relations  of 
the  primordial  fronds,  implied  by  the  endogenous  and  the 
exogenous  modes  of  growth?  We  shall  find  that  they  do. 

Starting,  as  before,  with  the  proliferous  form  shown  in 
Fig.  Ill,  it  is  clear  that  if  the  strength  required  for  main- 
taining the  vertical  attitude,  is  obtained  by  the  rolling  up  of 
the  fronds,  the  primary  frond  will  more  and  more  conceal  the 
secondary  frond  within  it.  At  the  same  time,  the  secondary 
frond  must  continue  to  be  dependent  on  the  first  for  its  nutri- 
tion ;  and,  being  produced  within  the  first,  must  be  prevented 
by  defective  supply  of  light  and  air,  from  ever  becoming  syn- 
chronous in  its  development  with  the  first.  Hence,  this 
infolding  which  -leads  to  the  endogenous  mode  of  growth, 
implies  that  there  must  always  continue  such  pre-eminence 
of  the  first-formed  frond  or  its  representative,  as  to  make  the 
germination  monocotyledonous.  Figs.  Ill  to  115,  show  the 
transitional  forms  that  would  result  from  the  infolding  of 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     71 


the  fronds.  In  Fig.  116  (a  vertical  section  of  the  form  repre- 
sented in  Fig.  115)  are  exhibited  the  relations  of  the  succes- 
sive fronds  to  each  other.  The  modified  relations  that  would 
result,  if  the  nutrition  of  the  embryo  admitted  of  anticipatory 
development  of  the  successive  fronds,  are  shown  in  Fig.  117. 
And  how  readily  the  structure  may  pass  into  that  of  the 
monocotyledonous  germ,  will  be  seen  on  inspecting  Fig.  118; 


=T 

HF^QJBSS 

/=!  <^> 

vf 


which  is  a  vertical  section  of  an  actual  monocotyledon  at  an 
early  stage — the  incomplete  lines  at  the  left  of  its  root,  indi- 
cating its  connexion  with  the  seed.*  Contrariwise, 

*  Since  these  figures  were  put  on  the  block,  it  has  occurred  to  me  that  the 
relations  would  be  still  clearer,  were  the  primary  frond  represented  as  not 
taking  part  in  these  processes  of  modification,  which  have  been  described  as 
giving  rise  to  the  erect  form ;  as,  indeed,  the  rooting  of  its  under  surface 
will  prevent  it  from  doing  in  any  considerable  degree.  In  such  case,  each  of 
the  Figs.  Ill  to  117,  should  have  a  horizontal  rooted  frond  at  its  base, 
homologous  with  the  pro-embryo  among  Acrogens.  This  primary  frond 
would  then  more  manifestly  stand  in  the  same  relation  to  the  rest,  as  the 


72        MORPHOLOGICAL  DEVELOPMENT. 

where  the  strength  required  for  maintaining  an  upright  atti- 
tude is  not  obtained  by  the  rolling  up  of  the  fronds,  but  by 
the  strengthening  of  the  continuous  mid-rib,  the  second 
frond,  so  far  from  being  less  favourably  circumstanced  than 
the  first,  becomes  in  some  respects  even  more  favourably 
circumstanced:  being  above  the  other,  it  gets  a  greater  share 
of  light,  and  it  is  less  restricted  by  surrounding  obstacles. 
There  is  nothing,  therefore,  to  prevent  it  from  rapidly  gaining 
an  equality  with  the  first.  And  if  we  assume,  as  the  truths  of 
embryology  entitle  us  to  do,  an  increasing  tendency  towards 
anticipation  in  the  development  of  subsequent  fronds — if 
we  assume  that  here,  as  in  other  cases,  structures  which  were 
originally  produced  in  succession  will,  if  the  nutrition  allows 
and  no  mechanical  dependence  hinders,  come  to  be  produced 
simultaneously;  there  is  nothing  to  prevent  the  passage 
of  the  type  represented  in  Fig.  Ill,  into  that  represented 
in  Fig.  122.  Or  rather,  there  is  everything  to  facilitate  it; 
seeing  that  natural  selection  will  continually  favour  the  pro- 
duction of  a  form  in  which  the  second  frond  grows  in  such 
way  as  not  to  shade  the  first,  and  in  such  way  as  allows  the 
axis  readily  to  assume  a  vertical  position. 

Thus,  then,  is  interpretable  the  universal  connexion  be- 
tween monocotyledonous  germination  and  endogenous  growth ; 
as  well  as  the  similarly-universal  connexion  between  exoge- 
nous growth  and  the  development  of  two  or  more  cotyledons. 
That  it  explains  these  fundamental  relations,  adds  very 
greatly  to  the  probability  of  the  hypothesis. 

§  196.  While  we  are  in  this  manner  enabled  to  discern 
the  kinship  that  exists  between  the  higher  vegetal  types 
themselves,  as  well  as  between  them  and  the  lower  types;  we 
cotyledon  does  to  the  plumule — both  by  position,  and  as  a  supplier  of  nutri- 
ment. Fig.  117a,  which  I  am  enabled  to  add,  shows  that  this  would  com- 
plete the  interpretation.  Of  the  dicotyledonous  series,  it  is  needful  to  add  no 
further  explanation  than  that  the  difference  in  habit  of  growth,  will  permit 
the  second  frond  to  root  itself  as  well  as  the  first ;  and  so  to  become  nn  addi- 
tional source  of  nutriment,  similarly  circumstanced  to  the  first  and  equal  with  it. 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     73 

are  at  the  same  time  supplied  with  a  rationale  of  those  truths 
which  vegetal  morphologists  have  established.  Those  homo- 
logies  which  Wolff  indicated  in  their  chief  outlines  and 
Goethe  followed  out  in  detail,  have  a  new  meaning  given  to 
them  when  we  regard  the  phaenogamic  axis  as  having  been 
evolved  in  the  way  described.  Forming  the  modified  con- 
ception which  we  are  here  led  to  do,  respecting  the  units  of 
which  a  flowering  plant  is  composed,  we  are  no  longer  left 
without  an  answer  to  the  question — What  is  an  axis?  And 
we  are  helped  to  understand  the  naturalness  of  those  cor- 
respondences which  the  successive  members  of  each  shoot 
display.  Let  us  glance  at  the  facts  from  our  present  stand- 
point. 

The  unit  of  composition  of  a  Phaenogam,  is  such  portion  of 
a  shoot  as  answers  to  one  of  the  primordial  fronds.  This 
portion  is  neither  one  of  the  foliar  appendages  nor  one  of  the 
internodes;  but  it  consists  of  a  foliar  appendage  together 
with  the  preceding  internode,  including  the  axillary  bud 
where  this  is  developed.  The  parts  intercepted  by  the  dotted 
lines  in  Fig.  123,  constitute  such  a  segment;  and  the  true 
homology  is  between  this  and  any  other  foliar  organ  with  the 
portion  of  the  axis  below  it.  And  now  observe  how,  when  we 
take  this  for  the  unit  of  composition,  the  metamorphoses 
which  the  phaenogamic  axis  displays,  are  inferable  from  known 
laws  of  development.  Embryology  teaches  us  that  arrest 

of  development  shows  itself  first  in  the  absence  of  those  parts 
that  have  arisen  latest  in  the  course  of  evolution;  that  if 
defect  of  nutrition  causes  an  earlier  arrest,  parts  that  are  of 
more  ancient  origin  abort;  and  that  the  part  alone  produced 
when  the  supply  of  materials  fails  near  the  outset,  is  the  prim- 
ordial part.  We  must  infer,  therefore,  that  in  each  seg- 
ment of  a  Phasnogam,  the  foliar  organ,  which  answers  to  the 
primordial  frond,  will  be  the  most  constant  element;  and 
that  the  internode  and  the  axillary  bud,  will  be  successively 
less  constant.  This  we  find.  Along  with  a  smaller  size  of 
foliar  surface  implying  lower  nutrition,  it  is  usual  to  see  a 


74  MORPHOLOGICAL  DEVELOPMENT. 

much-diminished  internode  and  a  less-pronounced  axillary 
bud,  as  in  Fig.  124.  On  approaching  the  flower,  the 
axillary  bud  disappears;  and  the  segment  is  reduced  to  a 
small  foliar  surface,  with  an  internode  which  is  in  most 


cases  very  short  if  not  absent,  as  in  125  and  126.  In  the 
flower  itself,  axillary  buds  and  internodes  are  both  want- 
ing: there  remains  only  a  foliar  surface  (127),  which, 
though  often  larger  than  the  immediately  preceding  foliar 
surface,  shows  failing  nutrition  by  absence  of  chlorophyll. 
And  then,  in  the  quite  terminal  organs  of  fructification  (129), 
we  have  the  foliar  part  itself  reduced  to  a  mere  rudiment. 
Though  these  progressive  degenerations  are  by  no  means 
regular,  being  in  many  cases  varied  by  adaptations  to  par- 
ticular requirements,  yet  it  cannot,  I  think,  be  questioned, 
that  the  general  relations  are  as  described,  and  that  they  are 
such  as  the  hypothesis  leads  us  to  expect.  Nor  are 

we  without  a  kindred  explanation  of  certain  remaining  traits 
of  foliar  organs  in  their  least-developed  forms.  Petals, 
stamens,  pistils,  &c.,  besides  reminding  us  of  the  primordial 
fronds  by  their  diminished  sizes,  and  by  the  want  of  those 
several  supplementary  parts  which  the  preceding  segments 
possess,  also  remind  us  of  them  by  their  histological  charac- 
ters: they  consist  of  simple  cellular  tissue,  scarcely  at  all 
differentiated.  The  fructifying  cells,  too,  which  here  make 
their  appearance,  are  borne  in  ways  like  those  in  which  the 
lower  Acrogens  bear  them — at  the  edge  of  the  frond,  or  at 
the  end  of  a  peduncle,  or  immersed  in  the  general  substance ; 
as  in  Figs.  128  and  129.  Nay,  it  might  even  be  said  that 


THE  MORPHOLOGICAL  COMPOSITION  OP  PLANTS.     75 

the  colours  assumed  by  these  terminal  folia,  call  to  mind  the 
plants  out  of  which  we  conclude  that  Phasnogams  have  been 
evolved;  for  it  is  said  of  the  fronds  of  the  Jungermanniacece, 
that,  "  though  under  certain  circumstances  of  a  pure  green, 
they  are  inclined  to  be  shaded  with  red,  purple,  chocolate,  or 
other  tints." 

As  thus  understood,  then,  the  homologies  among  the  parts 
of  the  pha?nogamic  axis  are  interpretable,  not  as  due  to  a 
needless  adhesion  to  some  typical  form  or  fulfilment  of  a  pre- 
determined plan;  but  as  the  inevitable  consequences  of  the 
mode  in  which  the  phanogamic  axis  originates. 

§  197.  And  now  it  remains  only  to  observe,  in  confirma- 
tion of  the  foregoing  synthesis,  that  it  at  once  explains  for  us 
various  irregularities.  When  we  see  leaves  sometimes  pro- 
ducing leaflets  from  their  edges  or  extremities,  we  recognize 
in  the  anomaly  a  resumption  of  -an  original  mode  of  growth : 
fronds  frequently  do  this.  When  we  learn  that  a  flowering 
plant,  as  the  Drosera  intermedia,  has  been  known  to  develop 
a  young  plant  from  the  surface  of  one  of  its  leaves,  we  are  at 
once  reminded  of  the  proliferous  growths  and  fructifying 
organs  in  the  Liverworts.  The  occasional  production  of  bul- 
bils by  Phasnogams,  ceases  to  be  so  surprising  when  we  find 
it  to  be  habitual  among  the  inferior  Acrogens,  and  when  we 
see  that  it  is  but  a  repetition,  on  a  higher  stage,  of  that  self- 
detachment  which  is  common  among  proliferously-produced 
fronds.  Nor  are  we  any  longer  without  a  solution  of  that 
transformation  of  foliar  organs  into  axial  organs,  which  not 
uncommonly  takes  place.  How  this  last  irregularity  of  de- 
velopment is  to  be  accounted  for,  we  will  here  pause  a  moment 
to  consider.  Let  us  first  glance  at  our  data. 

The  form  of  every  organism,  we  have  seen,  must  depend 
on  the  structures  of  its  physiological  [or  constitutional] 
units.  Any  group  of  such  units  will  tend  to  arrange  itself 
into  the  complete  organism,  if  uncontrolled  and  placed  in  fit 
conditions.  Hence  the  development  of  fertilized  germs;  and 


76        MORPHOLOGICAL  DEVELOPMENT. 

hence  the  development  of  those  self-detached  cells  which 
characterize  some  plants.  Conversely,  physiological  units 
which  form  a  small  group  involved  in  a  larger  group,  and  are 
subject  to  all  the  forces  of  the  larger  group,  will  become  sub- 
ordinate in  their  structural  arrangements  to  the  larger  group 
— will  be  co-ordinated  into  a  part  of  the  major  whole,  in- 
stead of  co-ordinating  themselves  into  a  minor  whole.  This 
antithesis  will  be  clearly  understood  on  remembering  how, 
on  the  one  hand,  a  small  detached  part  of  a  hydra  soon 
moulds  itself  into  the  shape  of  an  entire  hydra;  and  how, 
on  the  other  hand,  the  cellular  mass  that  buds  out  in  place 
of  a  lobster's  lost  claw,  gradually  assumes  the  form  of  a  claw 
— has  its  parts  so  moulded  as  to  complete  the  structure  of 
the  organism:  a  result  which  we  cannot  but  ascribe  to  the 
forces  which  the  rest  of  the  organism  exerts  upon  it.  Con- 
sequently, among  plants,  we  may  expect  that  whether  any 
portion  of  protoplasm  moulds  itself  into  the  typical  form 
around  an  axis  of  its  own,  or  is  moulded  into  a  part  subor- 
dinate to  another  axis,  will  depend  on  the  relative  mass  of 
its  physiological  units — the  accumulation  of  them  that  has 
taken  place  before  the  assumption  of  any  structural  arrange- 
ment. A  few  illustrations  will  make  clear  the  validity  of 
this  inference.  In  the  compound  leaf,  Fig.  65,  the  several 
lateral  growths  a,  b,  c,  d,  are  manifestly  homologous;  and 
on  comparing  a  number  of  such  leaves  together,  it  will  be 
seen  that  one  of  these  lateral  growths  may  assume  any  de- 
gree of  complexity,  according  to  the  degree  of  its  nutrition. 
Every  fern-leaf  exemplifies  the  same  general  truth  still  bet- 
ter. Whether  each  sub-frond  remains  an  undeveloped  wing 
of  the  main  frond,  or  whether  it  organizes  itself  into  a  group 
of  frondlets  borne  by  a  secondary  rib,  or  whether,  going 
further,  as  it  often  does,  it  gives  rise  to  tertiary  ribs  bear- 
ing frondlets,  is  determined  by  the  supply  of  materials  for 
growth;  since  such  higher  developments  are  most  marked 
at  points  where  the  nutrition  is  greatest;  namely,  next  the 
stem.  But  the  clearest  evidence  is  afforded  among  the  Algce, 


THE  MOPvPHOLOGICAL  COMPOSITION  OF  PLANTS.     77 

which,  not  drawing  nutriment  from  roots,  have  their  parts 
much  less  mutually  dependent;  and  are  therefore  capable  of 
showing  more  clearly,  how  any  part  may  remain  an  append- 
age or  may  become  the  parent  of  append- 
ages, according  to  circumstances.  In  the 
annexed  Fig.  130,  representing  a  branch  of 
Ptilota  plumosa,  we  see  how  a  wing  grows 
into  a  wing-bearing  branch  if  its  nutrition 
passes  a  certain  point.  This  form,  so  strik- 
ingly like  that  of  the  feathery  crystallizations /j* 
of  many  inorganic  substances,  implies  that, 
as  in  such  crystallizations,  the  simplicity 
or  complexity  of  structure  at  any  place 
depends  on  the  quantity  of  matter  that  has  to 
be  arranged  at  that  place  in  a  given  time.* 

Hence,  then,  we  are  not  without  an  interpretation  of  those 
over-developments  which  the  phaenogamic  axis  occasionally 
undergoes.  Fig.  104,  represents  the  phasnogamic  bud  in  its 
rudimentary  state.  The  lateral  process  6,  which  ordinarily 
becomes  a  foliar  appendage,  differs  very  little  from  the 
terminal  process  c,  which  is  to  become  an  axis — differs 
mainly  in  having,  at  this  period  when  its  form  is  being 
determined,  a  smaller  bulk.  If  while  thus  undifferentiated, 
its  nutrition  remains  inferior  to  that  of  the  terminal  process, 
it  becomes  moulded  into  a  part  that  is  subordinate  to  the 
general  axis.  But  if,  as  sometimes  happens,  there  is  supplied 
to  it  such  an  abundance  of  the  materials  needful  for  growth, 
that  it  becomes  as  large  as  the  terminal  process;  then  we 

*  How  the  element  of  time  modifies  the  result,  is  shown  by  the  familiar 
fact  that  crystals  rapidly  formed  are  small,  and  become  relatively  large  when 
left  to  form  more  slowly.  If  the  quantity  of  molecules  contained  in  a  solution 
is  relatively  great,  so  that  the  mutual  polarities  of  the  molecules  crowded 
together  in  every  place  throughout  the  solution  arc  intense,  there  arises  a 
crystalline  aggregation  around  local  axes ;  whereas,  in  proportion  as  the  local 
action  of  molecules  on  one  another  is  rendered  less  intense  by  their  wider 
dispersion,  they  become  relatively  more  subordinate  to  the  forces  exerted  on 
them  by  the  larger  aggregates  of  molecules  that  are  at  greater  distances,  and 
thus  are  left  to  arrange  themselves  round  fewer  axes  into  larger  crystals. 


78  MORPHOLOGICAL  DEVELOPMENT. 

may  naturally  expect  it  to  begin  moulding  itself  round  an 
axis  of  its  own:  a  foliar  organ  will  be  replaced  by  an  axial 
organ.  And  this  result  will  be  especially  liable  to  occur, 
when  the  growth  of  the  axis  has  been  previously  undergoing 
that  arrest  which  leads  to  the  formation  of  a  flower;  that  is 
when,  from  defect  of  materials,  the  terminal  process  has 
almost  ceased  to  increase,  and  when  some  concurrence  of 
favourable  causes  brings  a  sudden  access  of  sap  which  reaches 
the  lateral  processes  before  it  reaches  the  terminal  process.* 

§  198.  The  general  conclusion  to  which  these  various  lines 
of  evidence  converge,  is,  then,  that  the  shoot  of  a  flowering 
plant  is  an  aggregate  of  the  third  degree  of  composition. 
Taking  as  aggregates  of  the  first  order,  those  small  portions 
of  protoplasm  which  ordinarily  assume  the  forms  under 
which  they  are  known  as  cells ;  and  considering  as  aggregates 
of  the  second  order,  those  assemblages  of  such  cells  which, 
in  the  lower  cryptogams,  compose  the  various  kinds  of  thal- 
lus;  then  that  structure,  common  to  the  higher  cryptogams 
and  to  phaenogams,  in  which  we  find  a  series  of  such  groups 
of  cells  bound  up  into  a  continuous  whole,  must  be  regarded 
as  an  aggregate  of  the  third  order.  The  inference  drawn 
from  analysis,  and  verified  by  a  synthesis  which  corresponds 
in  a  remarkable  manner  with  the  facts,  is  that  those  com- 
pound parts  which,  in  Monocotyledons  and  Dicotyledons  are 
called  axes,  have  really  arisen  by  integration  of  such  simple 
parts  as  in  lower  plants  are  called  fronds.  Here,  on  a  higher 

*  It  is  objected  that  these  transformations  should  be  much  commoner  than 
they  are,  were  they  caused  solely  by  the  variations  of  nutrition  described. 
The  reply  is  that  they  are  comparatively  rare  in  uncultivated  plants,  where 
such  variations  are  not  frequent.  The  occurrence  of  them  is  chiefly  among 
cultivated  plants  which,  being  artificially  manured,  are  specially  liable  to 
immense  accessions  of  nutriment,  caused  now  by  sudden  supplies  of  fertilizing 
matters,  and  now  by  sudden  arrival  of  the  roots  at  such  matters  already 
deposited  in  the  soil.  It  is  to  these  great  chanffes  of  nutrition,  especially  apt 
to  take  place  in  gardens,  that  these  monstrosities  are  ascribed  ;  and  it  seems 
to  me  that  they  are  as  frequent  as  may  be  expected. 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     79 

level,  appears  to  have  taken  place  a  repetition  of  the  process 
already  observed  on  lower  levels.  The  formation  of  those 
small  groups  of  physiological  units  which  compose  the  lowest 
protophytes,  is  itself  a  process  of  integration;  and  the  con- 
solidation of  such  groups  into  definitely-circumscribed  and 
coherent  cells  or  morphological  units,  is  a  completing  of  the 
process.  In  those  coalescences  by  which  many  such  cells 
are  joined  into  threads,  and  discs,  and  solid  or  flattened- 
out  masses,  we  see  these  morphological  units  aggregating 
into  units  of  a  compound  kind:  the  different  phases  of  the 
transition  being  exemplified  by  groups  of  various  sizes, 
various  degrees  of  cohesion,  and  various  degrees  of  definite- 
ness.  And  now  we  find  evidences  of  a  like  process  on  a 
larger  scale:  the  compound  groups  are  again  compounded. 
Moreover,  as  before,  there  are  not  wanting  types  of  organi- 
zation by  which  the  stages  of  this  higher  integration  are 
shadowed  forth.  From  fronds  that  occasionally  produce 
other  fronds  from  their  surfaces,  we  pass  to  those  that 
habitually  produce  them;  from  those  that  do  so  in  an  in- 
definite manner,  to  those  that  do  so  in  a  definite  manner; 
and  from  those  that  do  so  singly,  to  those  that  do  so  doubly 
and  triply  through  successive  generations  of  fronds.  Even 
within  the  limits  of  a  sub-class,  we  find  gradations  between 
fronds  irregularly  proliferous,  and  groups  of  such  fronds 
united  into  a  regular  series. 

Nor  does  the  process  end  here.  The  flowering  plant  is 
rarely  uniaxial — it  is  nearly  always  multiaxial.  From  its 
primary  shoot  there  grow  out  secondary  shoots  of  like  kind. 
Though  occasionally  among  Phsenogams,  and  frequently 
among  the  higher  Cryptogams,  the  germs  of  new  axes  detach 
themselves  under  the  form  of  bulbils,  and  develop  separately 
instead  of  in  connexion  with  the  parent  axis;  yet  in  most 
Phffinogams  the  germ  of  each  new  axis  maintains  its  con- 
nexion with  the  parent  axis :  whence  results  a  group  of  axes 
— an  aggregate  of  the  fourth  order.  Every  tree,  by  the  pro- 


80  MORPHOLOGICAL  DEVELOPMENT. 

duction  of  branch  out  of  branch,  shows  us  this  integration 
repeated  over  and  over  again ;  forming  an  aggregate  having  a 
degree  of  composition  too  complex  to  be  any  longer  defined. 


[NOTE. — A  criticism  passed  on  the  general  argument  set 
forth  in  the  foregoing  sections,  runs  as  follows : — "  I  have 
already  pointed  out  that  the  process  of  evolution  by  which 
you  believe  the  Liverworts  with  a  distinct  axis  and  append- 
ages to  have  been  produced  from  the  thalloid  forms  is  not 
founded  on  sound  evidence  either  in  comparative  morphology 
or  development.  But  even  if  we  admit  that  such  an  inte- 
gration of  a  proliferously-produced  colony  might  have  given 
rise  to  the  leafy  Jungermanniacece,  there  are  even  more 
weighty  objections  to  the  supposition  that  the  same  process 
produced  the  shoot  structures  of  the  flowering  plants.  In  the 
first  place  the  flowering  plant-body  is  not  homologous  with 
the  liverwort  plant-body,  since  they  represent  different  genera- 
tions. The  liverwort  plant-body  or  gametophyte,  i.e.,  the 
generation  bearing  sexual  organs,  is  homologous  with  the 
prothallus  of  ferns  and  other  Pteridophytes,  and  in  the 
Flowering  Plants  with  reduced  structures  contained  within 
the  spores  (embryo-sac  and  pollen-grain)  but  still  giving  rise 
to  sexual  cells.  The  liverwort  spore-capsule  and  its  accessory 
parts  (in  fact  everything  produced  from  the  fertilized  egg)  is 
homologous  with  the  sporogonium  of  the  mosses,  and,  as  most 
botanists  think,  with  the  leafy  plant-body  of  Pteridophytes 
and  Phanerogams.  This  generation  is  called  the  sporophyte 
and  from  the  spores  which  it  produces  are  developed  the 
gametophytes  of  the  next  generation.  These  generalizations 
were  first  established  by  Hofmeister,  and  all  subsequent 
work  has  tended  to  establish  them  more  firmly.  The 
only  doubtful  question  is  (and  the  doubt  is  mainly,  I  think, 
peculiar  to  myself,  certainly  not  being  shared  by  the  majority 
of  botanists)  whether  the  sporophyte  of  Mosses  and  Liver- 
forts  is  really  homologous  with  that  of  Pteridophytes  and 


THE  MORPHOLOGICAL  COMPOSITION  OF  PLANTS.     81 

Phanerogams,  whether  it  may  not  rather  be  regarded  as  a 
parallel  development  along  another  line  of  descent  from  the 
Green  Alga?. 

"  Hence  we  must  look  for  the  origin  of  the  shoot-structure 
of  flowering  plants  in  the  sporophytes  of  the  Pteridophytes, 
from  which  group  there  is  no  reason  to  doubt  that  the 
phanerogams  have  arisen  in  descent.  The  various  groups  of 
Pteridophytes  vary  much  in  the  organization  of  these  shoot- 
systems,  as  a  mental  glance  at  the  types  exhibited  by  the 
Ferns,  Horse-tails,  Club-mosses,  Ophioglossacece,  and  the  iso- 
lated Isoetes  will  convince  you  at  once.  It  may  be  that 
some  of  these  groups  are  independent  in  descent,  i.e.,  that  the 
Pteridophyta  are  polyphyletic,  and  the  current  hypothesis 
with  regard  to  the  phanerogams  is  that  they  have  arisen  by 
two,  if  not  three,  separate  lines  of  descent  from  different 
groups  of  Pteridophytes  (this  is  indicated  in  the  classificatory 
diagram  on  p.  377  of  vol.  I).  I  should  not,  however,  care  to 
pin  my  faith  to  these  or  to  any  such  lines  of  ancestry.  Still 
I  think  we  must  look  for  the  ancestors  of  the  Flowering 
Plants  among  the  Pteridophytes,  and  the  latter  always 
have  a  good  distinction  between  axis  and  appendages.  The 
problem  of  the  evolution  of  these  differentiated  sporophytic 
shoots  is  undoubtedly  the  -  great  outstanding  problem  of 
morphology.  Various  attempts  have  been  made  to  solve  it, 
of  which  probably  the  most  important  is  the  theory  of  Profs. 
Bown  and  Campbell,  who  derive  the  Pteridophytes  from  some 
Liverwort  like  Anthoceros,  but  the  sporophyte  of  course 
from  the  sporophytic  portion  of  the  plant  (not  much  more 
than  a  spore-capsule),  the  prothallus  of  the  Fern  representing 
the  vegetative  thallus  of  Anthoceros.  I  am  not  wholly  con- 
vinced by  these  undoubtedly  ingenious  hypotheses,  in  support 
of  which  an  immense  amount  of  facts  have  been  collected; 
but  my  position  would,  I  know,  simply  '  put  us  to  ignorance 
again '  on  this  question. 

"  I  have  discussed  this  at  some  length  in  order  to  bring 
out  clearly  the  immense  difficulty  of  constructing  a  well- 
52 


82        MORPHOLOGICAL  DEVELOPMENT. 

grounded  theory  of  the  origin  of  the  differentiated  shoot- 
system  of  the  higher  plant.  I  confess  I  don't  think  it  can 
be  done  at  all  with  the  materials  at  present  at  our  disposal. 
Of  course  it  is  just  possible  to  suppose  that  some  ancestral 
sporophyte  had  the  structure  of  a  proliferous  thalloid  liver- 
wort gametophyte,  and  that  from  it  was  evolved  the  phanero- 
gamic shoot  in  the  ways  you  suggest.  This  gives  us  abso- 
lutely no  clue,  however,  to  any  Pteridophytic  shoot,  which 
ought  to  be  intermediate  (more  or  less)  between  the  hypo- 
thetical ancestor  and  the  Phanerogam,  and  is  furthermore, 
as  far  as  I  can  see,  not  supported  by  an  atom  of  evidence  of 
any  kind.  It  is  true  that  your  theory  fits  in  well  with  the 
phenomena  exhibited  by  phanerogamic  shoots  themselves, 
but  this  fact  you  will  see  must  lose  much  of  its  significance 
if  the  hypothesis  lacks  foundation. 

"With  regard  to  your  method  of  explaining  the  funda- 
mental characters  of  '  Exogens '  and  '  Endogens,'  this  of 
course  is  part  of  the  same  hypothesis;  but  I  may  point  out 
that  since  Von  Mohl  and  Sanio,  between  1855  and  1865, 
showed  (1)  that  the  growth  at  the  stem  apex  of  a  mono- 
cotyledon was  not  endogenous,  and  (2)  that  the  'thickening 
ring '  near  the  apex  of  a  dicotyledon  was  not  to  be  confused, 
as  had  been  done  up  till  then,  with  the  ring  of  secondary 
meristem  or  true  cambium,  which  arose  lower  down,  and  only 
in  woody  or  practically  woody  stem,  the  terms  '  Exogen '  and 
'  Endogen '  have  necessarily  fallen  into  disuse,  since  they 
imply  a  false  conception  of  what  happens.  Both  monocotyl- 
edons and  dicotyledons  have  a  '  thickening  ring,'  which 
gives-  rise  to  the  primary  vascular  cylinder  of  the  stem. 
When  the  stem  is  of  considerable  thickness,  as  in  Palms,  &c., 
it  grows  by  the  active  cell-division  of  its  outer  layers,  so  that 
both  classes  are  '  exogenous '  in  this  sense ;  while  the  addition 
of  a  centrifugal  zone  of  secondary  wood  is  confined  to  certain 
Dicotyledons  (Trees,  shrubs,  &c.). 

"  The  distinction  between  the  embryos,  moreover,  is  not 
absolute.  The  single  cotyledon  is  usually  terminal  in  mono- 


THE  MORPHOLOGICAL  COMPOSITION  OP  PLANTS.      83 

cotyledons,  but  not  always  (Dioscoracece  have  lateral  cotyl- 
edons), but  the  plumule  may  push  through  it  (Grasses)  or 
make  its  exit  sideways  (Palms),  or  be  formed  at  the  side 
(Alisma) ;  and  Dicotyledons  very  similarly. 

"  The  occurrence  of  completely  sheathing  leaves  in  grasses 
is  perhaps  correlated  with  the  absence  of  cambium,  but 
grasses  are  an  aberrant  type  among  monocotyledons,  and 
secondary  thickening  is  only  found  in  very  few  genera  of 
this  class,  so  that  the  correlation  is,  so  to  speak,  negative  and 
indirect It  is  clear  that  the  greater  part  of  the  dis- 
cussion will  have  to  be  re-written." 

For  the  reasons  assigned  in  the  preface  I  cannot  undertake 
to  re-write  the  discussion,  as  suggested.  It  must  stand  for 
what  it  is  worth.  All  I  can  do  is  here  to  include  along  with 
it  the  foregoing  criticisms. 

I  may,  however,  indicate  the  line  of  defence  I  should  take 
were  I  to  go  again  into  the  matter.  The  objections  are  based 
on  the  structure  of  existing  Liverworts  and  Phanogams.  But 
I  have  already  referred  to  the  probability — or,  indeed,  the 
certainty — that  in  conformity  with  the  general  principle  set 
forth  in  the  note  to  Chapter  I,  we,  must  conclude  that  the 
early  types  of  Liverworts  out  of  which  the  Phasnogams  are 
supposed  to  have  evolved,  as  well  as  the  early  types  of 
Pha?nogams  in  which  the  stages  of  evolution  were  presented, 
no  longer  exist.  We  must  infer  that  forms  simpler  than 
any  now  known,  and  more  intermediate  in  their  traits,  were 
the  forms  concerned;  and  if  so,  it  may  be  held  that  the 
incongruities  with  the  hypothesis  which  are  presented  by 
existing  forms,  do  not  negative  it.  The  scepticism  my  critic 
himself  expresses  respecting  the  current  interpretation  is  a 
partial  justification  of  this  view.  Moreover,  his  admission 
that  the  theory  set  forth  "  fits  in  well  with  the  phenomena 
exhibited  by  phanerogamic  shoots,"  must,  I  think,  be  regarded 
as  weighty  evidence.  On  the  Evolution-hypothesis  we  are 
obliged  to  suppose  that  the  Monocotyledons  and  Dicotyledons 
respectively  arose  by  integration  of  fronds;  and  if  to  the 


84:  MORPHOLOGICAL  DEVELOPMENT. 

question  after  what  manner  the  integration  took  place,  there 
is  an  hypothesis  which  renders  it  comprehensible,  and  agrees 
both  with  the  structures  of  the  two  kinds  of  shoots  and  the 
structures  of  the  two  kinds  of  seeds,  as  well  as  with  various 
of  the  other  phenomena  the  two  types  present,  it  has  strong 
claims  for  acceptance. 

Eegonsideration  suggests  the  following  remarks. 

1.  Alternation  of  generations  is  a  means  of  furthering 
multiplication.    To  be  effective  each  member  of  either  genera- 
tion must  be  a  self-supporting  centre  of  growth  or  diffusion 
or  both.    Hence  if,  as  in  the  Liverworts,  one  of  the  so-called 
alternating  generations  is  not  independent,  but  a  permanent 
growth  on  the  other — a  parasite — it  is  a  misuse  of  words 
to  call  the  arrangement  Alternation  of  generations.     (Since 
this  was  written  I  have  found  that  Sir  Edward  Fry  takes  the 
same  view.      He  approvingly  quotes  Professor  Bower,  who 
says  that  "the  alternation  of  generations  is  not  an  accurate 
statement  of  facts  or  a  useful  analogy.") 

2.  The  alternating  of  sexual  and  non-sexual  processes  is 
not   fundamentally   distinctive;    for,   as    shown    by    sundry 
Archegoniates,  it  is  an  inconstant  trait,  and  as  shown  by 
Klebs'   experiments   on    Vaucheria,  the  conditions   may   be 
varied  so  as  to  determine  its  occurrence  or  non-occurrence. 
Nay,  the  same  individual  may  reproduce  in  either  way. 

3.  Still  more  significant  is  the  fact  that  in  some  of  the 
marine  Thallophytes,  there  is  a  process  like  that  which  in  a 
moss  or  a  fern  is  considered  an  alternation  of  generations, 
whereas  in  others,  as  the  Brown  Wrack  (Fucus),  each  genera- 
tion is  sexual.    Thus  the  presence  or  absence  of  this  mode  of 
genesis  cannot  be  a  cardinal  distinction. 

4.  With  these  facts  before  us,  it  is  not  only  a  reasonable 
supposition  but  a  highly  probable  supposition,  that  there  have 
existed  plants  of  the  Liverwort  type  in  which  the  so-called 
alternation  of  generations  did  not  take  place.    If  so,  nearly  all 
the  foregoing  objections  to  my  hypothesis  fall  to  the  ground.] 


CHAPTER  IV. 

THE   MORPHOLOGICAL   COMPOSITION   OF   ANIMALS. 

§  199.  WHAT  was  said  in  §  180,  respecting  the  ultimate 
structure  of  organisms,  holds  more  manifestly  of  animals 
than  of  plants.  That  throughout  the  vegetal  kingdom  the 
cell  is  the  morphological  unit,  is  a  proposition  admitting  of  a 
better  defence,  than  the  proposition  that  the  cell  is  the  mor- 
phological unit  throughout  the  animal  kingdom.  The  quali- 
fications with  which,  as  we  saw,  the  cell-doctrine  must  be 
taken,  are  qualifications  thrust  upon  us  more  especially  by 
the  facts  which  zoologists  have  brought  to  light.  It  is  among 
the  Protozoa  that  there  occur  numerous  cases  of  vital  activity 
displayed  by  specks  of  protoplasm;  and  from  the  minute 
anatomy  of  all  creatures  above  these,  are  drawn  the  numer- 
ous proofs  that  non-cellular  tissues  may  arise  by  direct  meta- 
morphosis of  mixed  colloidal  substances.* 

*  Since  this  paragraph  was  published  in  1865,  much  has  been  learned  con- 
cerning cell-structure,  as  is  shown  in  Chapter  VI*  of  Part  I.  While  some 
assert  that  there  exist  portions  of  living  protoplasm  without  nuclei,  others 
assert  that  a  nucleus  is  in  every  case  present,  and  that  where  it  does  not  exist 
in  a  definite  aggregated  form  it  exists  in  a  dispersed  form.  As  remarked  in 
the  chapter  named,  "  the  evidence  is  somewhat  strained  to  justify  this  dogma." 
Words  are  taken  in  their  non-natural  senses,  if  one  which  connotes  an  indi- 
vidualized body  is  applied  to  the  widely-diffused  components  of  such  a  body ; 
and  this  perverting  of  proper  meanings  leads  to  obscuration  of  what  may 
perhaps  be  an  essential  truth.  As  argued  in  the  chapter  named  (§§  74c,  74/), 
jclear  matter  is,  as  shown  by  its  chemical  character,  an  extremely  unstable 
substance,  the  molecular  changes  of  which,  perpetually  going  on,  initiate 

85 


86  MORPHOLOGICAL   DEVELOPMENT. 

Our  survey  of  morphological  composition  throughout  the 
animal  kingdom,  must  therefore  begin  with  those  undiffer- 
entiated  aggregates  of  physiological  units  [or  constitutional 
units],  out  of  which  are  formed  what  we  call,  with  consider- 
able license,  morphological  units. 

§  200.  In  that  division  of  the  Protozoa  distinguished  as 
Rhizopoda,  are  presented,  under  various  modifications,  these 
minute  portions  of  living  organic  matter,  so  little  differenti- 
ated, if  not  positively  undifferentiated,  that  animal  individu- 
ality can  scarcely  be  claimed  for  them.  Figs.  131,  132,  and 
133,  represent  certain  nearly-allied  types  of  these — Amoeba, 


Actinophrys,  and  LieberJcuhnia.  The  viscid  jelly  or  sarcode, 
comparable  in  its  physical  properties  to  white  of  egg,  out  of 
which  one  of  these  creatures  is  mainly  formed,  shows  us  in 
various  ways,  the  feebleness  with  which  the  component 
physiological  units  are  integrated — shows  us  this  by  its  very 
slight  cohesion,  by  the  extreme  indefiniteness  and  mutability 
of  its  form,  and  by  the  absence  of  a  limiting  membrane. 
It  is  no  longer  held  even  by  unqualified  adherents  of  the 
cell-doctrine  that  the  Amoeba  has  an  investment.  Its  outer 
surface,  compared  to  the  film  which  forms  on  the  surface  of 
paste,  does  not  prevent  the  taking  of  solid  particles  into  the 
mass  of  the  body,  and  does  not,  in  such  kindred  forms  as 
Fig.  133,  prevent  the  pseudopodia  from  coalescing  when  they 
meet.  Hence  it  cannot  properly  have  the  name  of  a  cell-wall. 
A  considerable  portion  of  the  body,  however,  in  Difflugia, 

shocks,  producing  changes  all  around.  In  the  earlier  stages  of  cell-evolution 
this  unstable  substance  is  dispersed  throughout  the  cytoplasm;  whereas  in 
the  more  advanced  stages  it  is  gathered  together  in  one  mass.  If  so,  instead 
of  saying  there  is  a  dispersed  nucleus  we  should  say  there  are  the  materials 
of  a  nucleus  not  yet  integrated. 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.    87 

Fig.  134,  has  a  denser  coating  formed  of  agglutinated  foreign 
particles ;  so  that  the  protrusion  of  the  pseudopodia  is  limited 
to  one  part  of  it.  And  in  the  solitary  Foraminifera,  like 
Gromia,  the  sarcode  is  covered  over  most  of  its  surface  by  a 
delicate  calcareous  shell,  pierced  with  minute  holes,  through 
which  the  slender  pseudopodia  are  thrust.  The 

Gregarina  exhibits  an  advance  in  integration,  and  a  conse- 
quent greater  defmiteness.  Figs.  135  and  136,  exemplifying 
this  type,  show  the  complete  membrane  in  which  the  sub- 
stance of  the  creature  is  contained.  Here  there  has  arisen 
what  may  be  properly  called  a  cell:  under  its  solitary  form 
this  animal  is  truly  unicellular.  Its  embryology  has  con- 
siderable significance.  After  passing  through  a  certain  qui- 
escent, "  encysted "  state,  its  interior  breaks  up  into  small 
portions,  which,  after  their  exit,  assume  forms  like  that  of 
the  Amoeba;  and  from  this  young  condition  in  which  they 
are  undifferentiated,  they  pass  into  that  adult  condition  in 
which  they  have  limiting  membranes.  If  this  development 
of  the  individual  Gregarina  typifies  the  mode  of  evolution  of 
the  species,  it  yields  further  support  to  the  belief,  that  frag- 
ments of  sarcode  existed  earlier  than  any  of  the  structures 
which  are  called  cells.  Among  aggregates  of  the  first 

order,  there  are  some  much  more  highly  developed.  These 
are  the  Infusoria,  constituting  the  most  numerous  of  the 
Protozoa,  in  species  as  in  individuals.  Figs.  137,  138,  and 
139,  are  examples.  In  them  we  find,  along  with  greater 
definiteness,  a  considerable  heterogeneity.  The  sarcode  of 
which  the  body  consists,  has  an  indurated  outer  layer,  bearing 
cilia  and  sometimes  spines;  there  is  an  opening  serving  as 
mouth,  a  permanent  resophagus,  and  a  cavity  or  cavities, 
temporarily  formed  in  the  interior  of  the  sarcode,  to  serve  as 
one  or  more  stomachs;  and  there  is  a  comparatively  specific 
arrangement  of  these  and  various  minor  parts. 

Thus  in  the  animal  kingdom,  as  in  the  vegetal  kingdom, 
there  exists  a  class  of  minute  forms  having  this  peculiarity, 
that  no  one  of  them  is  separable  into  a  number  of  visible 


88 


MORPHOLOGICAL  DEVELOPMENT. 


components  homologous  with  one  another — no  one  of  them 
can  be  resolved  into  minor  individualities.  Its  proximate 
units  are  those  physiological  units  of  which  we  conclude  every 
organism  consists.  The  aggregate  is  an  aggregate  of  the  first 
order. 


§  201.  Among  plants  are  found  types  indicating  a  transi- 
tion from  aggregates  of  the  first  order  to  aggregates  of  the 
second  order;  and  among  animals  we  find  analogous  types. 
But  the  stages  of  progressing  integration  are  not  here  so  dis- 
tinct. The  reason  probably  is,  that  the  simplest  animals, 
having  individualities  much  less  marked  than  those  of  the 
simplest  plants,  do  not  afford  us  the  same  facilities  for  obser- 
vation. In  proportion  as  the  limits  of  the  minor  individuali- 
ties are  indefinite,  the  formation  of  major  individualities  out 
of  them,  naturally  leaves  less  conspicuous  traces. 

Be  this  as  it  may,  however,  in  such  types  of  Protozoa  as 
the  compound  Radiolaria,  we  find  that  though  there  is  reason 
to  regard  the  aggregate  as  an  aggregate  of  the  second  order, 
yet  its  divisibility  into  minor  individualities  like  those  just 
described,  is  less  manifest.  Fig.  140  representing  Sphcsrozoum 


punctatum,  one  of  the  group,  illustrates  this.  The  sceptic- 
ally-minded may  perhaps  doubt  whether  we  can  regard  the 
"  cellffiform  bodies "  contained  in  it,  as  the  morphological 
units  of  the  animal.  The  jelly-like  mass  in  which  they  are 
imbedded,  is  but  indefinitely  divisible  into  portions  having 
each  a  cell  or  nucleus  for  its  centre.*  Among  the 

*  This  statement  seems  at  variance  with  the  figure ;  but  the  figure  is  very 
inaccurate.  Its  inaccuracy  curiously  illustrates  the  vitiation  of  evidence. 
When  I  saw  the  drawing  on  the  block,  I  pointed  out  to  the  draughtsman, 


THE  MORPHOLOGICAL  COMPOSITION   OF  ANIMALS.    89 

Foraminifera,  we  find  only  indefinite  evidence  of  the  coal- 
escence of  aggregates  of  the  first  order,  into  aggregates  of  the 
second  order.  There  are  solitary  Foraminifers,  allied  to  the 
creature  represented  in  Fig.  134.  Certain  ideal  types  of 
combination  among  them,  are  shown  in  Fig.  141.  And 
setting  out  from  these,  we  may  ascend  in  various  directions 
to  kinds  compounded  to  an  immense  variety  of  degrees  in  an 
immense  variety  of  ways.  In  all  of  them,  however,  the 
separability  of  the  major  individuality  into  minor  indivi- 
dualities, is  very  incomplete.  The  portion  of  sarcode  con- 
tained in  one  of  these  calcareous  chambers,  gives  origin  to 
an  external  bud;  and  this  presently  becomes  covered,  like 
its  parent,  with  calcareous  matter:  the  position  in  which 
each  successive  chamber  is  so  produced,  determining  the 
form  of  the  compound  shell.  But  the  portions  of  sarcode 
thus  budded  out  one  from  another,  do  not  become  distinctly 
individualized.  Fig.  142,  representing  the  living  net-work 
which  remains  when  the  shell  of  an  Orbitolite  has  been  dis- 
solved, shows  the  continuity  that  exists  among  the  occupants 
of  its  aggregated  chambers.*  In  the  compound 

Infusoria,  the  component  units  remain  quite  distinct.  Being, 
as  aggregates  of  the  first  order,  much  more  definitely  or- 
ganized, their  union  into  aggregates  of  the  second  order  does 

that  he  had  made  the  surrounding  curves  much  more  obviously  related  to 
the  contained  bodies,  than  they  were  in  the  original  (in  Dr.  Carpenter's  For- 
aminifcra);  and  having  looked  on  while  he  in  great  measure  remedied  this 
defect,  thought  no  further  care  was  needed.  Now,  however,  on  seeing  the 
figure  in  the  printer's  proof,  I  find  that  the  engraver,  swayed  by  the  same 
supposition  as  the  draughtsman  that  such  a  relation  was  meant  to  be  shown, 
has  made  his  lines  represent  it  still  more  decidedly  than  those  of  the  draughts- 
man before  they  were  corrected.  Thus,  vague  linear  representations,  like 
vague  verbal  ones,  are  apt  to  grow  more  definite  when  repeated.  Hypothesis 
warps  perceptions  as  it  warps  thoughts. 

*  Though  the  subdivision  into  chambers  of  the  shell  does  not  correspond 
to  the  subdivision  into  cell-units  it  may  still  be  held  that  since  in  the  solitary 
types  the  subdivision  of  the  nucleus  is  followed  by  formation  of  new  indi- 
viduals which  separate,  and  since  in  the  compound  types  the  subdivision  of 
the  nucleus  is  followed  by  growth  and  formation  of  new  chambers,  the  com- 
pound type  must  be  regarded  as  an  aggregate  of  the  second  order. 


90         MORPHOLOGICAL  DEVELOPMENT. 

not  destroy  their  original  individualities.  Among  the  Vorti- 
cellce,  of  which  two  kinds  are  delineated  in  Figs.  144  and 
145,  there  are  various  illustrations  of  this :  the  members  of 
the  community  being  sometimes  appended  to  a  single  stem; 
sometimes  attached  by  long  separate  stems  to  a  common 
base;  and  sometimes  massed  together. 

Thus  far,  these  aggregates  of  the  second  order  exhibit  but 
indefinite  individualities.  The  integration  is  physical;  but 
not  physiological.  Though,  in  the  Polycytharia,  there  is  a 
shape  that  has  some  symmetry;  and  though,  in  the  Fora- 
minifera,  the  formation  of ,  successive  chambers  proceeds  in 
such  methodic  ways  as  to  produce  quite-regular  and  toler- 
ably-specific shells ;  yet  no  more  in  these  than  in  the  Sponges 
or  the  compound  Vorticellce,  do  we  find  such  co-ordination  as 
gives  the  whole  a  life  predominating  over  the  lives  of  its 
parts.  We  have  not  yet  reached  an  aggregate  of  the  second 
order,  so  individuated  as  to  be  capable  of  serving  as  a  unit  in 
still  higher  combinations.  But  in-the  class  Ccslenterata,,  this 
advance  is  displayed.  The  common  Hydra,  habitually  taken 
as  the  type  of  the  lowest  division  of  this  class,  has  specialized 
parts  performing  mutually-subservient  functions,  and  thus 
exhibiting  a  total  life  distinct  from  the  lives  of  the  units. 
Fig.  146  represents  one  of  these  creatures  in  its  contracted 
state  and  in  its  expanded  state;  while  Fig.  147  is  a  diagram 
showing  the  wall  of  this  crea- 
ture's sac-like  body  as  seen  in 
section  under  the  microscope: 
a  and  b  being  the  outer  and 
inner  cellular  layers;  while  be- 
tween them  is  the  "  mesogloea  " 
or  "  structureless  lamella,"  the 
supporting  or  skeletal  layer.  But  this  lowly-organized 
tissue  of  the  Hydra,  illustrates  a  phase  of  integration 
in  which  the  lives  of  the  minor  aggregates  are  only  par- 
tially-subordinated to  the  life  of  the  major  aggregate 
formed  by  them.  For  a  Hydra's  substance  is  separable 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.   91 

into  Amoeba-like  portions,  capable  of  moving  about  inde- 
pendently. If  we  bear  in  mind  how  analogous  are  the 
extreme  extensibility  and  contractility  of  a  Hydra's  body 
and  tentacles,  to  the  properties  displayed  by  the  sarcode 
among  Khizopods;  we  may  infer  that  probably  the  move- 
ments and  other  actions  of  a  Hydra,  are  due  to  the  half- 
independent  co-operation  of  the  Amceba-like  individuals 
composing  it. 

§  202.  A  truth  which  we  before  saw  among  plants,  we 
here  see  repeated  among  animals — the  truth  that  as  soon  as 
the  integration  of  aggregates  of  the  first  order  into  aggregates 
of  the  second  order,  produces  compound  wholes  so  specific  in 
their  shapes  and  sizes,  and  so  mutually  dependent  in  their 
parts,  as  to  have  distinct  individualities ;  there  simultaneously 
arises  the  tendency  in  them  to  produce,  by  gemmation,  other 
such  aggregates  of  the  second  order.  The  approach  towards 
definite  limitation  in  an  organism,  is,  by  implication,  an  ap- 
proach towards  a  state  in  which  growth  passing  a  certain 
point,  results,  not  in  the  increase  of  the  old  individual,  but 
in  the  formation  of  a  new  individual.  Thus  it  happens  that 
the  common  polype  buds  out  other  polypes,  some  of  which 
very  shortly  do  the  like,  as  shown  in  Fig.  148:  a  process 
paralleled  by  the  fronds  of 
sundry  Algae,  and  by  those 
of  the  lower  Jungermanni- 
acece.  And  just  as,  among 
these  last  plants,  the  pro- 
liferously-produced  fronds, 
after  growing  to  certain 
sizes  and  developing  root- 
lets, detach  themselves  from  their  parent  fronds;  so  among 
these  animals,  separation  of  the  young  ones  from  the  bodies 
of  their  parents  ensues  when  they  have  acquired  tolerably 
complete  organizations. 

There  is  reason  to  think  that  the  parallel  holds  still  fur- 


MORPHOLOGICAL  DEVELOPMENT. 


ther.  Within  the  limits  of  the  Jungermanniacece,  we  found 
that  while  some  genera  exhibit  this  discontinuous  develop- 
ment, other  genera  exhibit  a  development  that  is  similar  to 
it  in  all  essential  respects,  save  that  it  is  continuous.  And 
here  within  the  limits  of  the  Hydrozoa,  we  find,  along  with 
this  genus  in  which  the  gemmiparous  individuals  are  pre- 
sently cast  off,  other  genera  in  which  they  are  not  cast  off, 
but  form  a  permanent  aggregate  of  the  third  order.  Figs. 
149  and  150,  exemplify  these  compound  Hydrozoa — one  of 
them  showing  this  mode  of  growth  so  carried  out  as  to  pro- 
duce a  single  axis;  and  the  other  showing  how,  by  repeti- 
tions of  the  process,  lateral  axes  are  produced.  Integrations 
characterizing  certain  higher  genera 
of  the  Hydrozoa  which  swim  or  float 
instead  of  being  fixed,  are  indicated 
by  Figs.  151  and  152:  the  first  of 
them  representing  the  type  of  a 
group  in  which  the  polypes  growing 
from  an  axis,  or  coenosarc,  are  drawn 
through  the  water  by  the  rhythmi- 
cal contractions  of  the  organs  from 
which  they  hang;  and  the  second  of 
them  representing  a  Physalia  the 
component  polypes  of  which  are 
united  into  a  cluster,  attached  to  an 
air-vessel. 

A  parallel  series  of  illustrations  might  be  drawn  from  that 
second  division  of  the  Calentcrata,  known  as  the  Actinozoa. 
Here,'  too,  we  have  a  group  of  species — the  Sea-anemones — 
the  individuals  of  which  are  solitary.  Here,  too,  we  have 
agamogenetic  multiplication :  occasionally  by  gemmation,  but 
more  frequently  by  that  modified  process  called  spontaneous 
fission.  And  here,  too,  we  have  compound  forms  resulting 
from  the  arrest  of  this  spontaneous  fission  before  it  is  com- 
plete. To  give  examples  is  needless;  since  they  would  but 
show,  in  more  varied  ways,  the  truth  already  made  suffi- 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.   93 


ciently  clear,  that  the  compound  Ccelenterata  are 
of  the  third  order,  produced  by  integration  of  aggregates  of 
the  second  order  such  as  we  have  in  the  Hydra.  As  before, 
it  is  manifest  that  on  the  hypothesis  of  evolution,  these 
higher  integrations  will  insensibly  arise,  if  the  separation  of 
the  gemmiparous  polypes  is  longer  and  longer  postponed; 
and  that  an  increasing  postponement  will  result  by  survival 
of  the  fittest,  if  it  profits  the  group  of  individuals  to  remain 
united  instead  of  dispersing.* 

§  203.  The  like  relations  exist,  and  imply  that  the  like 
processes  have  been  gone  through,  among  those  more  highly- 
organized  animals  called  Polyzoa  and  Tunicata.  We  have 
solitary  individuals,  and  we  have  variously-integrated  groups 
of  individuals :  the  chief  difference  between  the  evidence 
here  furnished,  and  that  furnished  in  the  last  case,  being  the 
absence  of  a  type  obviously  linking  the  solitary  state  with 
the  aggregated  state. 

This  integration  of  aggregates  of  the  second  order,  is  car- 
ried on  among  the  Polyzoa  in  divers  ways,  and  with  different 
degrees  of  completeness.  The  little  patches  of  minute  cells, 
shown  as  magnified  in  Fig.  153,  so  common  on  the  fronds  of 
sea-weeds  and  the  surfaces  of  rocks  at  low-water  mark,  display 
little  beyond  mechanical  combination.  The  adjacent  indi- 


153 


IS* 


155 


I 


*  A  critic  says  the  question  is  "  what  are  the  forces  internal  or  external 
which  produce  union  or  separation."  A  proximate  reply  is — degree  of  nutri- 
tion. As  in  a  plant  new  individuals  or  rudiments  of  them  are  cast  off  where 
nutrition  is  failing,  so  in  a  compound  animal.  The  connecting  part  dwindles 
if  it  ceases  to  carry  nutriment. 


94        MORPHOLOGICAL  DEVELOPMENT. 

viduals,  though  severally  originated  by  gemmation  from  the 
same  germ,  have  but  little  physiological  dependence.  In 
kindred  kinds,  however,  as  shown  in  Figs.  154  and  155,  one 
of  which  is  a  magnified  portion  of  the  other,  the  integration 
is  somewhat  greater:  the  co-operation  of  the  united  indi- 
viduals being  shown  in  the  production  of  those  tubular 
branches  which  form  their  common  support,  and  establish 
among  them  a  more  decided  community  of  nutrition. 

Among  the  Ascidians  this  general  law  of  morphological 
composition  is  once  more  displayed.  Each  of  these  creatures 
subsists  on  the  nutritive  particles  contained  in  the  water 
which  it  draws  in  through  one  orifice  and  sends  out  through 
another;  and  it  may  thus  subsist  either  alone,  or  in  con- 
nexion with  others  that  are  in  some  cases  loosely  aggregated 
and  in  other  cases  closely  aggregated.  Fig.  156,  Phallusia 


mentula,  is  one  of  the  solitary  forms.  A  type  in  which  the 
individuals  are  united  by  a  stolon  that  gives  origin  to  them 
by  successive  buds,  is  shown  in  Perophora,  Fig.  157.  Among 
the  Botryllidce,  of  which  one  kind  is  drawn  on  a  small  scale 
in  Fig.  159,  and  a  portion  of  the  same  on  a  larger  scale  in 
Fig.  158,  there  is  a  combination  of  the  individuals  into  an- 
nular clusters,  which  are  themselves  imbedded  in  a  common 
gelatinous  matrix.  And  in  this  group  there  are  integrations 
even  a  stage  higher,  in  which  several  such  clusters  of  clusters 
grow  from  a  single  base.  Here  the  compounding  and  re- 
compounding  appears  to  be  carried  further  than  anywhere 
else  in  the  animal  kingdom. 

Thus  far,  however,  among  these  aggregates  of  the  third 
order,  we  see  what  we  before  saw  among  the  simpler  aggre- 
gates of  the  second  order — we  see  that  the  component  indi- 
vidualities are  but  to  a  very  small  extent  subordinated  to 


THE   MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  95 

the  individuality  made  up  of  them.  In  nearly  all  the  forms 
indicated,  the  mutual  dependence  of  the  united  animals  is  so 
slight,  that  they  are  more  fitly  comparable  to  societies,  of 
which  the  members  co-operate  in  securing  certain  common 
benefits.  There  is  scarcely  any  specialization  of  functions 
among  them.  Only  in  the  last  type  described  do  we  see  a 
number  of  individuals  so  completely  combined  as  to  simulate 
a  single  individual.  And  even  here,  though  there  appears  to 
be  an  intimate  community  of  nutrition,  there  is  no  physio- 
logical integration  beyond  that  implied  in  several  mouths  and 
stomachs  having  a  common  vent.* 

§  204.  We  come  now  to  an  extremely  interesting  question. 
Does  there  exist  in  other  sub-kingdoms  composition  of  the 
third  degree,  analogous  to  that  which  we  have  found  so 
prevalent  among  the  Codenterata  and  the  Polyzoa  and  Tuni- 
cata?  The  question  is  not  whether  elsewhere  there  are 
tertiary  aggregates  produced  by  the  branching  or  clustering 
of  secondary  aggregates,  in  ways  like  those  above  traced; 
but  whether  elsewhere  there  are  aggregates  which,  though 
otherwise  unlike  in  the  arrangement  of  their  parts,  never- 
theless consist  of  parts  so  similar  to  one  another  that  we 
may  suspect  them  to  be  united  secondary  aggregates.  The 
various  compound  types  above  described,  in  which  the  united 
animals  maintain  their  individualities  so  distinctly  that  the 
individuality  of  the  aggregate  remains  vague,  are  constructed 
in  such  ways  that  the  united  animals  carry  on  their  several 
activities  with  scarcely  any  mutual  hindrance.  The  members 
of  a  branched  Hydrozoon,  such  as  is  shown  in  Fig.  149  or 
Fig.  150,  are  so  placed  that  they  can  all  spread  their  tentacles 
and  catch  their  prey  as  well  as  though  separately  attached  to 
stones  or  weeds.  Packed  side  by  side  on  a  flat  surface  or 

*  It  has  been  pointed  out  that  I  have  here  understated  the  evidence  of 
physiological  integration.  An  instance  of  it  among  ffydrozoa  is  shown  in 
Fig.  151,  but  by  a  strange  oversight  I  have  forgotten  to  name  the  various 
cases  furnished  by  the  Siphonophorn  in  which  the  individual  polypes  of  a  com- 
pound aggregate  are  greatly  specialized  in  adaptation  to  different  functions. 


96  MORPHOLOGICAL  DEVELOPMENT. 

forming  a  tree-like  assemblage,  the  associated  individuals 
among  the  Polyzoa  are  not  unequally  conditioned:  or  if  one 
has  some  advantage  over  another  in  a  particular  case,  the 
mode  of  growth  and  the  relations  to  surrounding  objects  are 
so  irregular  as  to  prevent  this  advantage  re-appearing  with 
constancy  in  successive  generations.  Similarly  with  the 
Ascidians  growing  from  a  stolon  or  those  forming  an  annular 
cluster:  each  of  them  is  as  well  placed  as  every  other  for 
drawing  in  the  currents  of  sea-water  from  which  it  selects  its 
food.  In  these  cases  the  mode  of  aggregation  does  not  expose 
the  united  individuals  to  multiform  circumstances;  and 
therefore  is  not  calculated  to  produce  among  them  any 
structural  multiformity.  For  the  same  reason  no  marked 
physiological  division  of  labour  arises  among  them;  and 
consequently  no  combination  close  enough  to  disguise  their 
several  individualities.  But  under  converse  conditions  we 
may  expect  converse  results.  If  there  is  a  mode  of  integration 
which  necessarily  subjects  the  united  individuals  to  unlike 
sets  of  incident  forces,  and  does  this  with  complete  uniformity 
from  generation  to  generation,  it  is  to  be  inferred  that  the 
united  individuals  will  become  unlike.  They  will  severally 
assume  such  different  functions  as  their  different  positions 
enable  them  respectively  to  carry  on  with  the  greatest 
advantage  to  the  assemblage.  This  heterogeneity  of  function 
arising  among  them,  will  be  followed  by  heterogeneity  of 
structure;  as  also  by  that  closer  combination  which  the 
better  enables  them  to  utilize  one  another's  functions.  And 
hence,  while  the  originally-like  individuals  are  rendered 
unlike,  they  will  have  their  homologies  further  obscured  by 
their  progressing  fusion  into  an  aggregate  individual  of  a 
higher  order. 

These  converse  conditions  are  in  nearly  all  cases  fulfilled 
where  the  successive  individuals  arising  by  continuous  devel- 
opment are  so  budded-off  as  to  form  a  linear  series.  I  say 
in  nearly  all  cases,  because  there  are  some  types  in  which 
the  associated  individuals,  though  joined  in  single  file,  are 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.   97 

not  thereby  rendered  very  unlike  in  their  relations  to  the 
environment;  and  therefore  do  not  become  differentiated  and 
integrated  to  any  considerable  extent.  I  refer  to  such  Asci- 
dians  as  the  Salpidce.  These  creatures  float  passively  in  the 
sea,  attached  together  in  strings.  Being  placed  side  by  side 
and  having  mouths  and  vents  that  open  laterally,  each  of 
them  is  as  well  circumstanced  as  its  neighbours  for  absorb- 
ing and  emitting  the  surrounding  water;  nor  have  the  in- 
dividuals at  the  two  extremities  any  marked  advantages 
over  the  rest  in  these  respects.  Hence  in  this  type,  and  in 
the  allied  type  Pyrosoma,  which  has  its  component  indivi- 
duals built  into  a  hollow  cylinder,  linear  aggregation  may 
exist  without  the  minor  individualities  becoming  obscured 
and  the  major  individuality  marked:  the  conditions  under 
which  a  differentiation  and  integration  of  the  component 
individuals  may  be  expected,  are  not  fulfilled.  But  where 
the  chain  of  individuals  produced  by  gemmation,  is  either 
habitually  fixed  to  some  solid  body  by  one  of  its  extremities 
or  moves  actively  through  the  water  or  over  submerged 
stones  and  weeds,  the  several  members  of  the  chain  become 
differently  conditioned  in  the  way  above  described;  and  may 
therefore  be  expected  to  become  unlike  while  they  become 
united.  A  clear  idea  of  the  contrast  between  these  two 
linear  arrangements  and  their  two  diverse  results,  will  be 
obtained  by  considering  what  happens  to  a  row  of  soldiers, 
when  changed  from  the  ordinary  position  of  a  single  rank 
to  the  position  of  Indian  file.  So  long  as  the  men  stand 
shoulder  to  shoulder,  they  are  severally  able  to  use  their 
weapons  in  like  ways  with  like  efficiency;  and  could,  if 
called  on,  similarly  perform  various  manual  processes  directly 
or  indirectly  conducive  to  their  welfare.  But  when,  on  the 
word  of  command  "  right  face,"  they  so  place  themselves 
that  each  has  one  of  his  neighbours  before  him  and  another 
behind  him,  nearly  all  of  them  become  incapacitated  for 
fighting  and  for  many  other  actions.  They  can  walk  or  run 
one  after  another,  so  as  to  produce  movement  of  the  file  in 
53 


98  MORPHOLOGICAL  DEVELOPMENT. 

the  direction  of  its  length;  but  if  the  file  has  to  oppose  an 
enemy  or  remove  an  obstacle  lying  in  the  line  of  its  march, 
the  front  man  is  the  only  one  able  to  use  his  weapons  or 
hands  to  much  purpose.  And  manifestly  such  an  arrange- 
ment could  become  advantageous  only  if  the  front  man  pos- 
sessed powers  peculiarly  adapted  to  his  position,  while  those 
behind  him  facilitated  his  actions  by  carrying  supplies,  &c. 
This  simile,  grotesque  as  it  seems,  serves  to  convey  better 
perhaps  than  any  other  could  do,  a  clear  idea  of  the  relations 
that  must  arise  in  a  chain  of  individuals  arising  by  gemma- 
tion, and  continuing  permanently  united  end  to  end.  Such 
a  chain  can  arise  only  on  condition  that  combination  is  more 
advantageous  than  separation;  and  for  it  to  be  more  advan- 
tageous, the  anterior  members  of  the  series  must  become 
adapted  to  functions  facilitated  by  their  positions,  while  the 
posterior  members  become  adapted  to  functions  which  their 
positions  permit.  Hence,  direct  or  indirect  equilibration  or 
both,  must  tend  continually  to  establish  types  in  which  the 
connected  individuals  are  more  and  more  unlike  one  another, 
at  the  same  time  that  their  several  individualities  are  more 
and  more  disguised  by  the  integration  consequent  on  their 
mutual  dependence. 

Such  being  the  anticipations  warranted  by  the  general  laws 
of  evolution,  we  have  now  to  inquire  whether  there  are  any 
animals  which  fulfil  them.  Very  little  search  suffices;  for 
structures  of  the  kind  to  be  expected  are  abundant.  In  that 
great  division  of  the  animal  kingdom  at  one  time  called  An- 
nulosa,  but  now  grouped  into  Annelida  and  Arthropoda,  we 
find  a  variety  of  types  having  the  looked-for  characters.  Let 
us  contemplate  some  of  them. 

§  205.  An  adult  Chastopod  is  composed  of  segments  which 
repeat  one  another  in  their  details  as  well  as  in  their  general 
shapes.  Dissecting  one  of  the  lower  orders,  such  as  is 
shown  in  Fig.  160,  proves  that  the  successive  segments,  be- 
sides having  like  locomotive  appendages,  like  branchiae  and 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  99 

sometimes  even  like  pairs  of  eyes,  also  have  like  internal 
organs.  Each  has  its  enlargement  of  the  alimentary  canal; 
each  its  contractile  dilatation  of  the  great  blood-vessel;  each 
its  portion  of  the  double  nervous  cord,  with  ganglia  when 


these  exist ;  each  its  branches  from  the  nervous  and  vascular 
trunks  answering  to  those  of  its  neighbours;  each  its  simi- 
larly answering  set  of  muscles;  each  its  pair  of  openings 
through  the  body- wall;  and  so  on  throughout,  even  to  the 
organs  of  reproduction.  That  is  to  say,  every  segment  is  in 
great  measure  a  physiological  whole — every  segment  con- 
tains most  of  the  organs  essential  to  individual  life  and  mul- 
tiplication: such  essential  organs  as  it  does  not  contain, 
being  those  which  its  position  as  one  in  the  midst  of  a  chain, 
prevents  it  from  having  or  needing.  If  we 

ask  what  is  the  meaning  of  these  homologies,  no  adequate 
answer  is  supplied  by  any  current  hypothesis.  That  this 
"vegetative  repetition"  is  carried  out  to  fulfil  a  prede- 
termined plan,  was  shown  to  be  quite  an  untenable  notion 
(§§  133,  134),  On  the  one  hand,  we  found  nothing  satis- 
factory in  the  conception  of  a  Creator  who  prescribed  to  him- 
self a  certain  unit  of  composition  for  all  creatures  of  a  par- 
ticular class,  and  then  displayed  his  ingenuity  in  building  up 
a  great  variety  of  forms  without  departing  from  the  "  arche- 
typal idea."  On  the  other  hand,  examination  made  it  mani- 
fest that  even  were  such  a  conception  worthy  of  being  enter- 
tained, it  would  have  to  be  relinquished;  since  in  each  class 
there  are  numerous  deviations  from  the  supposed  "  archetypal 
idea."  Still  less  can  these  traits  of  structure  be  accounted 


100       MORPHOLOGICAL  DEVELOPMENT. 

for  teleologically.  That  certain  organs  of  nutrition  and  re- 
spiration and  locomotion  are  repeated  in  each  segment  of  a 
dorsibranchiate  annelid,  may  be  regarded  as  functionally  ad- 
vantageous for  a  creature  following  its  mode  of  life.  But 
why  should  there  be  a  hundred  or  even  two  hundred  pairs  of 
ovaries?  This  is  an  arrangement  at  variance  with  that 
physiological  division  of  labour  which  every  organism  pro- 
fits by — is  a  less  advantageous  arrangement  than  might  have 
been  adopted.  That  is  to  say,  the  hypothesis  of  a  designed 
adaptation  fails  to  explain  the  facts.  Contrariwise, 

these  structural  traits  are  just  such  as  might  naturally  be 
looked  for,  if  these  annulose  forms  have  arisen  by  the  in- 
tegration of  simpler  forms.  Among  the  various  compound 
animals  already  glanced  at,  it  is  very  general  for  the  united 
individuals  to  repeat  one  another  in  all  their  parts — repro- 
ductive organs  included.  Hence  if,  instead  of  a  clustered  or 
branched  integration,  such  as  the  Ccelenterata,  Polyzoa  and 
Tunicata  exhibit,  there  occurs  a  longitudinal  integration;  we 
may  expect  that  the  united  individuals  will  habitually  indi- 
cate their  original  independence  by  severally  bearing  germ- 
producing  or  sperm-producing  organs. 

The  reasons  for  believing  one  of  these  creatures  to  be  an 
aggregate  of  the  third  order,  are  greatly  strengthened  when 
we  turn  from  the  adult  structure  to  the  mode  of  develop- 
ment. Among  the  Dorsibranchiata  and  TubicolcR,  the  em- 
bryo leaves  the  egg  in  the  shape  of  a  ciliated  gemmule,  not 
much  more  differentiated  than  that  of  a  polype.  As  shown 
in  Fig.  162,  it  is  a  nearly  globular  mass;  and  its  interior 
consists  of  untransformed  cells.  The  first  appreciable  change 
is  an  elongation  and  a  simultaneous  commencement  of  seg- 
mentation. The  segments  multiply  by  a  modified  gemma- 
tion, which  takes  place  from  the  hinder  end  of  the  penultimate 
segment.  And  considerable  progress  in  marking  out  these 
divisions  is  made  before  the  internal  organization  begins. 
Figs.  163,  164,  165,  represent  some  of  these  early  stages.  In 
annelids  of  other  orders,  the  embryo  assumes  the  segmented 


THE   MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  101 

form  while  still  in  the  egg.  But  it  does  this  in  just  the 
same  manner  as  before.  Indeed,  the  essential  identity  of  the 
two  modes  of  development  is  shown  by  the  fact  that  the  seg- 
mentation within  the  egg  is  only  partially  carried  out:  in 


S6S 

all  these  types  the  segments  continue  to  increase  in  number 
for  some  time  after  hatching.  Now  this  process  is  as 

like  that  by  which  compound  animals  in  general  are  formed, 
as  the  different  conditions  of  the  case  permit.  When  new 
individuals  are  budded-out  laterally,  their  unfolding  is  not 
hindered — there  is  nothing  to  disguise  either  the  process  or 
the  product.  But  gemma3  produced  one  from  another  in  the 
same  straight  line,  and  remaining  connected,  restrict  one 
another's  developments;  and  that  the  resulting  segments  are 
so  many  gemmiparously-produced  individuals,  is  necessarily 
less  obvious. 

§  206.  Evidence  remains  which  adds  very  greatly  to  the 
weight  of  that  already  assigned.  Thus  far  we  have  studied 
only  the  individual  segmented  animal ;  considering  what  may 
be  inferred  from  its  mode  of  evolution  and  final  organization. 
We  have  now  to  study  segmented  animals  in  general.  Com- 
parison of  different  groups  of  them  and  of  kinds  within  each 
group,  will  disclose  various  phases  of  progressive  integration 
of  the  nature  to  be  anticipated. 

Among  the  simpler  Platylielmintlies,  as  in  some  kinds  of 
Planaria,  transverse  fission  occurs.  A  portion  of  a  Planaria 
separated  by  spontaneous  constriction,  becomes  an  inde- 


102        MORPHOLOGICAL  DEVELOPMENT. 

pendent  individual.  Sir  J.  G.  Dalyell  found  that  in  some 
cases  numerous  fragments  artificially  separated,  grew  into 
perfect  animals.*  In  these  creatures  which  thus  remind  us 
of  the  lowest  Hydrozoa  in  their  powers  of  agamogenetic 
multiplication,  the  individuals  produced  one  from  another 
do  not  continue  connected.  As  the  young  ones  laterally 
budded-off  by  the  Hydra  separate  when  complete,  so  do  the 
young  ones  longitudinally  budded-off  by  the  Planaria. 
Fig.  166  indicates  this.  But  there  are  allied  types  which 
show  us  a  more  or  less  persistent  union  of  homologous  parts, 
or  individuals,  similarly  arising  by  longitudinal  gemmation,  f 
The  cestoid  Entozoa  furnish  illustrations.  Without  dwelling 
on  the  fact  that  each  segment  of  a  Tcenia,  like  each  separate 
Planaria,  is  an  independent  hermaphrodite;  and  without 
specifying  the  sundry  common  structural  traits  which  add 
probability  to  the  suspicion  that  there  is  some  kinship  be- 
tween the  individuals  of  the  one  order  and  the  segments  of 
the  other;  it  will  suffice  to  point  out  that  the  two  types  are 
so  far  allied  as  to  demand  their  union  under  the  same  sub- 
class title.  And  recognizing  this  kinship,  we  see  significance 
in  the  fact  that  in  the  one  case  the  longitudinally-produced 
gemmaa  separate  as  complete  individuals,  and  in  the  other 
continue  united  as  segments  in  smaller  or  larger  numbers 
and  for  shorter  or  longer  periods.  In  Tcenia  ecliinococcus, 

*  Recently  Mr  T.  H.  Morgan  has  made  elaborate  experiments  which 
show  that  Planaria  Maculata  may  be  cut  into  many  pieces  from  various 
parts  and  of  various  shapes — even  a  slice  out  of  the  side — and  each,  if  not 
too  small,  will  produce  a  perfect  animal. 

t  Since  this  was  written  in  1865  there  has  come  to  light  evidence  more 
completely  to  the  point  than  any  at  that  time  known.  In  the  subdivision  of 
Plaiyhelminthes  known  as  Turbellaria,  there  are  some,  the  Microatomida 
which,  by  a  process  of  segmentation  form  "chains  of  4,  then  8,  then  16,  and 
sometimes  even  32  individuals."  "Each  forms  a  mouth  [lateral]  and  for 
some  time  the  chain  persists,  but  the  individuals  ultimately  become  sexually 
matured  and  then  separate."  (Shipley,  Zoology  of  the  Invertebrata,  p.  92.) 
Here  it  should  be  remarked  that  the  lateral  mouths  enable  the  members  of 
a  string  to  feed  separately,  and  that  nutrition  not  being  interfered  with  they 
doubtless  gain  some  advantage  by  temporary  maintenance  of  their  union — 
probably  in  creeping. 


THE   MORPHOLOGICAL  COMPOSITION  OP  ANIMALS.  1Q3 


represented  in  Fig.  167,  we  have  a  species  in  which  the 
number  of  segments  thus  united  does  not  exceed  four.  In 
Echinobothrium  typus  there  are  eight  or  ten;  and  in  cestoids 
generally  they  are  numerous.*  A  considerable 

hiatus  occurs  between  this  phase  of  integration  and  the  next 
higher  phase  which  we  meet  with;  but  it  is  not  greater 
than  the  hiatus  between  the  types  of  the  Platyhelminthes  and 
the  Chcetopoda,  which  present  the  two  phases.  Though  it  is 


doubtful  whether  separation  of  single  segments  occurs  among 
the  Annelida,\  yet  very  often  we  find  strings  of  segments, 

*  I  find  that  the  reasons  for  regarding  the  segment  of  a  Tcenia  as  answering 
to  an  individual  of  the  second  order  of  aggregation,  are  much  stronger  than 
I  supposed  when  writing  the  above.  Van  Beneden  says : — "  Le  Proglottis 
(segment)  ayant  acquis  tout  son  developpement,  se  detache  ordinairement  de 
la  colonie  et  continue  encore  a  croitre  dans  1'intestin  du  me'me  animal ;  il 
chance  menie  souvent  de  forme  et  semble  doue  d'une  nouvelle  vie ;  ses  angles 
s'effacent,  tout  le  corps  s'arrondit,  et  il  nage  comme  une  Planaire  au  milieu 
des  muscosites  intestinales." 

f  Though  this  was  doubtful  in  1866  it  is  no  longer  doubtful.  In  an  indi- 
vidual Ctenodrifus  monosfylus,  which  multiplies  by  dividing  and  subdividing 
itself,  "  parts  arise  which  are  destitute  of  both  head  and  anus  and  at  times 
consist  of  only  a  single  segment."  In  another  species,  C.  pardalis,  there  is 
separation  into  many  segments ;  and  each  segment  before  separating  forms 
a  budding  zone  out  of  which  other  segments  are  afterwards  produced,  com- 
pleting the  animal  (Korschelt  and  Heider,  Embryology,  i,  301-2). 


104  MORPHOLOGICAL  DEVELOPMENT. 

arising  by  repeated  longitudinal  budding,  which  after  reach- 
ing certain  lengths  undergo  spontaneous  fission:  in  some 
cases  doing  this  so  as  to  form  two  or  more  similar  strings 
of  segments  constituting  independent  individuals;  and  in 
other  cases  doing  it  so  that  the  segments  spontaneously 
separated  are  but  a  small  part  of  the  string.  Thus  a  Syllis, 
Fig.  168,  after  reaching  a  certain  length,  begins  to  trans- 
form itself  into  two  individuals :  one  of  the  posterior  segments 
develops  into  an  imperfect  head,  and  simultaneously  narrows 
its  connexion  with  the  preceding  segments,  from  which  it 
eventually  separates.  Still  more  remarkable  is  the  extent  to 
which  this  process  is  carried  in  certain  kindred  types;  which 
exhibit  to  us  several  individuals  thus  being  simultaneously 
formed  out  of  groups  of  segments.  Fig.  169,  copied  (omit- 
ting the  appendages)  from  one  contained  in  a  memoir 
by  M.  Milne-Edwards,  represents  six  worms  of  different 
ages  in  course  of  development:  the  terminal  one  being  the 
eldest,  the  one  having  the  greatest  number  of  segments, 
and  the  one  that  will  first  detach  itself;  and  the  success- 
ively anterior  ones,  with  their  successively  smaller  numbers 
of  segments,  being  successively  less  advanced  towards  fitness 
for  separation  and  independence.  Here  among  groups  of 
segments  we  see  repeated  what  in  the  previous  cases  occurs 
with  single  segments.  And  then  in  other  annelids  we  find  that 
the  string  of  segments  arising  by  gemmation  from  a  single 
germ  becomes  a  permanently  united  whole:  the  tendency  to 
any  more  complete  fission  than  that  which  marks  out  the  seg- 
ments, being  lost;  or,  in  other  words,  the  integration  having 
become  relatively  complete.  Leaving  out  of  sight  the 

question  of  alliance  among  the  types  above  grouped  together, 
that  which  it  here  concerns  us  to  notice  is,  that  longitudinal 
gemmation  does  go  on;  that  it  is  displayed  in  that  primitive 
form  in  which  the  gemma?  separate  as  soon  as  produced ;  that 
we  have  types  in  which  such  gemma?  hang  together  in 
groups  of  four,  or  in  groups  of  eight  and  ten,  from  which 
however  the  gemmae  successively  separate  as  individuals; 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  105 

that  among  higher  types  we  have  long  strings  of  similarly- 
formed  gemma?  which  do  not  become  individually  independ- 
ent, but  separate  into  organized  groups;  and  that  from 
these  we  advance  to  forms  in  which  all  the  gemmae  remain 
parts  of  a  single  individual.  One  other  significant 

fact  must  be  added.  There  are  cases  in  which  annelids 
multiply  by  lateral  gemmation.*  That  the  longitudinally- 
produced  gemma?  which  compose  an  annelid,  should  thus 
have,  one  of  them  or  several  of  them,  the  power  of  laterally 
budding-off  gemma?,  from  which  other  annelids  arise,  gives 
further  support  to  the  hypothesis  that,  primordially,  the  seg- 
ments were  independent  individuals.  And  it  suggests  this 
belief  the  more  strongly  because,  in  certain  types  of  Coden- 
terata,  we  see  that  longitudinal  and  lateral  gemmation  do 
occur  together,  where  the  longitudinally-united  gemma?  are 
demonstrably  independent  individuals. 

§  207.  Though  it  seems  next  to  impossible  that  we  shall 
ever  be  able  to  find  a  type  such  as  that  which  is  here  sup- 
posed to  be  the  unit  of  composition  of  the  annulose  type, 
since  we  must  assume  such  a  type  to  have  been  long  since 
extinct,  yet  the  foregoing  evidence  goes  far  towards  showing 
that  an  annulose  animal  is  an  aggregate  of  the  third  order. 
This  repetition  of  segments,  sometimes  numbering  several 
hundreds,  like  one  another  in  all  their  organs  even  down  to 
those  of  reproduction,  while  it  is  otherwise  unaccountable,  is 
fully  accounted  for  if  these  segments  are  homologous  with 
the  separate  individuals  of  some  lower  type.  The  gemma- 
tion by  which  these  segments  are  produced,  is  as  similar  as 
the  conditions  allow,  to  the  gemmation  by  which  compound 

*  In  place  of  those  originally  here  instanced  about  which  there  are  dis- 
putes, I  may  give  an  undoubted  one  described  by  Mclntosh,  the  Syllis 
ramosa,  a  species  of  chaetopod  living  in  hexactinellid  sponges  from  the  Ara- 
fura  Sea,  which  branches  laterally  repeatedly  so  as  to  extend  in  all  directions 
through  the  canals  of  the  sponge.  In  most  cases  the  buds  terminate  in  oval 
segments  with  two  long  cirri  each.  But  male  and  female  buds  were  found, 
provided  each  with  a  head,  and  containing  ovaries  and  testes.  Sometimes 
these  sexual  buds  had  become  separate  from  the  branched  stock. 


106        MORPHOLOGICAL  DEVELOPMENT. 

animals  in  general  are  produced.  As  among  plants,  and  as 
among  demonstrably-compound  animals,  we  see  that  the  only 
thing  required  for  the  formation  of  a  permanent  chain  of 
gemmiparously-produced  individuals,  is  that  by  remaining 
associated  such  individuals  will  have  advantages  greater  than 
are  to  be  gained  by  separation.  Further,  comparisons  of 
the  annuloid  and  lower  annulose  forms,  disclose  a  number 
of  those  transitional  phases  of  integration  which  the  hypo- 
thesis leads  us  to  expect.  And,  lastly,  the  differences  among 
these  united  individuals  or  successive  segments,  are  not 
greater  than  the  differences  in  their  positions  and  functions 
explain — not  greater  than  such  differences  are  known  to  pro- 
duce among  other  united  individuals:  witness  sundry  com- 
pound Hydrozoa. 

Indirect  evidence  of  much  weight  has  still  to  be  given. 
Thus  far  we  have  considered  only  the  less-developed  Annu- 
losa.  The  more  integrated  and  more  differentiated  types  of 
the  class  remain.  If  in  them  we  find  a  carrying  further  of 
the  processes  by  which  the  lower  types  are  here  supposed  to 
have  been  evolved,  we  shall  have  additional  reason  for  be- 
lieving them  to  have  been  so  evolved.  If  we  find  that  in 
these  superior  orders,  the  individualities  of  the  united  seg- 
ments are  much  less  pronounced  than  in  the  inferior,  we 
shall  have  grounds  for  suspecting  that  in  the  inferior  the 
individualities  of  the  segments  are  less  pronounced  than  in 
those  lost  forms  which  initiated  the  annulose  sub-kingdom. 


[N"OTE. — Partly  from  the  wish  to  incorporate  further  evi- 
dence, and  partly  from  the  wish  to  present  the  evidence,  old 
and  new,  in  a  more  effective  order,  I  decide  here  to  recast  the 
foregoing  exposition. 

Significant  traits  of  development  are  exhibited  in  common 
by  two  groups  otherwise  unallied — certain  of  the  PlatyJiel- 
minthes  and  certain  of  the  lower  Annulosa.  Of  the  Platyhel- 
minthes  the  ordinary  type  is  an  unsegmented  creature:  a 


THE   MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  107 

Planarian  or  a  Trematode  exemplifying  it.  Among  the  free 
forms,  as  in  some  Planarians,  there  occurs  transverse  fission, 
and  prompt  separation  of  the  segments;  while  among  some 
other  free  forms,  as  the  Microstomida,  the  two  segments  first 
produced,  themselves  become  segmented  while  still  adherent, 
and  this  process  is  repeated  until  a  string  is  formed.  Another 
group  of  the  Platyhelminthes,  the  Cestoid  Entozoa,  exhibit 
analogous  processes.  There  are  unsegmented  forms,  as  the 
Caryopliyllaeus,  and  there  are  forms  in  which  the  segments, 
now  few  now  many,  adhere  together  in  chains;  the  terminal 
members  of  which,  however,  eventually  separate,  and  having 
before  separation  approached  the  trematode  structure,  become 
independent  individuals  which  grow,  creep  about,  and  con- 
tinue the  race.  In  both  of  these  types  the  condition  under 
which  the  gemmiparously-produced  members  remain  con- 
nected, is  that  they  shall  be  able  to  feed  individually:  in 
the  one  case  by  lateral  mouths,  in  the  other  case  by  absorp- 
tion through  the  integument.  It  is  further  observable  that 
in  both  cases  separation  of  the  component  individuals  occurs 
at  sexual  maturity,  when  advantage  in  nutrition  has  ceased 
to  be  the  dominant  need  and  dispersion  of  the  species  has 
taken  its  place  in  degree  of  importance.  Among 

Annelids,  higher  though  they  are  in  type,  we  find  parallel- 
isms. Usually  in  its  first  stage  an  annelid  is  unsegmented, 
but  as  fast  as  it  elongates  lines  of  segmentation  indent  its 
surface.  This  segmentation  proceeds  in  various  ways,  and 
the  segments  exhibit  various  degrees  of  dependence.  In 
some  low  types,  spontaneous  fission  goes  on  to  the  extent  of 
producing  single  segments,  each  of  which  has  such  vitality 
that  it  buds  out  anterior  and  posterior  parts  at  its  two  ends. 
Thus  alike  in  the  simple  form  which  exists  before  segmenta- 
tion and  in  the  form  exhibited  by  a  detached  segment,  we 
have  a  unit  analogous  to  each  of  the  units  which  are  joined 
together  in  certain  free  Turbellaria  and  in  the  Cestoids:  the 
difference  being  that  in  the  Annelids  the  sexually-mature 
units  do  not  individually  disunite.  But  though  there  does 


108        MORPHOLOGICAL  DEVELOPMENT. 

not  take  place  separation  of  single  completed  segments,  there 
takes  place  separation  of  groups  of  segments,  which  are 
either  sexually  mature  at  the  time  they  drop  off  or  presently 
become  so.  And  the  groups  of  segments  which  have  become 
sexually  mature  before  they  drop  off,  have  simultaneously 
acquired  swimming  organs  and  developed  eyes,  enabling  them 
to  spread  and  diffuse  the  species.  Sundry  biologists  recognize 
a  parallelism  between  that  detachment  of  developed  segments 
which  goes  on  in  the  cestoid  Entozoa,  and  that  which  goes  on 
in  the  Scypliomedusce.  The  successively  detached  members 
of  the  strobila  are  sexually-matured  or  maturing  individuals 
which,  as  medusae,  are  fitted  for  swimming  about,  multiply- 
ing, and  reaching  other  habitats;  while  each  detached  pro- 
glottis  of  the  cestoid  is,  by  the  nature  of  its  medium,  limited 
to  creeping  about.  Clearly  this  fissiparous  process  in  such 
Annelids  as  the  Syllidce,  which  has  similarly  been  compared 
to  the  stabilization  of  the  Scypliomedusce,  differs  simply  in 
the  respect  that  single  segments  are  not  adapted  for  locomo- 
tion, and  it  therefore  profits  the  species  to  separate  in  groups. 
All  these  facts  and  analogies  point  to  the  conclusion  that  the 
remote  ancestor  of  the  Annelids  was  an  unsegmented  crea- 
ture homologous  with  each  of  the  segments  of  an  existing 
Annelid. 

This  conclusion  is  supported  by  other  kinds  of  evidence 
here  to  be  added.  The  Iarva3  of  Annelids  are  very  various ; 
but  amid  their  differences  there  is  a  recognizable  type.  "  The 
Trochophore  is  the  typical  larval  form  of  the  Annelid  stem  " : 
a  trochophore  being  a  curious  spheroidal  ciliated  structure 
suggestive  of  ccelenterate  affinities.  And  this  unsegmented 
larva,  representing  the  remote  ancestor  from  which  the  many 
Annelid  types  diverged,  is  similar  to  the  larvaa  of  the  Rotifera 
and  the  Mollusca :  a  trochophore  is  common  to  all  these  great 
classes.  Moreover  since,  among  the  Rhizota  (a  sub-class  of 
the  Rotiferce),  there  is  a  species,  Trochosphcera,  solitary  and 
free-swimming,  resembling  in  form  and  structure  a  trocho- 
phore, though  it  is  not  a  larva  but  an  adult,  we  get  further 


THE  MORPHOLOGICAL  COMPOSITION  OP  ANIMALS.  109 

evidence  that  there  was  a  primitive  creature  of  this  general 
character,  of  which  the  trochophores  of  Mollusca,  Rotifera, 
and  Annelida  are  divergent  modifications,  and  which  was 
unsegmented:  the  implication  being  that  the  segmentation 
or  the  Annelida  was  superinduced.  That  this  segmentation 
resulted  from  gemmation  is  implied  by  what  are  called  poly- 
trochal  larvae.  These  "  sometimes  appear  as  a  stage  succeed- 
ing other  larval  types.  Thus  those  of  Arenicola  marina  arise 
from  larva?  which  at  first  were  monotrochal,  later  became 
telotrochal,  and  finally,  by  the  appearance  of  new  ciliated 
rings  between  those  already  present,  assumed  the  stage  of 
polytrochal  larvas.  .  .  .  This  condition  warrants  the 
assumption  that  the  segmented  forms  are  to  be  looked  upon 
as  the  younger,  the  unsegmented,  on  the  other  hand,  as  the 
phylogenetically  older."  (Korschelt  and  Heider,  i,  278.)  And 
that  the  above-described  rings  of  cilia  mark  off  segments  is 
shown  by  the  case  of  Ophryotrocha  puerilis,  which  "  remains, 
as  it  were,  in  a  larval  condition,  since  the  segments  retain 
their  ciliation  throughout  life."  (/&.,  277.)  Yet  one  more 
significant  fact  must  be  named.  In  early  stages  of  develop- 
ment each  segment  of  an  archiannelidan  has  ccelomic  spaces 
separate  from  those  of  neighbouring  segments,  but  in  the 
adult  the  septa  "  generally  break  down  either  partially  or 
completely,  so  that  the  perivisceral  cavity  becomes  a  con- 
tinuous space  from  end  to  end  of  the  animal."  (Sedgwick, 
Text  Boole,  449.)  While  this  fact  is  congruous  with  the 
hypothesis  here  maintained,  it  is  incongruous  with  the  hypo- 
thesis that  the  annelid  was  originally  an  elongated  creature 
which  afterwards  became  segmented;  since  in  that  case  the 
implication  would  be  that  the  ccelomic  septa,  not  arising  from 
recapitulation  of  an  ancestral  structure,  but  originated  by  the 
process  of  segmentation,  were  first  superfluously  formed  and 
then  destroyed. 

Various  lines  of  evidence  thus  converge  to  the  conclusion 
that  an  annulose  animal  is  an  aggregate  of  the  third  order. 

In  June,  1865,  when  No.  14  of  my  serial  containing  the 


110  MORPHOLOGICAL  DEVELOPMENT. 

foregoing  chapter  was  issued,  I  supposed  myself  to  be  alone 
in  holding  this  belief  respecting  the  annulose  type,  and  long 
continued  to  suppose  so.  Over  thirty  years  later,  however,  in 
M.  Edmond  Terrier's  work,  La  Philosophic  Zoologique  avant 
Darwin,  I  found  mention  of  a  lecture  delivered  by  M.  Lacaze- 
Duthiers  at  the  Ecole  Normale  Superieure  in  Paris,  and  re- 
ported in  the  Revue  des  Cours  Scientifiques  for  January  28, 
1865,  in  which  he  enunciated  a  like  belief.  Judging,  how- 
ever, by  the  account  of  this  lecture  which  M.  Perrier  gives 
(he  was  present),  it  appears  that  M.  Lacaze-Duthiers  simply 
contended  that  this  view  of  the  annulose  structure  as  arising 
by  union  of  once-independent  units,  is  suggested  by  certain 
a  priori  considerations.  There  is  no  indication  that  he 
assigned  any  of  the  classes  of  facts  above  given,  which  go  to 
show  that  it  has  thus  arisen. 

For  further  facts  and  arguments  concerning  the  genesis 
of  the  annulose  type,  see  Appendix  D  2.] 


CHAPTEK  V. 

THE    MORPHOLOGICAL   COMPOSITION    OF   ANIMALS, 
CONTINUED. 

§  208.  INSECTS,  Arachnids,  Crustaceans,  and  Myriapods, 
are  all  members  of  that  higher  division  of  the  Annulosa  * 
called  Articulata  or  now  more  generally  Arthropoda.  Though 
in  these  creatures  the  formation  of  segments  may  be  inter- 
preted as  a  disguised  gemmation;  and  though,  in  some  of 
them,  the  number  of  segments  increases  by  this  modified  bud- 
ding after  leaving  the  egg,  as  it  does  among  the  Annelids; 
yet  the  process  is  not  nearly  so  dominant:  the  segments  are 
usually  much  less  numerous  than  we  find  them  in  the  types 
last  considered.  In  most  cases,  too,  the  segments  are  in  a 
greater  degree  differentiated  one  from  another,  at  the  same 
time  that  they  are  severally  more  differentiated  within  them- 
selves. Nor  is  there  any  instance  of  spontaneous  fission 
taking  place  in  the  series  of  segments  composing  an  articu- 
late animal.  On  the  contrary,  the  integration,  always  great 
enough  permanently  to  unite  the  segments,  is  frequently 
carried  so  far  as  to  hide  very  completely  the  individualities 
of  some  or  many  of  them;  and  occasionally,  as  among  the 
Acari,  the  consolidation,  or  the  arrest  of  segmentation,  is  so 

*  The  name  Annulosa,  once  used  to  embrace  the  Annelida  and  Arthro- 
poda, has  of  late  ceased  to  be  used.  It  seems  to  me  better  than  Appen- 
diculaia,  both  as  being  more  obviously  descriptive  and  as  being  more 
exclusive. 

Ill 


112 


MORPHOLOGICAL  DEVELOPMENT. 


decided  as  to  leave  scarcely  a  trace  of  the  articulate  struc- 
ture: the  type  being  in  these  cases  indicated  chiefly  by  the 
presence  of  those  characteristically-formed  limbs,  which  give 
the  alternative  name  Arthropoda  to  all  the  higher  Annulosa. 
Omitting  the  parasitic  orders,  which,  as  in  other  cases,  are 
aberrant  members  of  their  sub-kingdom,  comparisons  between 
the  different  orders  prove  that  the  higher  are  strongly  dis- 
tinguished from  the  lower,  by  the  much  greater  degree  in 
which  the  individuality  of  the  tertiary  aggregate  dominates 
over  the  individualities  of  those  secondary  aggregates  called 
segments  or  "  somites,"  of  which  it  is  composed.  The  suc- 
cessive Figs.  170 — 176,  representing  (without  their  limbs)  a 


Julus,  a  Scolopendra,  an  isopodous  Crustacean,  and  four 
kinds  of  decapodous  Crustaceans,  ending  with  a  Crab,  will 
convey  at  a  glance  an  idea  of  the  way  in  which  that  greater 
size  and  heterogeneity  reached  by  the  higher  types,  is  accom- 
panied by  an  integration  which,  in  the  extreme  cases,  nearly 
obliterates  all  traces  of  composite  structure.  In  the  Crab 
the  posterior  segments,  usually  folded  underneath  the  shell, 
alone  preserve  their  primitive  distinctness.  So  completely 
confluent  are  the  rest,  that  it  seems  absurd  to  say  that  a 
Crab's  carapace  is  composed  of  as  many  segments  as  there  are 
pairs  of  limbs,  foot-jaws,  and  antenna  attached  to  it;  and 
were  it  not  that  during  early  stages  of  the  Crab's  develop- 


THE  MORPHOLOGICAL  COMPOSITION   OP  ANIMALS.  H3 


ment  the  segmentation  is  faintly  marked,  the  assertion  might 
be  considered  illegitimate. 

That  all  articulate  animals  are  thus  composed  from  end  to 
end  of  homologous  segments,  is,  however,  an  accepted  doc- 
trine among  naturalists.  It  is  a  doctrine  that  rests  on  careful 
observation  of  three  classes  of  facts — the  correspondences 
of  parts  in  the  successive  "  somites  "  of  an  adult  articulate 
animal;  the  still  more  marked  correspondences  of  such  parts 
as  they  exist  in  the  embryonic  or  larval  articulate  animal; 
and  the  maintenance  of  such  correspondences  in  some  types, 
which  are  absent  in  types  otherwise  near  akin  to  them. 
The  nature  of  the  conclusion  which  these  evidences  unite  in 
supporting,  will  best  be  shown  by  the  annexed  copies  from 
the  lecture-diagrams  of  Prof.  Huxley;  exhibiting  the  typical 
structures  of  a  Myriapod,  an  Insect,  a  Spider,  and  a  Crust- 
acean, with  their  relations  to  a  common  plan,  as  interpreted 
by  him. 

Insecf 


Treating  of  these  homologies,  Prof.  Huxley  says  "  that  a 
striking  uniformity  of  composition  is  to  be  found  in  the  heads 
of,  at  any  rate,  the  more  highly  organized  members  of  these 
four  classes;  and  that,  typically,  the  head  of  a  Crustacean, 
an  Arachnid,  a  Myriapod,  or  an  Insect,  is  composed  of  six 


54 


114:  MORPHOLOGICAL  DEVELOPMENT. 

somites  (or  segments  corresponding  with  those  of  the  body) 
and  their  appendages,  the  latter  being  modified  so  as  to  serve 
the  purpose  of  sensory  and  manducatory  organs."  * 

Thus  even  in  the  higher  Arthropoda,  the  much  greater  con- 
solidation and  much  greater  heterogeneity  do  not  obliterate 
all  evidence  of  the  fact,  that  the  organism  is  an  aggregate  of 
the  third  order.  Comparisons  show  that  it  is  divisible  into  a 
number  of  proximate  units,  each  of  which  is  akin  in  certain 
fundamental  traits  to  its  neighbours,  and  each  of  which  is  an 
aggregate  of  the  second  order,  in  so  far  as  it  is  an  organized 
combination  of  those  aggregates  of  the  first  order  which  we 
call  morphological  units  or  cells.  And  that  these  segments 
or  somites,  which  make  up  an  annulose  animal,  were  origin- 
ally aggregates  of  the  second  order  having  independent  in- 
dividualities, is  an  hypothesis  which  gathers  further  support 
from  the  contrast  between  the  higher  and  the  lower  Arthro- 
pods, as  well  as  from  the  contrast  between  the  Arthropods 

*  The  fusion  of  the  segments  forming  the  Arthropod  head  and  the 
extreme  changes,  or  perhaps  in  some  cases  disappearances,  of  their  appen- 
dages, put  great  difficulties  in  the  way  of  identification ;  so  that  there  are 
differences  of  opinion  respecting  the  number  of  included  segments.  Prof. 
MacBride  writes: — "It  is  highly  probable  that  a  primary  head  (prseoral 
lobe  or  praestomium)  has  been  derived  from  annelid  ancestors,  but  the 
secondary  fusion  of  body-segments  with  this  head,  in  other  words  the  forma- 
tion of  a  secondary  head,  has  gone  on  independently  in  the  different  classes 
of  the  phylum  Arthropoda,  viz.,  Arachnida,  Crustacea,  and  Tracheata 
(including  Insects  and  Myriapods).  Judged  by  the  number  of  appendages 
(which  gives  an  inferior  limit)  the  head  of  a  malacostracous  Crustacean 
consists  of  praestomium  and  8  segments ;  the  head  of  an  insect  of  prsesto- 
miurn  and  4  segments;  the  head  of  a  Myriapod  of  prsestomium  and  8 
segments ;  and  the  head  of  an  Arachnid  of  praestomium  and  3  segments." 
Again,  the  comment  of  Mr.  J.  T.  Cunningham  is  : — "  According  to  Claus 
and  most  modern  authorities  there  are  only  5  segments  in  the  head  of  an 
Arthropod,  the  eyes  not  counting  as  appendages ;  and  further  it  should  be 
noted  that  the  second  pair  of  antennas  are  wanting  in  Insects." 

Of  course  difference  of  opinion  respecting  the  number  of  somites  in  the 
head  involves  difference  of  opinion  respecting  the  number  constituting  the 
entire  body,  which,  in  the  higher  Arthropods,  is  said  by  some  to  be  19  and 
by  others  20.  But  those  who  thus  differ  in  detail,  agree  in  regarding  all 
the  segments  of  head  and  body  as  homologous,  and  this  is  the  essential  point 
with  which  we  are  here  concerned. 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  H5 

in  general  and  the  Annelids.  For  if  that  masking  of  the 
individualities  of  the  segments  which  we  find  distinguishes 
the  higher  forms  from  the  lower,  has  been  going  on  from  the 
beginning,  as  we  may  fairly  assume;  it  is  to  be  inferred  that 
the  individualities  of  the  segments  in  the  lower  forms,  were 
originally  more  marked  than  they  now  are.  Eeversing  those 
processes  of  change  by  which  the  most  developed  Annulosa 
have  arisen  from  the  least  developed;  and  applying  in 
thought  this  reversed  process  to  the  least  developed,  as  they 
were  described  in  the  last  Chapter;  we  are  brought  to  the 
conception  of  attached  segments  that  are  all  completely  alike, 
and  have  their  individualities  in  no  appreciable  degree  sub- 
ordinated to  that  of  the  chain  they  compose.  From  which 
there '  is  but  one  step  to  the  conception  of  gemmiparously- 
produced  individuals  which  severally  part  one  from  another 
as  soon  as  they  are  formed. 

§  209.  We  must  now  return  to  a  junction  whence  we 
diverged  some  time  ago.  As  before  explained  under  the 
head  of  Classification,  organisms  do  not  admit  of  uni-serial 
arrangement,  either  in  general  or  in  detail;  but  everywhere 
form  groups  within  groups.  Hence,  having  traced  the  phases 
of  morphological  composition  up  to  the  highest  forms  in  any 
sub-kingdom,  we  find  ourselves  at  the  extremity  of  a  great 
branch,  from  which  there  is  no  access  to  another  great  branch, 
except  by  going  back  to  some  place  of  bifurcation  low  down 
in  the  tree. 

There  exist  such  similarities  of  shape"  and  structure  be- 
tween the  larval  forms  of  low  Molluscs  and  those  of  Annelids 
and  Rotifers,  as  to  show  that  there  was  an  early  type  common 
to  them  all;  and  its  probable  characters,  suggested  by  com- 
parison, seem  to  imply  that  it  had  arisen  from  some  coelen- 
terate  type,  intermediate  between  the  Cnidaria  and  the  Cteno- 
phora.  But  there  is  this  noteworthy  difference  between  the 
molluscan  larva  and  the  allied  larvae,  that  it  gives  origin  to 
only  one  animal  and  not  to  a  group  of  animals,  united  or 


116 


MORPHOLOGICAL  DEVELOPMENT. 


disunited.  No  true  Mollusc  multiplies  by  gemmation,  either 
continuous  or  discontinuous;  but  the  product  of  every  ferti- 
lized germ  is  a  single  individual. 

It  is  a  significant  fact  that  here,  where  for  the  first  time 
we  have  homogenesis  holding  throughout  an  entire  sub-king- 
dom, we  have  also  throughout  an  entire  sub-kingdom  no  case 
in  which  the  organism  is  divisible  into  two,  three,  or  more, 
like  parts.  There  is  neither  any  such  clustering  or  branch- 
ing as  a  ccelenterate  or  molluscoid  animal  usually  displays; 
nor  is  there  any  trace  of  that  segmentation  which  charac- 
terizes the  Annulosa.  Among  these  animals  in  which  no 
single  egg  produces  several  individuals,  no  individual  is 
separable  into  several  homologous  divisions.  This  connexion 
will  be  seen  to  have  a  probable  meaning,  on  remembering 
that  it  is  the  converse  of  the  connexion  which  obtains  among 
the  Annulosa,  considered  as  a  group. 

A  Mollusc,  then,  is  an  aggregate  of  the  second  order.  Not 
only  in  the  adult  animal  is  there  no  sign  of  a  multiplicity  of 
like  parts  that  have  become  obscured  by  integration ;  but 
there  is  no  sign  of  such  multiplicity  in  the  embryo.  And  this 
unity  is  just  as  conspicuous  in  the  lowest  Lamellibranch  as  in 
the  highest  Cephalopod. 

It  may  be  well  to  note,  however,  more  especially  because 
it  illustrates  a  danger  of  misinterpretation  presently  to  be 
guarded  against,  that  there  are  certain  Molluscs  which  simu- 
late the  segmented  structure.  Externally  a  Chiton,  Fig.  188, 

appears  to  be  made  up  of 
divisions  substantially  like 
those  of  the  creature  Fig. 
189;  and  one  who  judged 
only  by  externals,  would  say 
that  the  creature  Fig.  190 
differs  as  much  from  the 
creature  Fig.  189,  as  this 
does  from  the  preceding  one.  But  the  truth  is,  that  while 
190  and  189  are  closely-allied  types,  189  differs  from  188 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  117 

much  more  widely  than  a  man  does  from  a  fish.  And  the 
radical  distinction  between  them  is  this : — Whereas  in  the 
Crustacean  the  segmentation  is  carried  transversely  through 
the  whole  mass  of  the  body,  so  as  to  render  the  body  more 
or  less  clearly  divisible  into  a  series  of  parts  which  are  simi- 
larly composed;  in  the  Mollusc  the  segmentation  is  limited 
to  the  shell  carried  on  its  upper  surface,  and  leaves  its 
body  as  completely  undivided  as  is  that  of  a  common  slug.* 
Were  the  body  cut  through  at  each  of  the  divisions,  the 
section  of  it  attached  to  each  portion  of  the  shell  would  be 
unlike  all  the  other  sections.  Here  the  segmentation  has  a 
purely  functional  derivation — is  adaptive  instead  of  genetic. 
The  similarly-formed  and  similarly-placed  parts,  are  not 
homologous  in  the  same  sense  as  are  the  appendages  of  a 
phasnogamic  axis  or  the  limbs  of  an  insect. 

§  210.  In  studying  the  remaining  and  highest  sub-king- 
dom of  animals,  it  is  important  to  recognize  this  radical  dif- 
ference in  meaning  between  that  likeness  of  parts  which  is 
produced  by  likeness  of  modifying  forces,  and  that  likeness 
of  parts  which  is  due  to  primordial  identity  of  origin.  On 
our  recognition  of  this  difference  depends  the  view  we  take 
of  certain  doctrines  that  have  long  been  dominant,  and  have 
still  a  wide  currency. 

Among  the  Vertebrata,  as  among  the  Mollusca,  homogene- 
sis  is  universal.  The  two  sub-kingdoms  are  like  one  another 
and  unlike  the  remaining  sub-kingdoms  in  this,  that  in  all 
the  types  they  severally  include,  a  single  fertilized  ovum  pro- 
duces only  a  single  individual.  It  is  true  that  as  the  eggs  of 
certain  gasteropods  occasionally  exhibit  spontaneous  fission 

*  Prof.  MacBride  corrects  this  statement  by  saying  that  "  The  ctenidia  or 
gills  (which  in  Mollusca  generally  are  represented  only  by  a  single  pair)  are 
here  represented  by  a  large  number  of  pairs ;  they  do  not,  however,  correspond 
in  either  number  or  position  to  the  shell  plates."  It  may,  I  think,  be  con- 
tended that  if  these  had  any  morphological  significance,  they  would  not  differ 
in  arrangement  from  the  shell  plates,  and  would  not  be  limited  to  this  special 
type  of  Mollusc. 


118        MORPHOLOGICAL  DEVELOPMENT. 

of  the  vitelline  mass,  which  may  or  may  not  result  in  the 
formation  of  two  individuals;  so  among  vertebrate  animals 
we  now  and  then  meet  with  double  monsters,  which  appear 
to  imply  such  a  spontaneous  fission  imperfectly  carried  out. 
But  these  anomalies  serve  to  render  conspicuous  the  fact, 
that  in  both  these  sub-kingdoms  the  normal  process  is  the 
integration  of  the  whole  germ-mass  into  a  single  organism, 
which  at  no  phase  of  its  development  displays  any  tendency 
to  separate  into  two  or  more  parts. 

Equally  as  throughout  the  Mollusca,  there  holds  through- 
out the  Vertebrata  the  correlative  fact,  that  not  even  in  its  low- 
est any  more  than  in  its  highest  types,  is  the  body  divisible 
into  homologous  segments.  The  vertebrate  animal,  under  its 
simplest  as  under  its  most  complex  form,  is  like  the  mollusc- 
ous animal  in  this,  that  you  cannot  cut  it  into  transverse 
slices,  each  of  which  contains  a  digestive  organ,  a  respiratory 
organ,  a  reproductive  organ,  &c.  The  organs  of  the  least- 
developed  fish  as  well  as  those  of  the  most-developed 
mammal,  form  but  a  single  physiological  whole;  and  they 
show  not  the  remotest  trace  of  having  ever  been  divisible 
into  two  or  more  physiological  wholes.  That  segmentation 
which  the  vertebrate  animal  usually  exhibits  throughout 
part  of  its  organization,  is  the  same  in  origin  and  meaning 
as  the  segmentation  of  a  Chiton's  shell;  and  no  more  implies 
in  the  vertebrate  animal  a  composite  structure,  than  do  the 
successive  pairs  of  branchiaa  of  the  Doto,  or  the  transverse 
rows  of  branchiffi  in  the  Eolis,  imply  composite  structure  in 
the  molluscous  animal.  To  some  this  will  seem  a  very  ques- 
tionable proposition;  and  had  we  no  evidence  beyond  that 
which  adult  vertebrate  animals  of  developed  types  supply,  it 
would  be  a  proposition  not  easy  to  substantiate.  But  abundant 
support  for  it  is  to  be  found  in  the  structure  of  the  vertebrate 
embryo,  and  in  the  comparative  morphology  of  the  Vertebrata 
in  general. 

Embryologists  teach  us  that  the  primordial  relations  of 
parts  are  most  clearly  displayed  in  the  early  stages  of  evo- 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  H9 

lution;  and  that  they  generally  become  partially  or  com- 
pletely disguised  in  its  later  stages.  Hence,  were  the  verte- 
brate animal  on  the  same  level  as  the  annulose  animal  in 
degree  of  composition — did  it  similarly  consist  of  segments 
which  are  homologous  in  the  sense  that  they  are  the  prox- 
imate units  of  composition;  we  ought  to  find  this  funda- 
mental fact  most  strongly  marked  at  the  outset.  As  in 
the  annelid-embryo  the  first  conspicuous  change  is  the 
elongation  and  division  into  segments,  by  constrictions  that 
encircle  the  whole  body;  and  as  in  the  arthropod  embryo 
the  blastoderm  becomes  marked  out  transversely  into  pieces 
which  extend  themselves  round  the  yelk  before  the  internal 
organization  has  made  any  appreciable  progress;  so  in  the 
embryo  of  every  vertebrate  animal,  had  it  an  analogous  com- 
position,  the  first  decided  change  should  be  a  segmentation 
implicating  the  entire  mass.  But  it  is  not  so.  Sundry  im- 
portant differentiations  occur  before  any  divisions  begin  to 
show  themselves.  There  is  the  defining  of  that  elongated, 
elevated  area  with  its  longitudinal  groove,  which  becomes  the 
seat  of  subsequent  changes;  there  is  the  formation  of  the 
notochord  lying  beneath  this  groove;  there  is  the  growth 
upwards  of  the  boundaries  of  the  groove  into  the  dorsal 
lamina?,  which  rapidly  develop  and  fold  over  in  the  region  of 
the  head.  Eathke,  as  quoted  and  indorsed  by  Prof.  Huxley, 
describes  the  subsequent  changes  as  follows : — "  The  gelatin- 
ous investing  mass,  which,  at  first,  seems  only  to  constitute 
a  band  to  the  right  and  to  the  left  of  the  notochord  forms 
around  it,  in  the  further  course  of  development,  a  sheath, 
which  ends  in  a  point  posteriorly.  Anteriorly,  it  sends  out 
two  processes  which  underlie  the  lateral  parts  of  the  skull, 
but  very  soon  coalesce  for  a  longer  or  shorter  distance. 
Posteriorly,  the  sheath  projects  but  little  beyond  the  noto- 
chord ;  but,  anteriorly,  for  a  considerable  distance,  as  far  as  the 
infundibulum.  It  sends  upwards  two  plates,  which  embrace 
the  future  central  parts  of  the  nervous  system  laterally,  prob- 
ably throughout  their  entire  length."  That  is  to  say,  in  the 


120       MORPHOLOGICAL  DEVELOPMENT. 

Vertebrata  the  first  step  is  the  marking  out  on  the  blastoderm 
of  an  integrated  structure,  within  which  segments  subse- 
quently appear.  When  these  do  appear,  they  are  for  some 
time  limited  to  the  middle  region  of  the  spinal  axis;  and  no 
more  then  than  ever  after,  do  they  implicate  the  general 
mass  of  the  body  in  their  transverse  divisions.  On  the 
contrary,  before  vertebral  segmentation  has  made  much  pro- 
gress, the  rudiments  of  the  vascular  system  are  laid  down  in 
a  manner  showing  no  trace  of  any  primordial  correspondence 
of  its  parts  with  the  divisions  of  the  axis.  Equally 

at  variance  with  the  belief  that  the  vertebrate  animal  is 
essentially  a  series  of  homologous  parts,  is  the  heterogeneity 
which  exists  among  these  parts  on  their  first  appearance. 
Though  in  the  head  of  an  adult  articulate  animal  there  is 
little  sign  of  divisibility  into  segments  like  those  of  the 
body;  yet  such  segments,  with  their  appropriate  ganglia  and 
appendages,  are  easily  identifiable  in  the  articulate  embryo. 
But  in  the  Vertebrata  this  antithesis  is  reversed.  At  the 
time  when  segmentation  has  become  decided  in  the  dorsal 
region  of  the  spine,  there  is  no  trace  of  segments  in  the  parts 
which  are  to  form  the  skull — nothing  whatever  to  suggest  that 
the  skull  is  being  formed  out  of  divisions  homologous  with 
vertebrae.*  And  minute  observation  no  more  discloses  any 
such  homology  than  does  general  appearance.  "  Eemak," 
says  Prof.  Huxley,  "  has  more  fully  proved  than  any  other 
observer,  the  segmentation  into  '  urwirbel,'  or  proto-vertebrae, 
which  is  characteristic  of  the  vertebral  column,  stops  at 
the  occipital  margin  of  the  skull — the  base  of  which,  before 
ossification,  presents  no  trace  of  that  segmentation  which 
occurs  throughout  the  vertebral  column." 

Consider  next  the  evidence  supplied  by  comparative  mor- 
phology.    In  preceding  sections  (§§  206,  208,)  it  has  been 

*  Though  it  is  allcjred  that  at  a  later  stage  the  posterior  part  of  the  skull 
is  formed  by  fusion  of  divisions  which  are  assumed  to  represent  vertebrae,  yet 
it  is  admitted  that  the  anterior  part  of  the  skull  never  shows  any  signs  of 
such  division.  Moreover  in  both  parts  the  bones  show  no  trace  of  primitive 
segmentation. 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  121 

shown  that  among  annulose  animals,  the  divisibility  into 
homologous  parts  is  most  clearly  demonstrable  in  the  lowest 
types.  Though  in  decapodous  Crustaceans,  in  Insects,  in 
Arachnids,  there  is  difficulty  in  identifying  some  or  many  of 
the  component  somites;  and  though,  when  identified,  they 
display  only  partial  correspondences;  yet  on  descending  to 
Annelids,  the  composition  of  the  entire  body  out  of  such 
somites  becomes  conspicuous,  and  the  homology  between  each 
somite  and  its  neighbours  is  shown  by  the  repetition  of  one 
another's  structural  details,  as  well  as  by  their  common 
gemmiparous  origin:  indeed,  in  some  cases  we  have  the 
homology  directly  demonstrated  by  seeing  a  somite  of  the 
body  transformed  into  a  head.  If,  then,  a  vertebrate  animal 
had  a  segmental  composition  of  kindred  nature,  we  ought  to 
find  it  most  clearly  marked  in  the  lowest  Vertebrata  and 
most  disguised  in  the  highest  Vertebrata.  But  here,  as  before, 
the  fact  is  just  the  reverse.  Among  the  Vertebrata  of 
developed  type,  such  segmentation  as  really  exists  remains 
conspicuous — is  but  little  obscured  even  in  parts  of  the  spinal 
column  formed  out  of  integrated  vertebras.  Whereas  in  the 
undeveloped  vertebrate  type,  segmentation  is  scarcely  at  all 
traceable.*  The  Amphioxus,  Fig.  191,  is  not  only  without 


tff* 

ossified  vertebrae;  not  only  is  it  without  cartilaginous  repre- 
sentatives of  them;  but  it  is  even  without  anything  like 
distinct  membranous  divisions.  The  spinal  column  exists 
as  a  continuous  notochord:  the  only  signs  of  incipient  seg- 
mentation being  given  by  its  membranous  sheath,  in  the 
upper  part  of  which  "  quadrate  masses  of  somewhat  denser 
*  See  note  at  the  end  of  the  chapter. 


122        MORPHOLOGICAL  DEVELOPMENT. 

tissue  seem  faintly  to  represent  neural  spines."  Moreover, 
throughout  sundry  groups  of  fishes  and  amphibians,  the 
segmentation  remains  very  imperfect :  only  certain  peri- 
pheral appendages  of  the  vertebrae  becoming  defined  and 
solidified,  while  in  place  of  the  bodies  of  the  vertebrae  there 
still  continues  the  undivided  notochord.  Thus,  instead  of 
being  morphologically  composed  of  vertebral  segments,  the 
vertebrate  animal  in  its  primitive  form  is  entirely  without 
vertebral  segments;  and  vertebral  segments  begin  to  appear 
only  as  we  advance  towards  developed  forms.  Once 

more,  evidence  equally  adverse  to  the  current  hypothesis 
meets  us  on  observing  that  the  differences  between  the  parts 
supposed  to  be  homologous,  are  as  great  at  first  as  at  last. 
Did  the  vertebrate  animal  primordially  consist  of  homo- 
logous segments  from  snout  to  tail;  then  the  segments  said 
to  compose  the  skull  ought,  in  the  lowest  Vertebraia,  to  show 
themselves  much  more  like  the  remaining  segments  than 
they  do  in  the  highest  V ' ertebrata.  But  they  do  not.  Fishes 
have  crania  made  up  of  bones  that  are  no  more  clearly 
arrangeable  into  segments  like  vertebrae,  than  are  the  cranial 
bones  of  the  highest  mammal.  Nay,  indeed,  the  case  is 
much  stronger.  The  simplest  fish  possessing  a  skeleton, 
has  a  cranium  composed  of  cartilage  that  is  not  segmented 
at  all! 

Besides  being  inconsistent  with  the  leading  truths  of 
Embryology  and  Comparative  Morphology,  the  hypothesis  of 
Goethe  and  Oken  is  inconsistent  with  itself.  The  facts 
brought  forward  to  show  that  there  exists  an  archetypal 
vertebra,  and  that  the  vertebrate  animal  is  composed  of 
archetypal  vertebrae  arranged  in  a  series,  and  severally  modi- 
fied to  fit  their  positions — these  facts,  I  say,  so  far  from 
proving  as  much,  suffice,  when  impartially  considered,  to  dis- 
prove it.  No  assigned,  nor  any  conceivable,  attribute  of  the 
supposed  archetypal  vertebra  is  uniformly  maintained.  The 
parts  composing  it  are  constant  neither  in  their  number,  nor 
in  their  relative  positions,  nor  in  their  modes  of  ossification, 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  123 

nor  in  the  separateness  of  their  several  individualities  when 
present.  There  is  no  fixity  of  any  one  element,  or  con- 
nexion, or  mode  of  development,  which  justifies  even,  a 
suspicion  that  vertebrae  are  modelled  after  an  ideal  pattern. 
To  substantiate  these  assertions  here  would  require  too  much 
space,  and  an  amount  of  technical  detail  wearisome  to  the 
general  reader.  The  warrant  for  them  will  be  found  in  a 
criticism  on  the  osteological  works  of  Prof.  Owen,  originally 
published  in  the  British  and  Foreign  Medico-Chirurgical  Re- 
view for  Oct.  1858.  This  criticism  I  add  in  the  Appendices, 
for  the  convenience  of  those  who  may  wish  to  study  the 
question  more  fully.  ( See  Appendix  B. ) 

Everything,  then,  goes  to  show  that  the  segmental  compo- 
sition which  characterises  the  apparatus  of  external  relation 
in  most  Vertebrata,  is  not  primordial  or  genetic,  but  function- 
ally determined  or  adaptive.  Our  inference  must  be  that  the 
vertebrate  animal  is  an  aggregate  of  the  second  order,  in 
which  a  relatively  superficial  segmentation  has  been  pro- 
duced by  mechanical  intercourse  with  the  environment.  We 
shall  hereafter  see  that  this  conception  leads  us  to  a  con- 
sistent interpretation  of  the  facts — shows  us  why  there  has 
arisen  such  unity  in  variety  as  exists  in  every  vertebral  column, 
and  why  this  unity  in  variety  is  displayed  under  countless 
modifications  in  different  skeletons.* 

§  211.  On  glancing  back  at  the  facts  brought  together  in 
these  two  chapters,  we  see  it  to  be  probable  that  there  has  gone 
on  among  animals  a  process  like  that  which  we  saw  reason 
to  think  has  gone  on  among  plants.  Minute  aggregates  of 
those  physiological  units  which  compose  living  protoplasm, 

*  A  qualifying  fact  should  be  named.  When  the  production  of  vertebral 
segments  has  become  constitutionally  established,  so  that  there  is  an  innate 
tendency  to  form  them,  there  arises  a  liability  to  form  supernumerary  ones  ; 
and  this,  from  time  to  time  recurring,  may  lengthen  the  series,  as  in  the 
body  of  a  snake  or  the  neck  of  a  swan.  This  qualification,  however,  affects 
equally  the  hypothesis  of  an  ideal  type  and  the  hypothesis  of  mechanical 
genesis. 


124        MORPHOLOGICAL  DEVELOPMENT. 

exist  as  Protozoa:  some  of  them  incoherent,  indefinite,  and 
almost  homogeneous,  and  others  of  them  more  coherent,  de- 
finite, and  heterogeneous.  By  union  of  these  nucleated  parti- 
cles of  sarcode,  are  produced  various  indefinite  aggregates  of 
the  second  order — Sponges,  Polycytharia,  Foraminifers,  &c. ; 
in  which  the  compound  individuality  is  scarcely  enough 
marked  to  subordinate  the  primitive  individualities.  But  in 
other  types,  as  in  Hydra,  the  lives  of  the  morphological 
units  are  in  a  considerable  degree,  though  not  wholly,  merged 
in  the  life  of  the  integrated  body  they  form.  As  the  primary 
aggregate,  when  it  passes  a  certain  size,  undergoes  fission  or 
gemmation;  so  does  the  secondary  aggregate.  And  as  on 
the  lower  stage  so  on  the  higher,  we  see  cases  in  which  the 
gemmiparously-produced  individuals  part  as  soon  as  formed, 
and  other  cases  in  which  they  continue  united,  though  in 
great  measure  independent.  This  massing  of  secondary  aggre- 
gates into  tertiary  aggregates,  is  variously  carried  on  among 
the  Hydrozoa,  the  Actinozoa,  the  Polyzoa,  and  the  Tunicata. 
In  most  of  the  types  so  produced,  the  component  individu- 
alities are  very  little  subordinated  to  the  individuality  of  the 
composite  mass — there  is  only  physical  unity  and  not  physio- 
logical unity;  but  in  certain  of  the  oceanic  Hydrozoa,  the 
individuals  are  so  far  differentiated  and  combined  as  very 
much  to  mask  them.  Forms  showing  us  clearly  the  transi- 
tion to  well-developed  individuals  of  the  third  order,  are  not 
to  be  found.  Nevertheless,  in  the  great  sub-kingdom  Annu- 
losa,  there  are  traits  of  structure,  development,  and  mode  of 
multiplication,  which  go  far  to  show  that  its  members  are 
such  individuals  of  the  third  order;  and  in  the  relations  to 
external  conditions  involved  by  the  mode  of  union,  we  find 
an  adequate  cause  for  that  obscuration  of  the  secondary  indi- 
vidualities which  we  must  suppose  has  taken  place.  The 
two  other  great  sub-divisions,  Mollusca  and  Vertebrata, 
between  the  lower  members  of  which  there  are  suggestive 
points  of  community,  present  us  only  with  aggregates  of  the 
second  order,  that  have  in  many  cases  become  very  large  and 


THE  MORPHOLOGICAL  COMPOSITION  OF  ANIMALS.  125 

very  complex.  We  find  in  them  no  trace  of  the  union  of 
gemmiparously-produced  individuals.  Neither  the  molluscous 
nor  the  vertebrate  animal  shows  the  faintest  trace  of  a  seg- 
mentation affecting  the  totality  of  its  structure;  and  we  see 
good  grounds  for  concluding  that  such  segmentation  as  ex- 
ceptionally occurs  in  the  one  and  usually  occurs  in  the  other, 
is  superinduced. 


[NOTE  : — A  critic  calls  in  question  the  statement  on  p.  121 
respecting  the  Amphioxus.  At  the  outset,  however,  he 
admits  that  in  the  Amphioxus  "  the  central  nervous  system 
and  the  notochord  are  not  segmented."  In  the  Annelid, 
however,  the  central  nervous  system  is  segmented,  and  there 
is  segmentation  of  the  part  which,  as  a  supporting  structure, 
is  analogous  to  the  notochord  in  respect  of  function — the 
outer  part  which  represents  the  exo-skeleton  in  contrast  to 
the  endo-skeleton.  He  goes  on  to  say  that  "  the  gut  is  not 
involved  [in  the  segmentation]  and  exhibits  in  Amphioxus 
just  as  it  does  in  worms  differentiations  entirely  independent 
of  the  segmentation  of  the  mesoblast."  Part  of  this  state- 
ment is,  I  think,  not  congruous  with  all  the  facts.  In  Proto- 
drilus,  one  of  the  lowest  of  the  Archiannelida,  "  the  intestine 
is  moniliform,  there  being  a  constriction  between  each  seg- 
ment"  and  the  next.  (Shipley.)  Complete  segmentation  of 
the  intestine  is  obviously  impossible,  since,  were  the  canal 
divided  into  portions  by  septa,  no  food  could  pass.  But  the 
fact  that  the  gut  has  these  successive  expansions  and  con- 
strictions, corresponding  to  the  successive  segments,  and 
giving  to  each  segment  a  partially-separate  stomach,  shows 
that  segmentation  has  gone  as  far  as  consists  with  the 
carrying  on  of  the  lives  of  the  segments.  No  such  partial 
segmentation  exists  in  the  Amphioxus.  Thus,  then,  three 
fundamental  structures — the  directive  structure,  the  sup- 
porting structure,  and  the  alimentary  structure — are  respec- 
tively simple  in  the  lowest  vertebrate  and  segmented,  or 


126        MORPHOLOGICAL  DEVELOPMENT. 

partially  segmented,  in  the  lowest  Annelid.  Again,  while  it 
is  said  that  the  gill-clefts  exhibit  segmentation,  it  is  admitted 
that  this  has  no  relevance  to  any  constitutional  segmenta- 
tion :  "  they  are  segmented  on  a  plan  of  their  own "  irre- 
spective of  other  organs.  Another  allegation  is  that  the 
ovaries  of  Amphioxus  are  segmented.  Their  segmentation, 
however,  like  that  of  the  gills,  is  isolated,  and  may  be  con- 
sidered as  illustrating  those  repetitions  of  like  parts  seen  in 
supernumerary  vertebrae  in  various  creatures — a  repetition 
which  becomes  habitual  if  the  resulting  structure  is  advan- 
tageous to  the  species.  On  the  statement  that  while  the 
Amphioxus  has  no  rudiments  of  a  renal  system  the  Elasmo- 
branch  embryo  has  such  rudiments,  which  are  as  distinctly 
segmented  as  the  nephridia  of  a  worm,  two  comments  may 
be  made.  The  first  is  that  if  in  these  Vertebrates  the 
nephridia  bear  a  relation  to  the  general  structure  like  that 
which  they  do  in  Annelids,  then  one  would  expect  to  find 
the  segmental  arrangement  shown  in  the  lowest  type,  as  in 
Annelids,  rather  than  in  a  type  considerably  advanced  in 
development.  Should  it  be  replied  that  in  the  Amphioxus 
an  excretory  system  had  not  yet  arisen,  though  one  is  re- 
quired for  the  higher  organization  of  an  Elasmobranch,  then 
the  answer  may  be  that  since  the  segmental  arrangement  in 
the  Elasmobranch  corresponds  with  that  of  the  myotomes,  it 
has  no  reference  to  any  primordial  segmentation,  since  the 
myotomes  have  been  functionally  generated.  The  second 
comment  is  that  whereas  the  nephridia  of  the  Annelid  have 
independent  external  openings,  the  nephridia  in  the  Elasmo- 
branch have  not.  These  discharge  their  secretions  into  cer- 
tain general  tubes  of  exit  common  to  them  all;  showing  that 
each  of  them,  instead  of  being  a  member  of  a  partially  inde- 
pendent structure,  is  united  with  others  in  subordination  to 
a  general  structure.  That  is  to  say,  the  segmentations  are 
far  from  being  parallel  in  their  essential  natures.  The  asser- 
tion accompanying  these  criticisms,  that  there  is  "  no  differ- 
ence in  principle  between  the  segmentation  of  Amphioxus 


THE  MORPHOLOGICAL  COMPOSITION   OF  ANIMALS.  127 

and  Annelid  "  is  difficult  to  reconcile  with  the  visible  con- 
trast between  the  two.  Whatever  local  segmentations  there 
are  in  an  Amphioxus  appear  to  me  quite  unlike  "  in  prin- 
ciple "  to  those  which  an  Annelid  exhibits.  Could  its 
portion  of  gut  be  duly  supplied  with  nutriment,  the  segment 
of  a  low  Annelid  could  carry  on  its  vital  functions  independ- 
ently. In  the  parts  of  the  Amphioxus  we  see  nothing 
approaching  to  this.  Cut  it  into  transverse  sections  and  no 
one  of  them  contains  anything  like  the  assemblage  of  struc- 
tures required  for  living.  The  Amphioxus  is  a  physiological 
whole,  and  in  that  respect  differs  radically  from  the  Annelid, 
each  segment  of  which  is  in  chief  measure  a  physiological 
whole.  No  occurrence  of  local  segmentation  in  the  Am- 
phioxus can  obliterate  this  fundamental  contrast. 

An  accompanying  contrast  tells  the  same  story.  On  as- 
cending from  the  lowest  to  the  highest  annulose  types  we 
see  a  progressing  integration,  morphological  and  physiologi- 
cal; so  that  whereas  in  a  low  annelid  the  successive  parts 
are  in  large  measure  independent  in  their  structures  and  in 
their  lives,  in  a  high  arthropod,  as  a  crab,  most  of  the  parts 
have  lost  their  individualities  and  have  become  merged  in  a 
consolidated  organism  with  a  single  life.  Quite  otherwise  is 
it  in  the  vertebrate  series.  Its  lowest  member  is  at  the  very 
outset  a  complete  morphological  and  physiological  whole,  and 
the  formation  of  those  serial  parts  which  some  think  analo- 
gous to  the  serial  parts  of  an  Annelid,  begins  at  a  later  stage 
and  becomes  gradually  pronounced.  That  is  to  say,  the  course 
of  transformation  is  reversed.] 


CHAPTER  VI. 

MORPHOLOGICAL  DIFFERENTIATION    IN    PLANTS. 

§  212.  WHILE,  in  the  course  of  their  evolution,  plants  and 
animals  have  displayed  progressive  integrations,  there  have 
at  the  same  time  gone  on  progressive  differentiations  of  the 
resulting  aggregates,  both  as  wholes  and  in  their  parts. 
These  differentiations  and  the  interpretations  of  them,  form 
the  second  class  of  morphological  problems. 

We  commence  as  before  with  plants.  We  have  to  con- 
sider, first,  the  several  kinds  of  modification  in  shape  they 
have  undergone;  and,  second,  the  relations  between  these 
kinds  of  modification  and  their  factors.  Let  us  glance  at  the 
leading  questions  that  have  to  be  answered. 

§  213.  Irrespective  of  their  degrees  of  composition,  plants 
may,  and  do,  become  changed  in  their  general  forms.  Are 
their  changes  capable  of  being  formulated?  The  inquiry 
which  meets  us  at  the  outset  is — does  a  plant's  shape  admit 
of  being  expressed  in  any  universal  terms? — terms  that 
remain  the  same  for  all  genera,  orders,  and  classes. 

After  plants  considered  as  wholes,  have  to  be  considered 
their  proximate  components,  which  vary  with  their  degrees 
of  composition,  and  in  the  highest  plants  are  what  we  call 
branches.  Is  there  any  law  traceable  among  the  contrasted 
shapes  of  different  branches  in  the  same  plant  ?  Do  the  rela- 
tive developments  of  parts  in  the  same  branch  conform  to 
any  law?  And  are  these  laws,  if  they  exist,  allied  with  one 
128 


MORPHOLOGICAL  DIFFERENTIATION  IN  PLANTS.  129 

another  and  with  that  to  which  the  shape  of  the  whole  plant 
conforms  ? 

Descending  to  the  components  of  these  components,  which 
in  developed  plants  we  distinguish  as  leaves,  there  meet  us 
kindred  questions  respecting  their  relative  sizes,  their  rela- 
tive shapes,  and  their  shapes  as  compared  with  those  of 
foliar  organs  in  general.  Of  their  morphological  differentia- 
tions, also,  it  has  to  be  asked  whether  they  exemplify  any 
truth  that  is  exemplified  by  the  entire  plant  and  by  its  larger 
parts. 

Then,  a  step  lower,  we  come  down  to  those  morphological 
units  of  which  leaves  and  fronds  consist;  and  concerning 
these  arise  parallel  inquiries  touching  their  divergences  from 
one  another  and  from  cells  in  general. 

The  problems  thus  put  together  in  several  groups  cannot 
of  course  be  rigorously  separated.  Evolution  presupposes 
transitions  which  make  all  such  classings  more  or  less  con- 
ventional; and  adherence  to  them  must  be  subordinate  to 
the  needs  of  the  occasion. 

§  214.  In  studying  the  causes  of  the  morphological  differ- 
entiations thus  divided  out  and  prospectively  generalized, 
we  shall  have  to  bear  in  mind  several  orders  of  forces  which 
it  will  be  well  briefly  to  specify. 

Growth  tends  inevitably  to  initiate  changes  in  the  shape 
of  any  aggregate,  by  altering  both  the  amounts  of  the  inci- 
dent forces  and  the  forces  which  the  parts  exert  on  one 
another.  With  the  mechanical  actions  this  is  obvious. 
Matter  that  is  sensibly  plastic  cannot  be  increased  in  mass 
without  undergoing  a  change  in  its  proportions,  consequent 
on  the  diminished  ratio  of  its  cohesive  force  to  the  force  of 
gravitation.  With  the  physiological  actions  it  is  equally 
obvious.  Increase  of  size,  other  things  equal,  alters  the  rela- 
tions of  the  parts  to  the  material  and  dynamical  factors  of 
nutrition;  and  by  so  affecting  differently  the  nutrition  of 
different  parts,  initiates  further  changes  of  proportions. 
55 


130       MORPHOLOGICAL  DEVELOPMENT. 

In  plants  of  the  third  order  it  is  thus  with  the  proximate 
components:  they  are  subject  to  mutual  influences  that 
are  unlike  one  another  and  are  continually  changing.  The 
earlier-formed  units  become  mechanical  supporters  of  the 
later-formed  units,  and  so  experience  modifying  forces  from 
which  the  later-formed  units  are  exempt.  Further,  these 
elder  units  simultaneously  begin  to  serve  as  channels  through 
which  materials  are  carried  to  and  from  the  younger  units — 
another  cause  of  differentiation  that  goes  on  increasing  in  in- 
tensity. Once  more,  there  arise  ever-strengthening  contrasts 
between  the  amounts  of  light  which  fall  upon  the  youngest  or 
outermost  units  and  the  eldest  or  innermost  units;  whence 
result  structural  contrasts  of  yet  another  kind.  Evidently, 
then,  along  with  the  progressive  integration  of  cells  into 
fronds,  of  fronds  into  axes,  and  of  axes  into  plants  still  more 
composite,  there  come  into  play  sundry  causes  of  differen- 
tiation which  act  on  the  whole  and  on  each  of  its  parts, 
whatever  their  grade.  The  forces  to  be  overcome,  the  forces 
to  be  utilized,  and  the  matters  to  be  appropriated,  do  not 
remain  the  same  in  their  proportions  and  modes  of  action  for 
any  two  members  of  the  aggregate:  be  they  members  of  the 
first,  second,  third,  or  any  other  order. 

§  215.  Nor  are  these  the  only  kinds  and  causes  of  hetero- 
geneity which  we  have  to  consider.  Beyond  the  more 
general  changes  produced  in  the  relative  sizes  and  shapes  of 
plants  and  their  parts  by  progressive  aggregation,  there  are 
the  more  particular  changes  determined  by  the  more  par- 
ticular conditions. 

Plants  as  wholes  assume  unlike  attitudes  towards  their 
environments;  they  have  many  ways  of  articulating  their 
parts  with  one  another;  they  have  many  ways  of  adjusting 
their  parts  towards  surrounding  agencies.  These  are  causes 
of  special  differentiations  additional  to  those  general  differen- 
tiations that  result  from  increase  of  mass  and  increase  of  com- 
position. In  each  part  considered  individually,  there  arises 


MORPHOLOGICAL   DIFFERENTIATION  IN  PLANTS.  131 

a  characteristic  shape  consequent  on  that  relative  position 
towards  external  and  internal  forces,  which  the  mode  of 
growth  entails.  Every  member  of  the  aggregate  presents 
itself  in  a  more  or  less  peculiar  way  towards  the  light,  towards 
the  air,  and  towards  its  point  of  support;  and  according  to 
the  relative  homogeneity  or  heterogeneity  in  the  incidence  of 
the  agencies  thus  brought  to  bear  on  it,  will  be  the  relative 
homogeneity  or  heterogeneity  of  its  shape. 

§  216.  Before  passing  from  this  a  priori  view  of  the 
morphological  differentiations  which  necessarily  accompany 
morphological  integrations,  to  an  a  posteriori  view  of  them,  it 
seems  needful  to  specify  the  meanings  of  certain  descriptive 
terms  we  shall  have  to  employ. 

Taking  for  our  broadest  division  among  forms,  the  regular 
and  the  irregular,  we  may  divide  the  latter  into  those  which 
are  wholly  irregular  and  those  which,  being  but  partially 
irregular,  suggest  some  regular  form  to  which  they  approach. 
By  slightly  straining  the  difference  between  them,  two  cur- 
rent words  may  be  conveniently  used  to  describe  these  sub- 
divisions. The  entirely  irregular  forms  we  may  class  as 
asymmetrical — literally  as  forms  without  any  equalities  of 
dimensions.  The  forms  which  approximate  towards  regu- 
larity without  reaching  it,  we  may  distinguish  as  unsymmetri- 
cal:  a  word  which,  though  it  asserts  inequality  of  dimensions, 
has  been  associated  by  use  rather  with  such  slight  inequality 
as  constitutes  an  observable  departure  from  equality. 

Of  the  regular  forms  there  are  several  classes,  differing  in 
the  number  of  directions  in  which  equality  of  dimensions  is 
repeated.  Hence  results  the  need  for  names  by  which  sym- 
metry of  several  kinds  may  be  expressed. 

The  most  regular  of  figures  is  the  sphere :  its  dimensions 
are  the  same  from  centre  to  surface  in  all  directions;  and  if 
cut  by  any  plane  through  the  centre,  the  separated  parts  are 
equal  and  similar.  This  is  a  kind  of  symmetry  which  stands 
alone,  and  will  be  hereafter  spoken  of  as  spherical  symmetry.. 


132       MORPHOLOGICAL  DEVELOPMENT. 

When  a  sphere  passes  into  a  spheroid,  either  prolate  or 
oblate,  there  remains  but  one  set  of  planes  that  will  divide  it 
into  halves,  which  are  in  all  respects  alike;  namely,  the 
planes  in  which  its  axis  lies,  or  which  have  its  axis  for  their 
line  of  intersection.  Prolate  and  oblate  spheroids  may 
severally  pass  into  various  forms  without  losing  this  pro- 
perty. The  prolate  spheroid  may  become  egg-shaped  or  pyri- 
form,  and  it  will  still  continue  capable  of  being  divided  into 
two  equal  and  similar  parts  by  any  plane  cutting  it  down 
its  axis;  nor  will  the  making  of  constrictions  deprive  it  of 
this  property.  Similarly  with  the  oblate  spheroid.  The 
transition  from  a  slight  oblateness,  like  that  of  an  orange, 
to  an  oblateness  reducing  it  nearly  to  a  flat  disc,  does  not 
alter  its  divisibility  into  like  halves  by  every  plane  passing 
through  its  axis.  And  clearly  the  moulding  of  any  such 
flattened  oblate  spheroid  into  the  shape  of  a  plate,  leaves  it 
as  before,  symmetrically  divisible  by  all  planes  at  right 
angles  to  its  surface  and  passing  through  its  centre.  This 
species  of  symmetry  is  called  radial  symmetry.  It  is  familiar- 
ly exemplified  in  such  flowers  as  the  daisy,  the  tulip,  and  the 
dahlia. 

From  spherical  symmetry,  in  which  we  have  an  infinite 
number  of  axes  through  each  of  which  may  pass  an  infinite 
number  of  planes  severally  dividing  the  aggregate  into  equal 
and  similar  parts;  and  from  radial  symmetry,  in  which  we 
have  a  single  axis  through  which  may  pass  an  infinite  number 
of  planes  severally  dividing  the  aggregate  into  equal  and 
similar  parts;  we  now  turn  to  bilateral  symmetry,  in  which 
the  divisibility  into  equal  and  similar  parts  becomes  much 
restricted.  Noting,  for  the  sake  of  completeness,  that  there  is 
a  sextuple  bilateralness  in  the  cube  and  its  derivative  forms 
which  admit  of  division  into  equal  and  similar  parts  by  planes 
passing  through  the  three  diagonal  axes  and  by  planes  passing 
through  the  three  axes  that  join  the  centres  of  the  surfaces, 
let  us  limit  our  attention  to  the  three  kinds  of  bilateralness 
which  here  concern  us.  The  first  of  these  is  triple 


MORPHOLOGICAL  DIFFERENTIATION  IN   PLANTS.  133 

bilateral  symmetry.  This  is  the  symmetry  of  a  figure  having 
three  axes  at  right  angles  to  one  another,  through  each  of 
which  there  passes  a  single  plane  that  divides  the  aggregate 
into  corresponding  halves.  A  common  brick  will  serve  as  an 
example ;  and  of  objects  not  quite  so  simple,  the  most  familiar 
is  that  modern  kind  of  spectacle-case  which  is  open  at  both 
ends.  This  may  be  divided  into  corresponding  halves  along 
its  longitudinal  axis  by  cutting  it  through  in  the  direction 
of  its  thickness,  or  by  cutting  it  through  in  the  direction  of 
its  breadth;  or  it  may  be  divided  into  corresponding  halves 
by  cutting  it  across  the  middle.  Of  objects  which 

illustrate  double  bilateral  symmetry,  may  be  named  one  of 
those  boats  built  for  moving  with  equal  facility  in  either 
direction,  and  therefore  made  alike  at  stem  and  stern.  Ob- 
viously such  a  boat  is  separable  into  equal  and  similar  parts 
by  a  vertical  plane  passing  through  stem  and  stern ;  and  it  is 
also  separable  into  equal  and  similar  parts  by  a  vertical  plane 
cutting  it  amidships.  To  exemplify  single  bilateral 

symmetry  it  needs  but  to  turn  to  the  ordinary  boat  of  which 
the  two  ends  are  unlike.  Here  there  remains  but  the  one 
plane  passing  vertically  through  stem  and  stern,  on  the  oppo- 
site sides  of  which  the  parts  are  symmetrically  disposed. 

These  several  kinds  of  symmetry  as  placed  in  the  foregoing 
order,  imply  increasing  heterogeneity.  The  greatest  uni- 
formity in  shape  is  shown  by  the  divisibility  into  like  parts 
in  an  infinite  number  of  infinite  series  of  ways;  and  the 
greatest  degree  of  multiformity  consistent  with  any  regularity, 
is  shown  by  the  divisibility  into  like  parts  in  only  a  single 
way.  Hence,  in  tracing  up  organic  evolution  as  displayed  in 
morphological  differentiations,  we  may  expect  to  pass  from 
the  one  extreme  of  spherical  symmetry,  to  the  other  extreme 
of  single  bilateral  symmetry.  This  expectation  we  shall  find 
to  be  completely  fulfilled. 


CHAPTER  VII. 

THE  GENERAL  SHAPES  OF  PLANTS. 

§  217.  AMONG  protophytes  those  exemplified  by  Pleuro- 
coccus  vulgaris  are  by  general  consent  considered  the  simplest. 
As  shown  in  Fig.  1,  they  are  globular  cells  presenting  no 
obvious  differentiation  save  that  between  inner  and  outer 
parts.  Their  uniformity  of  figure  coexists  with  a  mode  of 
life  involving  the  uniform  exposure  of  all  their  sides  to 
incident  forces.  For  though  each  individual  may  have  its 
external  parts  differently  related  to  environing  agencies,  yet 
the  new  individuals  produced  by  spontaneous  fission,  whether 
they  part  company  or  whether  they  form  clusters  and  are 
made  polyhedral  by  mutual  pressure,  have  no  means  of  main- 
taining parallel  relations  of  position  among  their  parts. 
On  the  contrary,  the  indefiniteness  of  the  attitudes  into 
which  successive  generations  fall,  must  prevent  the  rise  of 
any  unlikeness  between  one  portion  of  the  surface  and 
another.  Spherical  symmetry  continues  because,  on  the 
average  of  cases,  incident  forces  are  equal  in  all  directions. 

Other  orders  of  Protophyta  have  much  more  special 
forms,  along  with  much  more  special  attitudes:  their  ho- 
mologous parts  maintaining,  from  generation  to  generation, 
unlike  relations  to  incident  forces.  The  Desmidiacece  and 
Diatomacece,  of  which  Figs.  2  and  3  show  examples,  severally 
include  genera  characterized  by  triple  bilateral  symmetry. 
A  Navicula  is  divisible  into  corresponding  halves  by  a  trans- 
134 


THE  GENERAL  SHAPES  OF  PLANTS.      135 

verse  plane  and  by  two  longitudinal  planes — one  cutting  its 
valves  at  right  angles  and  the  other  passing  between  its 
valves.  The  like  is  true 
of  those  numerous  trans- 
versely-constricted  forms  of 
Desmidiacece,  exemplified  by 
the  second  of  the  individuals 
represented  in  Fig.  2.  If  now  we  ask  how  a  Navicula  is 
related  to  its  environment,  we  see  that  its  mode  of  life  ex- 
poses it  to  three  different  sets  of  forces :  each  set  being 
resolvable  into  two  equal  and  opposite  sets.  A  Navicula 
moves  in  the  direction  of  its  length,  with  either  end  foremost. 
Hence,  on  the  average,  its  ends  are  subject  to  like  actions 
from  the  agencies  to  which  its  motions  subject  it.  Further, 
either  end  while  moving  exposes  its  right  and  left  sides  to 
amounts  of  influence  which  in  the  long  run  must  be  equal. 
If,  then,  the  two  ends  are  not  only  like  one  another,  but  have 
corresponding  right  and  left  sides,  the  symmetrical  distribu- 
tion of  parts  answers  to  the  symmetrical  distribution  of 
forces.  Passing  to  the  two  edges  and  the  two  flat  surfaces, 
we  similarly  find  a  clue  to  their  likenesses  and  differences  in 
their  respective  relations  to  the  things  around  them.  These 
locomotive  protophytes  move  through  the  entangled  masses 
of  fragments  and  fibres  produced  by  decaying  organisms  and 
confervoid  growths.  The  interstices  in  such  matted  accu- 
mulations are  nearly  all  of  them  much  longer  in  one  dimen- 
sion than  in  the  rest — form  crevices  rather  than  regular 
meshes.  Hence,  a  small  organism  will  have  much  greater 
facility  of  insinuating  itself  through  this  debris,  in  which  it 
finds  nutriment,  if  its  transverse  section  is  flattened  instead 
of  square  or  circular.  And  while  we  see  how,  by  survival  of 
the  fittest,  a  flattened  form  is  likely  to  be  acquired  by 
diatoms  having  this  habit;  we  also  see  that  likeness  will  be 
maintained  between  the  two  flat  surfaces  and  between  the 
two  edges.  For,  on  the  average,  the  relations  of  the  two  flat 
surfaces  to  the  sides  of  the  openings  through  which  the 


136  MORPHOLOGICAL  DEVELOPMENT. 

diatom  passes,  will  be  alike;  and  so,  too,  on  the  average, 
will  be  the  relations  of  the  two  edges.  In  desmids 

of  the  type  exemplified  by  the  second  individual  in  Fig.  2, 
a  kindred  equalization  of  dimensions  is  otherwise  insured. 
There  is  nothing  to  keep  one  of  the  two  surfaces  uppermost 
rather  than  the  other;  and  hence,  in  the  long  succession  of 
individuals,  the  two  surfaces  are  sure  to  be  similarly  exposed 
to  light  and  agencies  in  general.  When  to  this  is  added  the 
fact  that  spontaneous  fission  occurs  transversely  in  a  constant 
way,  it  becomes  manifest  that  the  two  ends,  while  they  are 
maintained  in  conditions  like  one  another,  are  maintained  in 
conditions  unlike  those  of  the  two  edges.  Here  then,  as 
before,  triple  bilateral  symmetry  in  form,  coexists  with  a  triple 
bilateral  symmetry  in  the  average  distribution  of  actions. 

Still  confining  our  attention  to  aggregates  of  the  first 
order,  let  us  next  note  what  results  when  the  two  ends  are 
permanently  subject  to  different  conditions.  The  fixed 
unicellular  plants,  of  which  examples  are  given  in  Figs.  4, 
5,  and  6,  severally  illustrate  the  contrast  in  shape  arising 


6 


between  the  part  that  is  applied  to  the  supporting  surface 
and  the  part  that  extends  into  the  surrounding  medium. 
These  two  parts  which  are  the  most  unlike  in  their  relations 
to  incident  forces,  are  the  most  unlike  in  the  forms.  Observe, 
next,  that  the  part  which  lifts  itself  into  the  water  or  air,  is 
more  or  less  decidedly  radial.  Each  outward  growing  tubule 
of  Codium  adhcerens,  Fig.  4,  has  its  parts  disposed  with  some 
regularity  around  its  axis;  the  upper  stem  and  spore- vessel 


THE  GENERAL  SHAPES  OF  PLANTS.      137 

of  Botrydium,  Fig.  5,  display  a  lateral  growth  that  is  approxi- 
mately equal  in  every  direction;  and  the  stems  of  the 
Mucor,  Fig.  6,  shoot  up  with  an  approach  to  evenness  on  all 
sides.  Plants  of  this  low  type  are  naturally  very  variable 
in  their  modes  of  growth :  each  individual  being  greatly  modi- 
fied in  form  by  its  special  circumstances.  But  they  neverthe- 
less show  us  a  general  likeness  between  parts  exposed  to  like 
forces,  as  well  as  a  general  unlikeness  between  parts  exposed 
to  unlike  forces. 

Eespecting  the  forms  of  these  aggregates  of  the  first  order, 
it  has  only  to  be  added  that  they  are  asymmetrical  where 
there  is  total  irregularity  in  the  incidence  of  forces.  We 
have  an  example  in  the  indefinitely  contorted  and  branched 
shape  of  a  fungus-cell,  growing  as  a  mycelium  among  the 
particles  of  soil  or  through  the  interstices  of  organic  tissue. 

§  218.  Ee-illustrations  of  the  general  truths  which  the 
forms  of  these  vegetal  aggregates  of  the  first  order  display, 
are  furnished  by  vegetal  aggregates  of  the  second  order.  The 
equalities  and  inequalities  of  growth  in  different  directions, 
prove  to  be  similarly  related  to  the  equalities  and  inequalities 
of  environing  actions  in  different  directions. 

Of  spherical  symmetry  an  instance  occurs  in  Eudorina 
elegans.  The  ciliated  cells  are  here  so  united  as  to  produce  a 
small,  mulberry-shaped,  hollow  ball  which,  being  similarly 
conditioned  on  all  sides,  shows  no  unlikenesses  of  structure. 
An  allied  form,  however,  Volvox  globator,  presents  a  highly 
instructive,  though  very  trifling,  modification.  It  is  not 
absolutely  homogeneous  in  its  structure  and  is  not  absolutely 
homogeneous  in  its  motions.  The  waving  cilia  of  its  compo- 
nent cells  have  fallen  into  such  slight  heterogeneities  of 
action  as  to  cause  rotation  in  a  constant  direction;  and 
along  with  a  fixed  axis  of  rotation  there  has  arisen  a  fixed 
axis  of  progression.  A  concomitant  fact  is  that  the  cells 
of  the  colony  exhibit  an  appreciable  differentiation  in  relation 
to  the  fixed  axis.  There  is  an  incipient  divergence  from 


138 


MORPHOLOGICAL  DEVELOPMENT. 


spherical  uniformity  along  with  this  slight  divergence  from 
uniformity  of  conditions. 

Vegetal  aggregates  of  the  second  order  are  usually  fixed : 
locomotion  is  exceptional.  Fixity  implies  that  the  surface 
of  attachment  is  differently  circumstanced  from  the  free  sur- 
face. Hence  we  may  expect  to  find,  as  we  do  find,  that 
among  these  rooted  aggregates  of  the  second  order,  as  among 
those  of  the  first  order,  the  primary  contrast  of  shape  is 
between  the  adherent  part  and  the  loose  part.  Sea-weeds 
variously  exemplify  this.  In  some  the  fronds  are  very 
irregular  and  in  some  tolerably  regular;  in  some  the  form  is 
pseudo-foliar  and  in  some  pseud-axial;  but  differing  though 
they  do  in  these  respects,  they  agree  in  having  the  end 
which  is  attached  to  a  solid  body  unlike  the  other  end.  The 
same  truth  is  seen  in  such  secondary  aggregates  as  the  com- 
mon Agarics,  or  rather  in  their  immensely-developed  organs 
of  fructification.  A  puff-ball,  Fig.  192,  presents  no  other 
obvious  unlikeness  of  parts  than  that  between  its  under  and 
upper  surfaces.  So  too  with  the  stalked  kinds  that  frequent 
our  woods  and  pastures.  In  the  types  which  Figs.  193, 
194,  195,  delineate,  the  unlikenesses  between  the  rooted 
ends  and  the  expanded  ends,  as  well  as  between  the  under 
and  upper  surfaces  of  the  expanded  ends,  are  obviously 
related  to  this  fundamental  contrast  of  conditions.  N~or  is 
this  relation  less  clearly  displayed  in  the  sessile  fungi  which 
grow  out  from  the  sides  of  trees,  as  shown  at  a,  b,  Fig.  196. 


That  which  is  common  to  this  and  the  preceding  types,  is  the 
contrast  between  the  attached  end  and  the  free  end. 


THE  GENERAL  SHAPES  OF  PLANTS.       139 

From  what  these  forms  have  in  common,  let  us  turn  to 
that  which  they  have  not  in  common,  and  observe  the  causes 
of  the  want  of  community.  A  puff-ball  shows  us  in  the 
simplest  way,  the  likeness  of  parts  accompanying  likeness  of 
conditions,  along  with  the  unlikeness  of  parts  accompanying 
unlikeness  of  conditions.  For  while,  if  we  cut  vertically 
through  its  centre,  we  find  a  difference  between  top  and 
bottom,  if  we  cut  horizontally  through  its  centre,  we  find  no 
differences  among  its  several  sides.  Being,  on  the  average  of 
cases,  similarly  related  to  the  environment  all  round,  it 
remains  the  same  all  round.  The  radial  symmetry  of  the 
mushroom  and  other  vertically-growing  fungi,  illustrates 
this  connexion  of  cause  and  effect  still  better.  But  now 
mark  what  happens  in  the  group  of  Agaricus  noli-tangere, 
shown  in  Fig.  195.  Eadially  symmetrical  as  is  the  type,  and 
radially  symmetrical  as  are  those  centrally-placed  individuals 
which  are  equally  crowded  all  round,  we  see  that  the  peri- 
pheral individuals,  dissimilarly  circumstanced  on  their  outer 
sides  and  on  their  sides  next  the  group,  have  partially 
changed  their  radial  symmetry  into  bilateral  symmetry.  It 
is  no  longer  possible  to  make  two  corresponding  halves  by 
any  vertical  plane  cutting  down  through  the  pileus  and  the 
stem;  but  there  is  only  one  vertical  plane  that  will  thus  pro- 
duce corresponding  halves — the  plane  on  the  opposite  sides 
of  which  the  relations  to  the  environment  are  alike.  And 
then  mark  that  the  divergence  from  all-sided  symmetry 
towards  two-sided  symmetry,  here  caused  in  the  individual 
by  special  circumstances,  is  characteristic  of  the  race  where 
the  habits  of  the  race  constantly  involve  two-sidedness  of 
conditions.  Besides  being  exemplified  by  such  comparatively 
undifferentiated  types  as  certain  Polypori,  Fig.  196,  a,  &,  this 
truth  is  exemplified  by  members  of  the  genus  just  named. 
In  Agaricus  Jiorizontalis,  Fig.  196,  c,  we  have  a  departure 
from  radial  symmetry  that  is  conspicuous  only  in  the  form 
of  the  stem.  A  more  decided  bilateralness  exists  in  A.  sub- 
palmatus,  shown  in  elevation  at  d  and  in  section  at  d'.  And 


140        MORPHOLOGICAL  DEVELOPMENT. 

Lentinus  flabelliformis,  of  which  e  and  e'  are  different  views, 
exhibits  complete  bilateralness — a  bilateralness  in  which 
there  is  the  greatest  likeness  of  the  parts  that  are  most  simi- 
larly conditioned,  and  the  greatest  unlikeness  of  the  parts 
that  are  most  dissimilarly  conditioned. 

Among  plants  of  the  second  order  of  composition,  it  will 
suffice  to  note  one  further  class  of  facts  which  are  the  con- 
verse of  the  foregoing  and  have  the  same  implications.  These 
are  the  facts  showing  that  along  with  habitual  irregularity  in 
the  relations  to  external  forces,  there  is  habitual  irregularity 
in  the  mode  of  growth.  Besides  finding  such  facts  among 
Thallophytes,  as  in  the  tubers  of  underground  fungi  and  in 
the  creeping  films  of  sessile  lichens,  which  severally  show  us 
variations  of  proportions  obviously  caused  by  variations  in 
the  amounts  of  the  influences  on  their  different  sides,  we  also, 
among  Archegoniates  of  inferior  types,  find  irregularities  of 
form  along  with  irregularities  in  environing  actions.  The 
fronds  of  the  Marcliantiacece  or  such  Jungermanniacece  as  are 
shown  in  Figs.  41,  42,  43,  illustrate  the  way  in  which  each 
lowly-organized  aggregate  of  the  second  order,  not  individuated 
by  the  mutual  dependence  of  its  parts,  has  its  form  deter- 
mined by  the  balance  of  facilities  and  resistances  which  each 
side  of  the  frond  meets  with  as  it  spreads. 

§  219.  Among  plants  displaying  integration  of  the  third 
degree,  and  among  plants  still  further  compounded,  these 
same  truths  are  equally  manifest.  In  the  forms  of  such  plants 
we  see  primary  contrasts  and  secondary  contrasts  which,  no 
less  clearly  than  the  foregoing,  are  related  to  contrasts  of 
conditions. 

That  flowering  plants  from  the  daisy  up  to  the  oak,  have 
in  common  the  fundamental  unlikeness  between  the  upward 
growing  part  and  the  downward  growing  part;  and  that 
this  most  marked  unlikeness  corresponds  with  the  most 
marked  unlikeness  between  the  two  parts  of  their  environ- 
ment, soil  and  air;  are  facts  too  conspicuous  to  be  named 


THE  GENERAL  SHAPES  OP  PLANTS.       141 

were  they  not  important  items  in  the  argument.  More 
instructive  perhaps,  because  less  familiar,  is  the  fact  that  we 
miss  this  extreme  contrast  in  flowering  plants  which  have  not 
their  higher  and  lower  portions  exposed  to  conditions  thus 
extremely  contrasted.  A  parasite  like  the  Dodder,  growing 
in  entangled  masses  upon  other  plants,  from  which  it  sucks 
the  juices,  is  not  thus  divisible  into  two  strongly-distinguished 
halves. 

Leaving  out  of  consideration  the  difference  between  the 
supporting  part  and  the  supported  part  in  phaenogams,  and 
looking  at  the  supported  part  only,  we  observe  between  its 
form  and  the  habitual  incidence  of  forces,  a  relation  like  that 
which  we  observed  in  the  simpler  plants.  Phasnogams  that 
are  practically  if  not  literally  uniaxial,  and  those  which  de- 
velop their  lateral  axes  only  in  the  shape  of  axillary  flowers, 
when  uninterfered  with  commonly  send  up  vertical  stems 
round  which  the  leaves  and  flowers  are  disposed  with  a  more 
or  less  decided  radial  symmetry.  Gardens  and  fields  supply 
us  with  such  instances  as  the  Tulip  and  the  Orchis;  and,  on 
a  larger  scale,  the  Palms  and  the  Aloes  are  fertile  in  ex- 
amples. The  exceptions,  too,  are  instructive.  Besides  the 
individual  divergences  arising  from  special  interferences,  there 
are  to  be  traced  general  divergences  where  the  habits  of  the 
plants  expose  them  to  general  interferences  in  anything 
approaching  to  constant  ways.  Plants  which,  like  the  Fox- 
glove, have  spikes  of  flowers  that  are  borne  on  flexible  foot- 
stalks, have  their  flowers  habitually  bent  round  to  one  face  of 
the  stem:  an  unlikeness  of  distribution  probably  caused  by 
unlikeness  in  the  relation  to  the  Sun's  rays.  The  wild  Hya- 
cinth, too,  with  stem  so  flexible  that  its  upper  part  droops, 
shows  us  how  a  consequent  difference  in  the  action  of  gravity 
on  the  flowers,  causes  them  to  deviate  from  their  typically- 
radial  arrangement  towards  a  bilateral  arrangement. 

Much  more  conspicuous  are  these  general  and  special  rela- 
tions of  form  to  general  and  special  actions  in  the  environ- 
ment, among  phasnogams  that  are  multiaxial.  That  when 


142        MORPHOLOGICAL  DEVELOPMENT. 

standing  alone,  and  in  places  where  the  winds  do  not  injure 
them  nor  adjacent  things  shade  them,  shrubs  and  trees  develop 
with  tolerable  evenness  on  all  sides,  is  an  obvious  truth. 
Equally  obvious  is  the  truth  that,  when  growing  together  in  a 
wood,  and  mutually  interfered  with  on  all  sides,  trees  still 
show  obscurely  radial  distributions  of  parts;  though,  under 
such  conditions,  they  have  tall  taper  stems  with  branches 
directed  upwards — a  difference  of  shape  clearly  due  to  the 
different  incidence  of  forces.  And  almost  equally  obvious  is 
the  truth,  that  a  tree  of  this  same  kind  growing  at  the  edge 
of  the  wood,  has  its  outer  branches  well  developed  and  its 
inner  branches  comparatively  ill-developed.  Fig.  197,  which 


taa 

inaccurately  represents  this  difference,  will  serve  to  make  it 
manifest  that  while  one  of  the  peripheral  trees  can  be  cut 
into  something  like  two  similar  halves  by  a  vertical  plane 
directed  towards  the  centre  of  the  wood — a  plane  on  each  side 
of  which  the  conditions  are  alike — it  cannot  be  cut  into  simi- 
lar halves  by  any  other  plane.  A  like  divergence  from  an 
indefinitely-radial  symmetry  towards  an  indefinitely-bilateral 
symmetry,  occurs  in  trees  that  have  their  conditions  made 
bilateral  by  growing  on  inclined  surfaces.  Two  of  the  common 
forms  observable  in  such  cases  are  given  in  Fig.  198.  Here 
there  is  divisibility  into  parts  that  are  tolerably  similar,  by 
a  vertical  plane  running  directly  down  the  hill;  but  not  by 
any  other  plane.  Then,  further,  there  is  the  bilateralness, 
similar  in  general  meaning  though  differently  caused,  often 
seen  in  trees  exposed  to  strong  prevailing  winds.  Almost 


THE   GENERAL  SHAPES  OF   PLANTS.  143 

every  sea-coast  has  abundant  examples  of  stunted  trees  which, 
like  the  one  shown  in  Fig.  199,  have  been  made  to  deviate 
from  their  ordinary  equal  growth  on  all  sides  of  a  vertical 
axis,  to  a  growth  that  is  equal  only  on  the  opposite  sides  of  a 
vertical  plane  directed  towards  the  wind's  eye. 

From  among  vegetal  aggregates  of  the  third  order,  we  have 
now  only  to  add  examples  of  the  entirely  asymmetrical  form 
which  accompanies  an  entirely  irregular  distribution  of  inci- 
dent forces.  Creeping  plants  furnish  such  examples.  They 
show,  both  when  climbing  up  vertical  or  inclined  surfaces  and 
when  trailing  on  the  ground,  that  their  branches  grow  hither 
and  thither  as  the  balance  of  forces  aids  or  opposes ;  and  the 
general  outline  is  without  symmetry  of  any  kind,  because 
the  environing  influences  have  no  kind  of  regularity  in  their 
arrangement. 

§  220.  Along  with  some  unfamiliar  facts,  I  have  here  set 
down  facts  which  are  so  familiar  as  to  seem  scarcely  worth 
noting.  It  is  because  these  facts  have  become  meaningless 
to  perceptions  deadened  by  infinite  repetitions  of  them,  that 
it  is  needful  here  to  point  out  their  meanings.  Not  alone  for 
its  intrinsic  importance  has  the  unlikeness  between  the 
attached  ends  and  the  free  ends  been  traced  among  plants  of 
all  degrees  of  integration.  Nor  is  it  simply  because  of  the 
significance  they  have  in  themselves,  that  instances  have 
been  given  of  those  varieties  of  symmetry  and  asymmetry 
which  the  free  ends  of  plants  equally  display:  be  they  plants 
of  the  first,  second,  third,  or  any  higher  order.  Neither  has* 
the  only  other  purpose  been  that  of  showing  how,  in  the  radial 
symmetry  of  some  vegetal  aggregates  and  the  single  bilateral 
symmetry  of  others,  there  are  traceable  the  same  ultimate 
principles  as  in  the  spherical  symmetry  and  triple  bilateral 
symmetry  of  certain  minute  plants  first  described.  But  the 
main  object  has  been  to  present,  under  their  simplest  aspects, 
those  general  laws  of  morphological  differentiation  which  are 
fulfilled  by  the  component  parts  of  each  plant. 


144  MORPHOLOGICAL  DEVELOPMENT. 

If  organic  form  is  determined  by  the  distribution  of  forces, 
and  the  approach  in  every  case  towards  an  equilibrium  of 
inner  actions  with  outer  actions;  then  this  relation  between 
forms  and  forces  must  hold  alike  in  the  organism  as  a  whole 
in  its  proximate  units,  and  in  its  units  of  lower  orders.  For- 
mulas which  express  the  shapes  of  entire  plants  in  terms  of 
surrounding  conditions,  must  be  formulas  which  also  express 
the  shapes  of  their  several  parts  in  terms  of  surrounding 
conditions.  If,  therefore,  we  find  that  a  plant  as  a  whole  is 
radially  symmetrical  or  bilaterally  symmetrical  or  asymme- 
trical, according  as  the  incident  forces  affect  it  equally  on  all 
sides  of  an  axis,  or  affect  it  equally  only  on  the  opposite  sides 
of  one  plane,  or  affect  it  equally  in  no  two  directions;  then, 
we  may  expect  that,  in  like  manner,  each  member  of  a  plant 
will  display  radial  symmetry  where  environing  influences  are 
alike  along  many  radii,  bilateral  symmetry  where  there  is 
bilateralness  of  environing  influences,  and  unsymmetry  or 
asymmetry  where  there  is  partial  or  entire  departure  from  a 
balance  of  surrounding  actions. 

To  show  that  this  expectation  is  borne  out  by  the  facts, 
will  be  the  object  of  the  following  four  chapters.  Let  us 
begin  with  the  largest  parts  into  which  plants  are  divisible; 
and  proceed  to  the  successively  smaller  parts. 


CHAPTER  VIII. 


THE    SHAPES   OF   BRANCHES. 

§221.  AGGREGATES  of  the  first  order  supply  a  few  examples 
of  forms  ramified  in  an  approximately-regular  manner,  under 
conditions  which  subject  their  parts  to  approximately-regu- 
lar distributions  of  forces.  Some  unicellular  Algae,  becoming 
elaborately  branched,  assume  very  much  the  aspects  of  small 
trees;  and  show  us  in  their  branches  analogous  relations  of 
forms  to  forces.  Bryopsis  plumosa  may 
be  instanced.  Fig.  200  represents  the 
end  of  one  of  its  lateral  ramifications, 
above  and  beneath  which  come  others  of 
like  characters.  Here  it  will  be  seen  that 
the  attached  and  free  ends  differ;  that 
the  two  sides  are  much  alike;  and  that  they  are  unlike  the 
upper  and  under  surfaces,  which  resemble  one  another.  The 
more  highly  developed  members  of  the  same  group  of  Algce, 
the  Siphonea,  show  a  marked  radial  symmetry  coexisting 
with  very  elaborate  branching,  e.g.,  Neo- 
meris,  Cymopolia,  and  others. 

§  222.  Fig.  201  shows  us  how,  in  an 
aggregate  of  the  second  order,  each  proxi-       ^ 
mate  component  is  modified  by  its  rela- 
tions to  the  rest;  just  as  we  before  saw 
a  whole  fungus  of  the  same  type  modified 

56  145 


14:6 


MORPHOLOGICAL  DEVELOPMENT. 


by  its  relations  to  environing  objects.  If  a  branch  of  the 
fungus  here  figured,  be  compared  with  one  of  the  fungi 
clustered  together  in  Fig.  195,  or,  still  better,  with  one  of  the 
laterally-growing  fungi  shown  in  Fig.  196,  there  will  be  per- 
ceived a  kindred  transition  from  radial  to  bilateral  symmetry, 
occurring  under  kindred  conditions.  The  portion  of  the 
pileus  next  to  the  side  of  attachment  is  undeveloped  in  this 
branched  form  as  in  the  simpler  form ;  and  in  the  one  case  as 
in  the  other,  the  stem  is  modified  towards  the  side  of  attach- 
ment. A  division  into  similar  halves,  which,  as  shown  in 
Fig.  196  e,  might  be  made  of  the  whole  fungus  by  a  vertical 
plane  passing  through  the  centre  of  the  pileus  and  the  axis 
of  the  supporting  body,  might  here  be  made  of  the  branch, 
by  a  vertical  plane  passing  through  the  centre  of  its  pileus 
and  the  axis  of  the  main  stem.  Among  aggregates  of  this 
order,  the  Algae  furnish  cases  of  kindred  nature.  In  the 
branches  of  Lessonia,  Fig.  37,  may  be  observed  a  substantially- 
similar  relationship.  As  their  inner  parts  are  less  developed 
than  their  outer  parts,  while  their  two  sides  are  developed  in 
approximately  equal  degrees,  they  are  rendered  bilateral. 

§  223.  These  few  cases  introduce  us  to  the  more  familiar 
but  more  complex  cases  which  plants  of  the  third  degree  of 


aggregation  present.    At  a,  6,  c,  Fig.  202,  are  sketched  three 
homologous  parts  of  the  same  tree:   a   being  the   leading 


THE  SHAPES  OF  BRANCHES.  147 

shoot ;  b  a  lateral  branch  near  the  top,  and  c  a  lateral  branch 
lower  down.  There  is  here  a  double  exemplification.  While 
the  branch  a,  as  a  whole,  has  its  branchlets  arranged  with 
tolerable  regularity  all  round,  in  correspondence  with  its 
equal  exposure  on  all  sides,  each  branchlet  shows  by  its 
curve  as  much  bilateral  symmetry  as  its  simple  form  permits. 
The  branch  b,  dissimilarly  circumstanced  on  the  side  next 
the  main  stem  and  on  the  side  away  from  it,  has  an  approxi- 
mate bilateralness  as  a  whole,  while  the  bilateralness  of  its 
branchlets  varies  with  their  respective  positions.  And  in  the 
branch  c,  having  its  parts  still  more  differently  conditioned, 
these  traits  of  structure  are  still  more  marked.  Extremely 
strong  contrasts  of  this  kind  occur  in  trees  having  very 
regular  modes  of  growth.  The  uppermost  branches  of  a 
Spruce-fir  have  radially-arranged  branchlets:  each  of  them, 
if  growing  vigorously,  repeats  the  type  of  the  leading  shoot, 
as  shown  in  Fig.  203,  a,  6.  But  if  we  examine  branches 
lower  and  lower  down  the  tree,  we  find  the  vertically-growing 
branchlets  bear  a  less  and  less  ratio  to  the  horizontally- 
growing  ones;  until,  towards  the  bottom,  the  radial  arrange- 
ment has  wholly  merged  into  the  bilateral.  Shaded  and 
confined  by  the  branches  above  them,  these  eldest  branches 
develop  their  offshoots  in  those  directions  where  there  is 
most  space  and  light:  becoming  finally  quite  flattened  and 
fan-shaped,  as  shown  at  Fig.  203,  c.  And  on  remembering 
that  each  of  these  eldest  branches,  when  first  it  diverged 
from  the  main  stem,  was  radial,  we  see  not  only  that  between 
the  upper  and  lower  branches  does  this  contrast  in  structure 
hold,  but  also  that  each  branch  is  transformed  from  the 
radial  to  the  bilateral  by  the  progressive  change  in  its  en- 
vironment. Other  forces  besides  those  which  aid  or 
hinder  growth,  conspire  to  produce  this  two-sided  character 
in  lateral  branches.  The  annexed  Fig.  204,  sketched  from 
an  example  of  the  Pinus  Coulterii  at  Kew,  shows  very  clearly 
how,  by  mere  gravitation,  the  once  radially-arranged  branch- 
lets  may  be  so  bent  as  to  produce  in  the  branch  as  a  whole  a 


148        MORPHOLOGICAL  DEVELOPMENT. 

decided  bilateralness.  A  full-grown  Araucaria,  too,  exhibits 
in  its  lower  branches  modifications  similarly  caused;  and  in 
each  of  such  branches  there  may  be  remarked  the  further 
fact,  that  its  upward-bending  termination  has  a  partially- 
modified  radialness,  at  the  same  time  that  its  drooping  lateral 
branchlets  give  to  the  part  nearer  the  trunk  a  completely 
bilateral  character. 

Now  in  these  few  instances,  typical  of  countless  instances 
which  might  be  given,  we  see,  as  we  saw  in  the  case  of 
the  fungi,  that  the  same  thing  is  true  of  the  parts  in  their 
relations  to  the  whole  and  to  one  another,  which  is  true  of 
the  whole  in  its  relations  to  the  environment  at  large.  Entire 
trees  become  bilateral  instead  of  radial,  when  exposed  to 
forces  that  are  equal  only  on  opposite  sides  of  one  plane ;  and 
in  their  branches,  parallel  changes  of  form  occur  under 
parallel  changes  of  conditions. 

§  224.  There  remains  to  be  said  something  respecting  the 
distribution  of  leaves.  How  a  branch  carries  its  leaves 
constitutes  one  of  its  characters  as  a  branch,  and  is  to  be 
considered  apart  from  the  characters  of  the  leaves  them- 
selves. The  principles  hitherto  illustrated  we  shall  here  find 
illustrated  still  further. 

The  leading  shoot  and  all  the  upper  twigs  of  a  fir-tree, 
have  their  pin-shaped  leaves  evenly  distributed  all  round,  or 
placed  radially ;  *  but  as  we  descend  we  find  them  beginning 
to  assume  a  bilateral  distribution;  and  on  the  lower,  horizon- 
tally-growing branches,  their  distribution  is  quite  bilateral,  f 
Between  the  Irish  and  English  kinds  of  Yew,  there  is  a  con- 
trast of  like  significance.  The  branches  of  the  one,  shooting 
up  as  they  do  almost  vertically,  are  clothed  with  leaves 

*  Here  and  throughout,  the  word  radial  is  applied  equally  to  the  spiral 
and  the  whorled  structures.  These,  as  being  alike  on  all  sides,  arc  similarly 
distinguished  from  arrangements  that  are  alike  on  two  sides  only. 

f  It  should  be  added  that  this  change  of  distribution  is  not  due  to 
change  in  the  relative  positions  of  the  insertions  of  the  leaves  but  to  their 
twisting?. 


THE   SHAPES   OF   BRANCHES.  149 

all  round;  while  those  of  the  other,  which  spread  laterally, 
bear  their  leaves  on  the  two  sides.  In  trees  with  better- 
developed  leaves,  the  same  principle  is  more  or  less  manifest 
in  proportion  as  the  leaves  are  more  or  less  enabled  by  their 
structures  to  maintain  fixed  positions.  Where  the  foot-stalks 
are  long  and  slender,  and  where,  consequently,  each  leaf, 
according  to  its  weight,  the  flexibility  and  twist  of  its  foot- 
stalk, and  the  direction  of  the  branch  it  grows  from,  falls 
into  some  indefinite  attitude,  the  relations  are  obscured.  But 
where  the  foot-stalks  are  stiff,  as  in  the  Laurel,  it  will  be 
found,  as  before,  that  from  the  topmost  and  upward-growing 
branches  the  leaves  diverge  on  all  sides;  while  the  under- 
most branches,  growing  out  from  the  shade  of  those  above, 
have  their  leaves  so  turned  as  to  bring  them  into  rows  hori- 
zontally spread  out  on  the  two  sides  of  each  branch. 

A  kindred  truth,  having  like  implications,  comes  into  view 
when  we  observe  the  relative  sizes  of  leaves  on  the  same 
branch,  where  their  sizes  differ. 
Fig.  205  represents  a  branch  of  a 
Horse-chestnut,  taken  from  the 
lowermost  fringe  of  the  tree,  where 
the  light  has  been  to  a  great  extent 
intercepted  from  all  but  the  most 
protruded  parts.  Beyond  the  fact 
that  the  leaves  become  by  appro- 
priate growths  of  their  foot-stalks 
bilaterally  distributed  on  this  droop- 
ing branch,  instead  of  being  distributed  symmetrically  all 
round,  as  on  one  of  the  ascending  shoots,  we  have  here  to 
note  the  fact  that  there  is  unequal  development  on  the  upper 
and  lower  sides.  Each  of  the  compound  leaves  acquires  a 
foot-stalk  and  leaflets  that  are  large  in  proportion  to  the 
supply  of  light;  and  hence,  as  we  descend  towards  the  bot- 
tom of  the  tree,  the  clusters  of  leaves  display  increasing 
contrasts.  How  marked  these  contrasts  become  will  be  seen 
on  comparing  a  and  &,  which  form  one  pair  of  leaves  that 


150        MORPHOLOGICAL  DEVELOPMENT. 

are  normally  equal,  or  c  and  d,  which  form  another  pair  nor- 
mally equal. 

Let  us  not  omit  to  note,  while  we  have  this  case  before  us, 
the  proof  it  affords  that  these  differences  of  development  are 
in  a  considerable  degree  determined  by  the  different  con- 
ditions of  the  parts  after  they  have  been  unfolded.  Though 
those  inequalities  of  dimensions  whence  the  differentiations 
of  form  result,  may  be  in  many  cases  largely  due  to  the 
inequalities  in  the  circumstances  of  the  parts  while  in  the 
bud  (which  are,  however,  representative  of  inequalities  in 
ancestral  circumstances) ;  yet  these  are  clearly  not  the  sole 
causes  of  the  unlikenesses  which  eventually  arise.  This  bi- 
lateralness  resulting  from  the  unequal  sizes  of  the  leaves, 
must  be  considered  as  due  to  the  differential  actions  that 
come  into  play  after  the  leaves  have  assumed  their  typical 
structures. 

§  225.  How,  in  the  arrangement  of  their  twigs  and  leaves, 
branches  tend  to  lapse  from  forms  that  are  approximately 
symmetrical  to  forms  that  are  quite  asymmetrical,  need  not 
be  demonstrated:  it  is  sufficiently  conspicuous.  But  it  may 
be  well  to  point  out  how  the  tendency  to  do  this  further 
enforces  our  argument.  The  comparatively  regular  budding- 
out  of  secondary  axes  and  tertiary  axes,  does  not  usually 
produce  an  aggregate  which  maintains  its  regularity,  for 
the  simple  reason  that  many  of  the  axes  abort.  Terminal 
buds  are  some  of  them  destroyed  by  birds;  others  are  bur- 
rowed into  by  insects;  others  are  nipped  by  frost;  others 
are  broken  off  or  injured  during  gales  of  wind.  The  envi- 
ronment of  each  branch  and  its  branchlets  is  thus  ever 
being  varied  on  all  sides:  here,  space  being  left  vacant  by 
the  death  of  some  shoot  that  would  ordinarily  have  occupied 
it;  and  there,  space  being  trenched  on  by  the  lateral  growth 
of  some  adjacent  branch  that  has  had  its  main  axis  broken. 
Hence  the  asymmetry,  or  heterogeneity  of  form,  assumed 
by  the  branch,  is  caused  by  the  asymmetrical  distribution 


THE  SHAPES  OF  BRANCHES.  151 

of  incident  forces — a  result  and  a  cause  which  go  on  ever 
complicating. 

§  226.  One  conspicuous  trait  in  the  shapes  of  branches 
has  still  to  be  named.  Their  proximal  or  attached  ends 
differ  from  their  distal  or  free  ends,  in  the  same  way  that 
the  lower  ends  of  trees  differ  from  their  upper  ends.  This 
fact,  like  the  fact  to  which  it  is  here  paralleled,  has  had  its 
significance  obscured  by  its  extreme  familiarity.  But  it 
shows  in  a  striking  way  how  the  most  differently-conditioned 
parts  become  the  most  strongly  contrasted  in  their  struc- 
tures. A  phaenogamic  axis  is  made  up  of  homologous  seg- 
ments, marked  off  from  one  another  by  the  nodes;  and  a 
compound  branch  consists  of  groups  of  such  segments.  The 
earliest-formed  segments,  alike  of  the  tree  and  of  each 
branch,  serve  as  mechanical  supports  and  channels  for  sap  to 
the  successive  generations  of  segments  that  grow  out  of 
them;  and  become  more  and  more  shaded  by  their  progeny 
as  these  increase.  Hence  the  progressively-increasing  con- 
trasts which,  while  mainly  due  to  the  unlikenesses  of  bulk 
accompanying  differences  of  age,  are  in  part  due  to  the  un- 
likenesses of  structure  which  differences  of  relation  to  the 
environment  have  caused. 

§  227.  Thus,  then,  it  is  with  the  proximate  parts  of  plants 
as  it  is  with  plants  as  wholes.  The  radial  symmetry,  the 
bilateral  symmetry,  and  the  asymmetry,  which  branches  dis- 
play in  different  trees,  in  different  parts  of  the  same  tree,  and 
at  different  stages  of  their  own  growths,  prove  to  be  all  con- 
sequent on  the  ways  in  which  they  stand  towards  the  entire 
plexus  of  surrounding  actions.  The  principle  that  the 
growths  are  unequal  in  proportion  as  the  relations  of  parts 
to  the  environment  are  unequal,  serves  to  explain  all  the 
leading  traits  of  structure. 


CHAPTEK  IX. 

THE    SHAPES   OF   LEAVES. 

§  228.  NEXT  in  the  descending  order  of  composition  come 
compound  leaves.  The  relative  sizes  and  distributions  of 
their  leaflets,  as  affecting  their  forms  as  wholes,  have  to  be 
considered  in  their  relations  to  conditions.  Figs.  206,  207, 
represent  leaves  of  the  common  Oxalis  and  of  the  Marsilea, 
in  which  radial  symmetry  is  as  completely  displayed  as  the 
small  number  of  leaflets  permits.  This  equal  development 
of  the  leaflets  on  all  sides,  occurs  where  the  foot-stalks,  grow- 
ing up  vertically  from  creeping  or  underground  stems,  are 
so  long  that  the  leaves  either  do  not  interfere  with  one 
another  or  do  it  in  an  inconstant  way:  the  leaflets  are  not 
differently  conditioned  on  different  sides,  as  they  are  where 
the  foot-stalks  grow  out  in  the  ordinary  manner.  How  un- 
likeness  of  position  influences  the  leaflets  is  clearly  shown  in 
a  Clover-leaf,  Fig.  208,  which  deviates  from  the  Oxalis-leaf 
but  slightly  towards  bilateralness,  as  it  deviates  from  it  but 
slightly  in  the  attitude  of  its  petiole;  which  is  a  little  in- 
clined away  from  the  others  borne  by  the  same  procumbent 
axis.  A  familiar  example  of  an  almost  radial  symmetry 
along  with  almost  equal  relations  to  surrounding  conditions, 
occurs  in  the  root-leaves  of  the  Lupin,  Fig.  209  b.  Here 
though  we  have  lateral  divergence  from  a  vertical  axis,  yet 
the  long  foot-stalks  preserve  nearly  erect  positions,  and 
carry  their  leaves  to  such  distances  from  the  axis,  that  the 
development  of  the  leaflets  on  the  side  next  it  is  not  much 
152 


THE   SHAPES  OF  LEAVES. 


153 


hindered.  Still  the  interference  of  the  leaves  with  one 
another  is,  on  the  average,  somewhat  greater  on  the  proximal 
side  than  on  the  distal  side;  and  hence  the  interior  leaflets 
are  rather  less  than  the  exterior  leaflets.  In  further  proof  of 
which  influence,  let  it  be  added  that,  as  shown  in  the  figure, 
at  a,  the  leaves  growing  out  of  the  flowering-stem  deviate 
towards  the  two-sided  form  more  decidedly.  Two-sidedness 
is  much  greater  where  there  is  a  greater  relative  proximity 
of  the  inner  leaflets  to  the  axis,  or  where  the  foot-stalk 
approaches  towards  a  horizontal  position.  The  Horse-chest- 
nut, Fig.  205,  already  instanced  as  showing  how  the  arrange- 
ments and  sizes  of  leaflets  are  determined  by  the  incidence  of 
forces,  serves  also  to  show  how  the  incidence  of  forces  deter- 


mines the  relative  sizes  and  arrangements  of  leaflets.  Fig. 
210,  which  shows  a  leaf  of  the  Bombax,  further  illustrates 
this  relation  of  structure  to  conditions. 

Compound  leaves  that  -are  completely  bilateral,  present  us 
with  modifications  of  form  exemplifying '  the  same  general 
truth  in  another  way.  In  them  the  proximal  and  distal 
parts  have  none  of  that  resemblance  which  we  see  in  those 
intermediate  forms  just  described.  The  portion  next  the  axis 
and  the  portion  furthest  from  the  axis  are  entirely  different; 
and  the  only  likeness  is  between  the  wings  or  leaflets  on 
opposite  sides  of  the  main  foot-stalk  or  mid-rib.  On  turning 
back  to  Fig.  65,  it  will  be  seen  that  the  compound  leaf  there 


154       MORPHOLOGICAL  DEVELOPMENT. 

drawn  to  exemplify  another  truth,  serves  also  to  exemplify  this 
truth:  the  homologous  parts  a,  b,  c,  d,  while  they  are  unlike 
one  another,  are,  in  their  main  proportions,  severally  like 
the  parts  with  which  they  are  paired.  And  here  let  us  not 
overlook  a  characteristic  which  is  less  conspicuous  but  not 
less  significant.  Each  of  the  lateral  wings  has  winglets 
that  are  larger  on  the  one  side  than  on  the  other;  and  in 
each  case  the  two  sides  are  dissimilarly  conditioned.  Even 
in  the  several  components  of  each  wing  may  be  traced  a  like 
divergence  from  symmetry,  along  with  a  like  inequality  in 
the  relations  to  the  rest:  the  proximal  half  of  each  leaflet 
is  habitually  larger  than  the  distal  half.  In  the  leaves  of 
the  Bramble,  previously  figured,  kindred  facts  are  presented. 
How  far  such  differences  of  development  are  due  to  the  posi- 
tions of  the  parts  in  the  bud;  how  far  the  respective 
spaces  available  for  the  parts  when  unfolded  affect  them; 
and  how  far  the  parts  are  rendered  unlike  by  unlikenesses 
in  their  relations  to  light;  it  is  difficult  to  say.  Probably 
these  several  factors  operate  in  all  varieties  of  proportion. 
That  the  habitual  shading  of  some  parts  by  others  largely 
aids  in  causing  these  divergences  from  symmetry,  is  very 
instructively  shown  by  the  compound  leaves  of  the  Cow- 
parsnip.  Fig.  211  represents  one  of  these.  While  the  leaf  as  a 

m 


whole  is  bilaterally  symmetrical,  each  of  the  wings  has  an  un- 
symmetrical  bilateralness :  the  side  next  the  axis  being  larger 
than  the  remoter  side.  How  does  this  happen?  Fig.  212, 
which  is  a  diagrammatic  section  down  the  mid-rib  of  the 
leaf,  showing  its  inclined  attitude  and  the  positions  of  the 


THE  SHAPES  OP  LEAVES.  155 

wings  a,  ~b,  c,  will  make  the  cause  clear.  As  the  wings 
overlap,  like  the  bars  of  a  Venetian  blind,  each  intercepts 
some  light  from  the  one  below  it;  and  the  one  below  it 
thus  suffers  more  on  its  distal  side  than  on  its  proximal  side. 
Hence  the  smaller  development  of  the  distal  side.  That  this 
is  the  cause  is  further  shown  by  the  proportion  that  is  main- 
tained between  the  degree  of  obscuration  and  the  degree  of 
non-development;  for  this  unlikeness  is  greater  between  the 
two  sides  a  and  a',  than  between  b  and  &'  or  c  and  c',  at  the 
same  time  that  the  interference  is  greater  in  the  lower  wings 
than  in  the  upper.  Of  course  in  this  case  and  in  the  kindred 
cases  hereafter  similarly  interpreted,  it  is  not  meant  that 
this  differentiation  is  consequent  solely,  or  even  chief!}',  on 
the  differential  actions  experienced  by  the  individual  plant. 
Though  there  is  good  reason  to  believe  that  the  rate  of  growth 
in  each  part  of  each  leaf  is  affected  by  the  incidence  of  light, 
yet  contrasts  so  marked  and  so  systematic  as  these  are  not 
explicable  without  taking  into  account  the  inheritance  of 
modifications  either  functionally  caused  or  caused  by  spon- 
taneous variation.  Clearly,  the  tendency  will  be  towards 
the  preservation  of  a  plant  which  distributes  its  chlorophyll 
in  the  most  advantageous  way ;  and  hence  there  will  always  be 
a  gravitation  towards  a  form  in  which  shaded  parts  of  leaves 
are  undeveloped. 

§  229.  From  compound  leaves  to  simple  ones,  we  find 
transitions  in  leaves  of  which  the  divisions  are  partial  in- 
stead of  total;  and  in  these  we  see,  with  equal  clearness,  the 
relations  between  forms  and  positions  that  have  been  traced 
thus  far.  Fig.  213  is  the  leaf  of  a  Winter-aconite  in  which, 
round  a  vertical  petiole,  there  is  a  radial  distribution  of  half- 
separated  leaflets.  The  Cecropia-leaf,  Fig.  214,  shows  us  a 
two-sided  development  of  the  parts  beginning  to  modify, 
but  not  obliterating,  the  all-sided  arrangement;  and  this 
mixed  symmetry  occurs  under  conditions  that  are  interme- 
diate. A  more  marked  degree  of  the  same  relation  is  pre- 


156 


MORPHOLOGICAL   DEVELOPMENT. 


sented  in  the  leaf  of  the  Lady's  Mantle,  Fig.  215.      And 
then  in  the  Sycamore  and  the  Vine,  we  have  a  cleft  type  of 


leaf  in  which  a  decided  hilateralness  of  form  co-exists  with  a 
decided  bilateralness  of  conditions. 

The  quite  simple  leaves  to  which  we  now  descend,  exhibit, 
very  distinctly,  a  parallel  series  of  facts.  Where  they  grow 
up  on  long  and  completely-independent  foot-stalks,  without 
definite  subordination  to  some  central  vertical  axis,  the 
leaves-  of  water-plants  are  symmetrically  peltate.  Of  this 
the  sacred  Indian-bean,  Fig.  216,  furnishes  an  example.  Here 
there  is  only  a  trace  of  bilateralness  in  the  venation  of  the 
leaf,  corresponding  to  the  very  small  difference  of  the  con- 
ditions on  the  proximal  and  distal  sides.  In  the  Victoria 
regia,  Fig.  217,  the  foot-stalks,  though  radiating  almost 
horizontally  from  a  centre,  are  so  long  as  to  keep  the  leaves 
quite  remote  from  one  another;  and  in  it  each  leaf  is  almost 
symmetrically  peltate,  with  a  bilateralness  indicated  only  by 
a  seam  over  the  line  of  the  foot-stalk.  The  leaves  of  the 
Nymphcea,  Fig.  218,  more  closely  clustered,  and  having  less 


room  transversely  than  longitudinally,  exhibit  a  marked 
advance  to  the  two-sided  form;  not  only  in  the  excess  of 
the  length  over  the  breadth,  but  in  the  existence  of  a  cleft, 


THE  SHAPES  OP  LEAVES. 


157 


where  in  the  Victoria  regia  there  is  merely  a  seam.  Among 
land-plants  similar  forms  are  found  under  analogous  condi- 
tions. The  common  Hydrocotyle,  Fig.  219,  which  sends 


up  direct  from  its  roots  a  few  almost  upright  leaf-stalks,  has 
these  surmounted  by  peltate  leaves;  which  leaves,  however, 
diverge  slightly  from  radial  symmetry  in  correspondence  with 
the  slight  contrast  of  circumstances  which  their  grouping  in- 
volves. Another  case  is  supplied  by  the  Nasturtium,  Fig. 
220,  which  combines  the  characters — a  creeping  stem,  long 
leaf-stalks  growing  up  at  right  angles  to  it,  and  unsymme- 
trically  peltate  leaves,  of  which  the  least  dimension  is,  on 
the  average,  towards  the  stem.  But  perhaps  the  most 
striking  illustration  is  that  furnished  by  the  Cotyledon  umbi- 
licus, Fig.  221,  in  which  different  kinds  of  symmetry  occur 
in  the  leaves  of  the  same  plant,  along  with  differences  in  their 
relations  to  conditions.  The  root-leaves,  a,  growing  up  on 
vertical  petioles  before  the  flower-stalk  makes  its  appearance, 
are  symmetrically  peltate;  while  the  leaves  which  subse- 
quently grow  out  of  the  flower-stalk,  &,  are  at  the  bottom 
transitionally  bilateral,  and  higher  up  completely  bilateral. 

That  the  bilateral  form  of  leaf  is  the  ordinary  form, 
corresponds  with  the  fact  that,  ordinarily,  the  circum- 
stances of  the  leaf  are  different  in  the  direction  of  the  plant's 
axis  from  what  they  are  in  the  opposite  direction,  while 


158        MORPHOLOGICAL  DEVELOPMENT. 

transversely  the  circumstances  are  alike.  It  is  needless  to 
give  diagrams  to  illustrate  this  extremely  familiar  truth. 
Whether  they  are  broad  or  long,  oval  or  heart-shaped,  pointed 
or  obtuse,  the  leaves  of  most  trees  and  plants  will  be  remem- 
bered by  all  as  having  the  ends  by  which  they  are  attached 
unlike  the  free  ends,  while  the  two  sides  are  alike.  And  it  will 
also  be  remembered  that  these  equalities  and  inequalities  of 
development  correspond  with  the  equalities  and  inequalities 
in  the  incidence  of  forces. 

§  230.  A  confirmation  that  is  interesting  and  important, 
is  furnished  by  the  cases  in  which  leaves  present  unsymme- 
trical  forms  in  positions  where  their  parts  are  unsymmetri- 
cally  related  to  the  environment.  A  considerable  deviation 
from  bilateral  symmetry  may  be  seen  in  a  leaf  which  habitu- 
ally so  carries  itself,  that  the  half  on  the  one  side  of  the  mid-rib 
is  more  shaded  than  the  other  half.  The  drooping  branches  of 
the  Lime,  delineated  in  Fig.  222,  show  us  leaves  so  arranged 


and  so  modified.  On  examining  their  attitudes  and  their 
relations  one  to  another,  it  will  be  found  that  each  leaf  is  so 
inclined  that  the  half  of  it  next  the  shoot  grows  over  the 
shoot  and  gets  plenty  of  light;  while  the  other  half  so  hangs 
down  that  it  comes  a  good  deal  into  the  shade  of  the  pre- 
ceding leaf.  The  result  is  that  having  leaves  which  fall  into 
these  positions,  the  species  profits  by  a  large  development  of 
the  exposed  halves;  and  by  survival  of  the  fittest,  acting 
along  with  the  direct  effect  of  extra  exposure,  this  modifi- 
cation becomes  established.  How  unquestionable  is  the 
connexion  between  the  relative  positions  of  the  halves  and 
their  relative  developments,  will  be  admitted  on  observing  a 


THE  SHAPES  OF  LEAVES.  159 

converse  ease.  Fig.  223  represents  a  shoot  of  Strobilantlies 
glomeratus.  Here  the  leaves  are  so  set  on  the  stem  that  the 
inner  half  of  each  leaf  is  shaded  by  the  subsequently-formed 
leaf,  while  its  outer  half  is  not  thus  shaded ;  and  here  we  find 
the  inner  half  less  developed  than  the  outer  half.  But  the 
most  conclusive  evidence  of  this  relation  between  unsymme- 
trical  form  and  unsymmetrical  distribution  of  surrounding 
forces,  is  supplied  by  the  genus  Begonia;  for  in  it  we  have 
a  manifest  proportion  between  the  degree  of  the  alleged 
effect  and  the  degree  of  the  alleged  cause.  These  plants 
produce  their  leaves  in  pairs,  in  such  ways  that  the  connate 
leaves  interfere  with  one  another,  much  or  little  according 
as  the  foot-stalks  are  short  or  long;  and  the  result  is  a  cor- 
relative divergence  from  symmetry.  In  Begonia  nelumbii- 
folia,  which  has  petioles  so  long  that  the  connate  leaves  are  not 
kept  close  together,  there  is  but  little  deviation  from  a  bilater- 
ally-peltate form;  whereas,  accompanying  the  comparatively 
marked  and  constant  proximity  in  B.  pruinata,  Fig.  224?  we 
see  a  more  decidedly  unsymmetrical  shape;  and  in  B. 
mahringii,  Fig.  225,  the  modification  thus  caused  is  pushed 
so  far  as  to  destroy  the  peltate  structure.* 

§  231.  Again,  then,  we  are  taught  the  same  truth.    Here, 
as  before,  we  see  that  homologous  units  of  any  order  become 

*  We  may  note  that  some  of  these  leaves,  as  those  of  the  Lime,  furnish 
indications  of  the  ratio  which  exists  between  the  effects  of  individual  circum- 
stances and  those  of  typical  tendencies.  On  the  one  hand,  the  leaves  borne 
by  these  drooping  branches  of  the  Lime  are  with  hardly  an  exception  unsym- 
metrical more  or  less  decidedly,  even  in  positions  where  the  causes  of  unsym- 
metry  are  not  in  action :  a  fact  showing  us  the  repetition  of  the  type  irrespec- 
tive of  the  conditions.  On  the  other  hand,  the  degree  of  deviation  from 
symmetry  is  extremely  variable,  even  on  the  same  shoot :  a  fact  proving  that 
the  circumstances  of  the  individual  leaf  are  influential  in  modifying  its  form. 
But  the  most  striking  evidence  of  this  direct  modification  is  afforded  by  the 
suckers  of  the  Lime.  Growing,  as  these  do,  in  approximately  upright  atti- 
tudes, the  leaves  they  bear  do  not  stand  to  one  another  in  the  way  above 
described,  and  the  causes  of  unsymmetry  are  not  in  action ;  and  here,  though 
there  is  a  general  leaning  to  the  unsymmetrical  form,  a  large  proportion  of  the 
leaves  become  quite  symmetrical. 


160        MORPHOLOGICAL  DEVELOPMENT. 

differentiated  in  proportion  as  their  relations  to  incident 
forces  become  different.  And  here,  as  before,  we  see  that  in 
each  unit,  considered  by  itself,  the  differences  of  dimension 
are  greatest  in  those  directions  in  which  the  parts  are  most 
differently  conditioned;  while  there  are  no  differences  be- 
tween the  dimensions  of  the  parts  that  are  not  differently 
conditioned.* 

*  It  was  by  an  observation  on  the  forms  of  leaves,  that  I  was  first  led  to 
the  views  set  forth  in  the  preceding  and  succeeding  chapters  on  the  mor- 
phological differentiation  of  plants  and  animals.  In  the  year  1851,  during 
a  country  ramble  in  which  the  structures  of  plants  had  been  a  topic  of  con- 
versation with  a  friend — Mr.  G.  H.  Lewes — I  happened  to  pick  up  the  leaf 
of  a  buttercup,  and,  drawing  it  by  its  foot-stalk  through  my  fingers  so  as  to 
thrust  together  its  deeply-cleft  divisions,  observed  that  its  palmate  and  almost 
radial  form  was  changed  into  a  bilateral  one ;  and  that  were  the  divisions  to 
grow  together  in  this  new  position,  an  ordinary  bilateral  leaf  would  result. 
Joining  this  observation  with  the  familiar  fact  that  leaves,  in  common  with 
the  larger  members  of  plants,  habitually  turn  themselves  to  the  light,  it 
occurred  to  me  that  a  natural  change  in  the  circumstances  of  the  leaf  might 
readily  cause  such  a  modification  of  form  as  that  which  I  had  produced  arti- 
ficially. If,  as  they  often  do  with  plants,  soil  and  climate  were  greatly  to 
change  the  habit  of  the  buttercup,  making  it  branched  and  shrub-like ;  and  if 
these  palmate  leaves  were  thus  much  overshadowed  by  one  another ;  would 
not  the  inner  segments  of  the  leaves  grow  towards  the  periphery  of  the  plant 
where  the  light  was  greatest,  and  so  change  the  palmate  form  into  a  more 
decidedly  bilateral  form  ?  Immediately  I  began  to  look  round  for  evidence  of 
the  relation  between  the  forms  of  leaves  and  the  general  characters  of  the 
plants  they  belong  to ;  and  soon  found  some  signs  of  connexion.  Certain 
anomalies,  or  seeming  anomalies,  however,  prevented  me  from  then  pursuing 
the  inquiry  much  further.  But  consideration  cleared  up  these  difficulties; 
and  the  idea  afterwards  widened  into  the  general  doctrine  here  elaborated. 
Occupation  with  other  things  prevented  me  from  giving  expression  to  this 
general  doctrine  until  Jan.  1859;  when  I  published  an  outline  of  it  in  the 
Medico-Chirurgical  Review. 


CHAPTER  X. 


THE   SHAPES   OF   FLOWERS. 


§  232.  FOLLOWING  an  order  like  that  of  preceding  chap- 
ters, let  us  first  note  a  few  typical  facts  respecting  the  forms  of 
clusters  of  flowers,  apart  from  the  forms  of  the  flowers  them- 
selves. Two  kindred  kinds  of  Leguminosce  serve  to  show  how 
the  members  of  clusters  are  distributed  in  an  all-sided  manner 
or  in  a  two-sided  manner,  according  as  the  circumstances 
are  alike  on  all  sides  or  alike  on  only  two  sides.  In  Hippo- 
crepis,  represented  in  Fig.  226,  the  flowers  growing  at  the  end 
of  a  vertical  stem,  are  arranged 
round  it  in  radial  symmetry. 
Contrariwise  in  Melilotus,  Fig. 
227,  where  the  axillary  stem 
bearing  the  flowers  is  so 
placed  in  relation  to  the  main 
stem,  that  its  outer  and  inner 
faces  are  differently  condi- 
tioned, the  flowers  are  all  on 
the  outer  face :  the  cluster  is 
bilaterally  symmetrical,  since 
it  may  be  cut  into  approx- 
imately equal  and  similar 
groups  by  a  vertical  plane  passing  through  the  main  axis. 

Plants  of  this  same  tribe  furnish  clusters  of  intermediate 
characters   having   intermediate   conditions.       Among   these, 
as  among  the  clusters  which  other  types  present,  may  be 
&  161 


162       MORPHOLOGICAL  DEVELOPMENT. 

found  some  in  which  conformity  to  the  general  law  is  not 
obvious.  The  discussion  of  these  apparent  anomalies  would 
carry  us  too  much  out  of  our  course.  A  clue  to  the  expla- 
nation of  them  will,  I  believe,  be  found  in  the  explanation 
presently  to  be  given  of  certain  kindred  anomalies  in  the 
forms  of  individual  flowers. 

§  233.  The  radially-symmetrical  form  is  common  to  all 
individual  flowers  that  have  vertical  axes.  In  plants  which 
are  practically  if  not  literally  uniaxial,  and  bear  their  flowers 
at  the  ends  of  upright  stalks,  so  that  the  faces  open  hori- 
zontally, the  petals  are  disposed  in  an  all-sided  way.  Cro- 
cuses, Tulips,  and  Poppies  are  familiar  examples  of  this 
structure  occurring  under  these  conditions.  A  Bammculus 
flower,  Fig.  228,  will  serve  as  a  typical 
one.  Similarly,  flowers  which  have 
peduncles  flexible  enough  to  let  them 
hang  directly  downwards,  and  are  not 
laterally  incommoded,  are  also  radial; 
as  in  the  Fuchsia,  Fig.  229,  as  in  Cycla- 
men, Hyacinth,  &c.  These  relations  of 
form  to  position  are,  I  believe,  uniform.  Though  some  flowers 
carried  at  the  ends  of  upright  or  downright  stems  have 
oblique  shapes,  it  is  only  when  they  have  inclined  axes  or 
are  not  equally  conditioned  all  round.  No  solitary  flower 
having  an  axis  habitually  vertical,  presents  a  bilateral  form. 
This  is  as  we  should  expect;  since  flowers  which  open  out 
their  faces  horizontally,  whether  facing  upwards  or  down- 
wards, are,  on  the  average,  similarly  affected  on  all  sides. 

At  first  it  seems  that  flowers  thus  placed  should  alone  be 
radial;  but  further  consideration  discloses  conditions  under 
which  this  type  of  symmetry  may  exist  in  flowers  otherwise 
placed.  Kemembering  that  the  radial  form  is  the  primitive 
form — that,  morphologically  speaking,  it  results  from  the 
contraction  into  a  whorl,  of  parts  that  are  originally  arranged 
in  the  same  spiral  succession  as  the  leaves;  we  must  expect 


THE  SHAPES  OP  FLOWERS.  163 

it  to  continue  wherever  there  are  no  forces  tending  to  change 
it.  What  now  must  be  the  forces  tending  to  change  it? 
They  must  be  forces  which  do  not  simply  affect  differently 
the  different  parts  of  an  individual  flower.  They  must  be 
forces  which  affect  in  like  contrasted  ways  the  homologous 
parts  of  other  individual  flowers,  both  on  the  same  plant  and 
on  surrounding  plants  of  the  same  species.  A  permanent 
modification  can  be  expected  only  in  cases  where,  by  inherit- 
ance, the  effects  of  the  modifying  causes  accumulate.  That 
they  may  accumulate  the  flowers  must  keep  themselves  so 
related  to  the  environment,  that  the  homologous  parts  may, 
generation  after  generation,  be  subjected  to  like  differentiating 
forces.  Hence,  among  a  plant's  flowers  which  maintain  no 
uniformity  in  the  relations  of  their  parts  to  surrounding  in- 
fluences, the  radial  form  will  continue.  Let  us  glance  at  the 
several  causes  which  entail  this  variability.  When 

flowers  are  borne  on  many  branches,  which  have  all  inclina- 
tions from  the  vertical  to  the  horizontal — as  are  the  flowers 
of  the  Apple,  the  Plum,  the  Hawthorn — they  are  placed  in 
countless  different  attitudes.  Consequently,  any  spontaneous 
variation  in  shape  which  might  be  advantageous  were  the 
attitude  constant,  is  not  likely  to  be  advantageous;  and  any 
functionally-produced  modification  in  one  flower,  is  likely  to 
be  neutralized  in  offspring  by  some  opposite  functionally-pro- 
duced modification  in  another  flower.  It  is  quite  compre- 
hensible, therefore,  that  irregularly-branched  plants  should 
thus  preserve  their  laterally-borne  flowers  from  under- 
going permanent  devia- 
tions from  their  primi- 
tive radial  symmetry. 
Fig.  230,  representing  a 
blossoming  twig  of  the 
Blackthorn,  illustrates 
this.  Again,  upright 
panicles,  such  as  those  of  the  Saxifrage  exemplified  in 
Fig.  231,  and  irregular  terminal  groups  of  flowers  other- 


164        MORPHOLOGICAL  DEVELOPMENT. 

wise  named,  furnish  conditions  under  which  there  is  simi- 
larly an  absence  of  determinate  relations  between  the 
parts  of  the  flowers  and  the  incident  forces;  and  hence  an 
absence  of  bilateralness.  This  inconstancy  of  rela- 

tive position  is  produced  in  various  other  ways — by  extreme 
flexibility  of  the  stems,  as  in  the  Blue- bell;  by  the  ten- 
dency of  the  peduncles  to  curl  to  a  greater  or  less  extent 
in  diverse  directions,  as  in  Pyrola;  by  special  twistings  of 
the  peduncles,  differing  in  degree  in  different  individuals, 
as  in  Convolvulus;  by  unusual  laxity  of  the  petals,  as  in 
Ly  thrum.  Elsewhere  the  like  general  result  arises  from  a 
progressive  change  of  attitude,  as  in  Myosotis,  the  stem  of 
which  as  it  unfolds  causes  each  flower  to  undergo  a  transition 
from  an  upward  position  of  the  mouth  to  a  lateral  position; 
or  as  in  most  Cruciferce,  where  the  like  effect  follows  from  an 
altered  direction  of  the  peduncle. 

There  are,  however,  certain  seemingly-anomalous  cases 
where  radial  symmetry  is  maintained  by  laterally-placed 
flowers,  which  keep  their  parts  in  relative  positions  that  are 
tolerably  constant.  The  explanation  of  these  exceptions  is 
not  manifest.  It  is  only  when  we  take  into  account  certain 
incident  actions  liable  to  be  left  unremembered,  that  we  find 
a  probable  solution.  It  will  be  most  convenient  to  postpone 
the  consideration  of  these  cases  until  we  have  reached  the 
general  rule  to  which  they  are  exceptions. 

§  234.  Transitions  varying  in  degree  from  the  radial  to- 
wards the  bilateral,  are  common  in  flowers  that  are  borne  at 
the  ends  of  branches  or  axes  which  are  inclined  in  tolerably 
constant  ways.  We  may  see  this  in  sundry  garden  flowers 
233  __  .J-/X  such  as  Petunia,  or  such  as 
Isoloma  and  Acliimenes, 
shown  in  Figs.  232  and  233. 
If  these  plants  be  examined, 
it  will  be  perceived  that  the 
mode  of  growth  makes  the  flower  unfold  in  a  partially  one- 


THE  SHAPES  OP   FLOWERS.  165 

sided  position;  that  its  parts  of  attachment  have  rigidity 
sufficient  to  prevent  this  attitude  from  being  very  much  in- 
terfered with;  and  that  though  the  individual  flowers  vary 
somewhat  in  their  attitudes,  they  do  not  vary  to  the  extent  of 
neutralizing  the  differentiating  conditions  —  there  remains  an 
average  divergence  from  a  horizontal  unfolding  of  the  flower, 
to  account  for  its  divergence  from  radial  symmetry. 

We  pass  insensibly  from  forms  like  these,  to  forms  having 
bilateral  symmetry  strongly  pronounced.  Some  such  forms 
occur  among  flowers  that  grow  at  the  ends  of  upright  stems; 
as  in  Pinguicula,  and  in  the  Violet  tribe.  But  this  happens 
only  where,  in  successive  generations,  the  flower  unfolds  its 
parts  sideways  in  constant  relative  positions.  And  in  the 
immense  majority  of  flowers  having  well-marked  two-sided 
forms,  the  habitual  exposure  of  the  different  parts  to  different 
sets  of  forces,  is  effectually  secured  by  the  mode  of  placing. 
As  illustrations,  I  may  name  the  genera  —  Orchis,  Utricularia, 
Salvia,  Salix,  Delphinium,  Mentha,  Teucrium,  Ajuga,  Ballota, 
GaUopsis,  Lamium,  Stachys,  Nepeta,  Marrubium,  Calamintha, 
Melittis,  Prunella,  Scutellaria,  Bartsia,  Euphrasia,  Rhinan- 
thus,  Melampyrum,  Pedicularis,  Linaria,  Digitalis,  Oro- 
banche,  Fumaria,  &c.;  to  which  may  be  added  all  the  Grasses 
and  all  the  Papilionacece.  In  most  of  these  cases  the  flowers, 
being  sessile  on  the  sides  of  upright  stems,  are  kept  in  quite 
fixed  attitudes  ;  and  in  the  other  cases  the  peduncles  are  very 
short,  or  else  stiff  enough  to  secure  general  uniformity  in  the 
positions.  A  few  of  the  more  marked  types  are  shown  in 
Figs.  234  to  241. 


234 


Very  instructive  evidences  here  meet  us.  Sometimes  within 
the  limits  of  one  genus  we  find  radial  flowers,  bilateral 
flowers,  and  flowers  of  intermediate  characters.  The  genus 


166        MORPHOLOGICAL  DEVELOPMENT. 

Begonia  may  be  instanced.  In  B.  rigida  the  flowers,  various 
in  their  attitudes,  are  in  their  more  conspicuous  characters 
radial:  though  there  is  a  certain  bilateralness  in  the  calyx, 
the  five  petals  are  symmetrically  disposed  all  round.  B. 
Wageneriana  furnishes  two  forms  of  flowers.  On  the  same  in- 
dividual plant  may  be  found  radial  flowers  like  Fig.  242,  and 
others,  like  Fig.  243,  which  are  merging  into  the  bilateral. 
More  decided  is  the  bilateralness  in  B.  albo-coccinea,  Fig.  244; 
and  still  more  in  B.  nitida,  Fig.  245.  While  in  B.  heraclei- 


Z42  343  Z44 


folia,  Fig.  246,  the  change  reaches  its  extreme  by  the  dis- 
appearance of  the  lateral  petals.  On  examining  the  modes  of 
growth  in  these  several  species,  they  will  be  seen  to  explain 
these  changes  in  the  manner  alleged.  Even 

more  conclusive  are  the  nearly-allied  transformations  occur- 
ring in  artificially-produced  varieties  of  the  same  species. 
Gloxinia  may  be  named  in  illustration.  In  Fig.  247  is  repre- 
sented one  of  the  ordinary  forms,  which  shows  us  bilateralness 
of  shape  along  with  a  mode  of  growth  that  renders  the  condi- 
tions alike  on  the  two  sides  while  different  above  and  below. 
But  in  G.  erecta,  Fig.  248,  we 
have  the  flower  assuming  an 
upright  attitude,  and  at  the 
same  time  assuming  the  radial 
type.  This  is  not  to  be  inter- 
preted as  a  production  of  ra- 
dial symmetry  out  of  bilateral  symmetry,  under  the  action  of 
the  appropriate  conditions.  It  is  rather  to  be  taken  as  a  case 
of  what  is  termed  "peloria" — a  reversion  to  the  primitive 
radial  type,  from  which  the  bilateral  modification  had  been 
derived.  The  significant  inference  to  be  drawn  from  it  is, 


THE  SHAPES  OF  FLOWERS.  167 

that  this  primitive  radial  type  had  an  upright  attitude;  and 
that  the  derivation  of  a  bilateral  type  from  it,  occurred  along 
with  the  assumption  of  an  inclined  attitude. 

We  come  now  to  a  group  of  cases  above  referred  to,  in 
which  radial  symmetry  continues  to  co-exist  with  that  con- 
stant lateral  attitude  ordinarily  accompanied  by  the  two- 
sided  form.  Two  examples  will  suffice:  one  a  very  large 
flower,  the  Hollyhock,  and  the  other  a  very  small  flower,  the 
Agrimony.  Why  does  the  radial  form  here  remain  unchanged  ? 
and  how  does  its  continuance  consist  with  the  alleged  general 
law? 

Until  quite  recently  I  have  been  unable  to  find  any  prob- 
able answers  to  these  questions.  When  the  difficulty  first 
presented  itself,  I  could  think  of  no  other  possible  cause  for 
the  anomaly,  than  that  the  parts  of  the  Hollyhock-flower, 
unfolding  spirally  as  they  do,  might  have  different  degrees 
of  spiral  twist  in  different  flowers,  and  might  thus  not  be 
unfolded  in  sufficiently-constant  positions.  But  this  seemed 
a  questionable  interpretation;  and  one  which  did  not  ob- 
viously apply  to  the  case  of  the  Agrimony.  It  was  only  on 
inquiring  what  are  the  special  causes  of  modifications  in  the 
forms  of  flowers,  that  a  more  feasible  explanation  suggested 
itself;  and  this  would  probably  never  have  suggested  itself, 
had  not  Mr.  Darwin's  investigations  into  the  fertilization  of 
Orchids  led  me  to  take  into  account  an  unnoticed  agency. 

The  actions  which  affect  the  forms  of  leaves,  affect  much 
less  decidedly  the  forms  of  flowers;  and  the  forms  of  flowers 
are  influenced  by  actions  which  do  not  influence  the  forms  of 
leaves.  Partly  through  the  direct  action  of  incident  forces 
and  partly  through  the  indirect  action  of  natural  selection, 
leaves  get  their  parts  distributed  in  ways  that  most  facilitate 
their  assimilative  functions,  under  the  circumstances  in  which 
they  are  placed;  and  their  several  types  of  symmetry  are  thus 
explicable.  But  in  flowers,  the  petals  and  fructifying  organs 
of  which  do  not  contain  chlorophyll,  the  tendency  to  grow 
most  where  the  supply  of  light  is  greatest,  is  less  decided,  if 


168  MORPHOLOGICAL  DEVELOPMENT. 

not  absent;  and  a  shape  otherwise  determined  is  hence  less 
liable  to  alter  in  consequence  of  altered  relations  to  sun  and 
air.  Gravity,  too,  must  be  comparatively  ineffective  in  caus- 
ing modifications:  the  smaller  sizes  of  the  parts,  as  well  as 
their  modes  of  attachment,  giving  them  greater  relative 
rigidity.  Not,  indeed,  that  these  incident  forces  of  the  inor- 
ganic world  are  here  quite  inoperative.  Fig. 
249,  representing  a  species  of  Campanula, 
shows  that  the  developments  of  individual 
flowers  are  somewhat  modified  by  the  rela- 
tions of  their  parts  to  general  conditions.  But 
the  fact  to  be  observed  is,  that  the  extreme 
transformations  which  flowers  undergo  are 
not  likely  to  be  thus  caused:  some  further 
cause  must  be  sought.  And  if  we  bear  in 
mind  the  functions  of  flowers,  we  shall  find  in 
their  adaptations  to  these  functions,  under  conditions  that  are 
extremely  varied,  an  adequate  cause  for  the  different  types 
of  symmetry,  as  well  as  for  the  exceptions  to  them.  Flowers 
are  parts  in  which  fertilization  is  effected;  and  the  active 
agents  of  this  fertilization  are  insects — bees,  moths,  butter- 
flies, &c.  Mr.  Darwin  has  shown  in  many  cases,  that  the 
forms  and  positions  of  the  essential  organs  of  fructification, 
are  such  as  to  facilitate  the  actions  of  insects  in  trans- 
ferring pollen  from  the  anthers  of  one  flower  to  the  pistil  of 
another — an  arrangement  produced  by  natural  selection. 
And  here  we  shall  find  reason  for  concluding,  that  the  forms 
and  positions  of  those  subsidiary  parts  which  give  their 
shapes  to  flowers,  similarly  arise  by  the  survival  of  indi- 
viduals which  have  the  subsidiary  parts  so  adjusted  as  to  aid 
this  fertilizing  process — the  deviations  from  radial  symmetry 
being  among  such  adjustments.  The  reasoning  is  as  fol- 
lows. So  long  as  the  axis  of  a  flower  is  vertical  and 
the  conditions  are  similar  all  round,  a  bee  or  butterfly  alight- 
ing on  it,  will  be  as  likely  to  come  from  one  side  as  from 
another;  and  hence,  hindrance  rather  than  facilitation  would 


THE  SHAPES  OF   FLOWERS.  169 

result  if  the  several  sides  of  the  flower  did  not  afford  it  equally 
free  access.  In  like  manner,  flowers  which  are  distributed 
over  a  plant  in  such  ways  that  their  discs  open  out  on 
planes  of  all  directions  and  inclinations,  will  have  no  tend- 
ency to  lose  their  radial  symmetry;  since,  on  the  average, 
no  part  of  the  periphery  is  differently  related  to  insect- 
agency  from  any  other  part.  But  flowers  so  fixed  as  to 
open  out  sideways  in  tolerably-constant  attitudes,  have 
their  petals  differently  related  to  insect-agency.  A  bee  or 
butterfly  coming  to  a  laterally-growing  flower,  does  not  settle 
on  it  in  one  way  as  readily  as  in  another;  but  almost  of 
necessity  settles  with  the  axis  of  its  body  inclined  upwards 
towards  the  stem  of  the  plant.  Hence  the  side-petals  of  a 
flower  so  fixed,  habitually  stand  to  the  alighting  insect  in 
relations  different  from  those  in  which  the  upper  and  lower 
petals  stand;  and  the  upper  and  lower  petals  differ  from  one 
another  in  their  relations  to  it.  If,  then,  there  so  arises  an 
habitual  attitude  of  the  insect  towards  the  petals,  there  is 
likely  to  be  some  arrangement  of  the  petals  that  will  be 
most  convenient  to  the  insect — will  most  facilitate  its  entrance 
into  the  flower.  Thus  we  see  in  many  cases,  that  a  long 
undermost  petal  or  lip,  by  enabling  the  insect  to  settle  in 
such  way  as  to  bring  its  head  opposite  to  the  opening  of  the 
tube,  aids  its  fertilizing  agency.  But  whatever  be  the  special 
modifications  of  the  corolla  which  facilitate  the  actions  of 
the  particular  insects  concerned,  all  of  them  will  conduce  to 
bilateral  symmetry;  since  they  will  be  alike  for  the  two  sides 
but  unlike  for  the  top  and  bottom.  And  now  we 

are  prepared  for  understanding  the  exceptions.  Flowers 
growing  sideways  can  become  thus  adapted  by  survival  of 
the  fittest,  only  if  they  are  of  such  sizes  and  structures  that 
insect-agency  can  affect  them  in  the  way  described.  But 
in  the  plants  named  above,  this  condition  is  not  fulfilled.  A 
Hollyhock-flower  is  so  open,  as  well  as  so  large,  that  its  petals 
are  not  in  any  appreciable  degree  differently  related  to  the 
insects  which  visit  it.  On  the  other  hand,  the  flower  of  the 


170        MORPHOLOGICAL  DEVELOPMENT. 

Agrimony  is  so  small,  that  unless  visited  by  insects  of  a 
corresponding  size  which  settle  as  bees  and  butterflies  settle, 
its  parts  will  not  be  affected  in  the  alleged  manner.  That 
all  anomalies  of  this  kind  can  at  once  be  satisfactorily  ex- 
plained, is  scarcely  to  be  expected :  the  circumstances  of  each 
case  have  to  be  studied.  But  it  seems  not  improbable  that 
they  are  due  to  causes  of  the  kind  indicated.* 

§  235.  We  have  already  glanced  at  clusters  of  flowers  for 
the  purpose  of  considering  their  shapes  as  clusters.  We  must 
now  return  to  them  to  observe  the  modifications  undergone 
by  their  component  flowers.  Among  these  occur  illustrations 
of  great  significance. 

An  example  of  transition  from  the  radial  to  the  bilateral 
form  in  clustered  flowers  of  the  same  species,  is  furnished  by 
the  cultivated  Geraniums,  called  by  florists  Pelargoniums. 
Some  of  these,  bearing  somewhat  small  terminal  clusters  of 
flowers,  which  are  closely  packed  together  with  their  faces 
almost  upwards,  have  radially-symmetrical  flowers.  But 
among  other  varieties  having  terminal  clusters  of  which  the 
members  are  mutually  thrust  on  one  side  by  crowding,  the 
flowers  depart  very  considerably  from  the  radial  shape 

*  It  is  objected  to  the  above  interpretation  that  "  many  flowers  of  sizes 
intermediate  between  the  Hollyhock  and  the  Agrimony  are  radially  sym- 
metrical and  yet  prow  sideways.  I  may  mention  various  Liliacece,  e.g.  Ch'o- 
rophytum,  Encomia,  Muscari,  AntJiericiim.  Sagittaria,  also,  has  many  of  its 
flowers  in  this  position.  Further,  if  the  higher  insects  alight  on  flowers  in 
a  definite  way.  as  they  do,  the  parts  of  the  flower  must  bear  different  rela- 
tions to  the  visiting  insect,  however  large,  so  that  flowers  unvisited  ought 
all  to  be  zygomorphic."  My  reply  is  that  in  the  sense  which  here  concerns 
UP,  the  different  petals  of  the  Hollyhock-flower  do  not  bear  different  rela- 
tions to  the  visiting  insect;  since,  practically,  the  upper  and  lateral  petals 
bear  no  physical  relations  at  all :  in  so  far  as  the  visiting  bee  is  concerned 
they  are  non-existent.  The  argument  implies  that  change  in  the  form  of  a 
flower  from  the  radial  to  the  bilateral  is  likely  to  take  place  only  when  the 
contact-relations  of  the  petals  to  the  visiting  insect,  arc  such  as  to  make  some 
forms  facilitate  its  action  more  than  others;  and  the  large  petals  of  the 
Hollyhock  cannot  facilitate  its  action  at  all.  In  respect  of  the  LViaccce 
instanced,  it  is  needful  to  inquire  whether  the  structures  are  such  that  this 
alleged  cause  of  bilateral  symmetry  can  come  into  play. 


THE  SHAPES  OF  FLOWERS.  171 

towards  the  bilateral  shape.  A  like  result  occurs  under  like 
conditions  in  Rhododendrons  and  Azaleas.  The  Verbena,  too, 
furnishes  an  illustration  of  radial  flowers  rendered  slightly 
two-sided  by  the  slight  two-sidedness  of  their  relations  to 
other  flowers  in  the  cluster.  And  among  the  Cruciferce  a 
kindred  case  occurs  in  the  cultivated  Candytuft. 

Evidence  of  a  somewhat  different  kind  is  offered  us  by 
clustered  flowers  in  which  the  peripheral  members  of  the 
clusters  differ  from  the  central  members;  and  this  evidence 
is  especially  significant  where  we  find  allied  species  that  do 
not  exhibit  the  deviation,  at  the  same  time  that  they  do  not 
fulfil  the  conditions  under  which  it  may  be  expected.  Thus, 
in  Scabiosa  succisa,  Fig.  250,  which  bears  its  numerous  small 
flowers  in  a  hemispherical  knob,  the  component  flowers, 
similarly  circumstanced,  are  all  equal  and  all  radial;  but  in 
Scabiosa  arvensis,  Fig.  251,  in  which  the  numerous  small 
flowers  form  a  flattened  disk 
only  the  confined  central  ones  ^ 
are  radial:  round  the  edge  the 
flowers  are  much  larger  and 
conspicuously  bilateral. 

But  the  most  remarkable 
and  most  conclusive  proofs  of  these  relations  between  forms 
and  positions,  are  those  given  by  the  clustered  flowers  called 
Umbelliferce.  In  some  cases,  as  where  the  component  flowers 
have  all  plenty  of  room,  or  where  the  surface  of  the  umbel  is 
more  or  less  globular,  the  modifications  are  not  conspicuous; 
but  where,  as  inViburnum, CJicerophyllum,  Anthriscus,  Torilis, 
Caucalis,  Daucus,  Tordylium,  &c.,  we  have  flowers  clustered 
in  such  ways  as  to  be  differently  conditioned,  we  find  a  num- 
ber of  modifications  that  are  marked  and  varied  in  propor- 
tion as  the  differences  of  conditions  are  marked  and  varied. 
In  Chcerophyllum,  where  the  flowers  of  each  umbellule  are 
closely  placed  so  as  to  form  a  flat  surface,  but  where  the 
umbellules  are  wide  apart  and  form  a  dispersed  umbel,  the 
umbellules  do  not  differ  from  one  another ;  though  among  the 


172 


MORPHOLOGICAL  DEVELOPMENT. 


flowers  of  each  umbellule  there  are  decided  differences:  the 
central  flowers  being  small  and  radial,  while  the  peripheral 
ones  are  large  and  bilateral.  But  in  other  genera,  where  not 
only  the  flowers  of  each  umbellule  but  also  the  umbellules 
themselves,  are  closely  clustered  into  a  flat  surface,  the  umbel- 
lules themselves  become  contrasted;  and  many  remarkable 
secondary  modifications  arise.  In  an  umbel  of  Heradeum, 
for  instance,  there  are  to  be  noted  the  facts; — first,  that  the 
external  umbellules  are  larger  than  the  internal  ones; 
second,  that  in  each  umbellule  the  central  flowers  are  less 
developed  than  the  peripheral  ones;  third,  that  this  greater 
development  of  the  peripheral  flowers  is  most  marked  in  the 
outer  umbellules ;  fourth,  that  it  is  most  marked  on  the  outer 
sides  of  the  outer  umbellules;  fifth,  that  while  the  interior 
flowers  of  each  umbellule  are  radial,  the  exterior  ones  are 
bilateral ;  sixth,  that  this  bilateralness  is  most  marked  in 
the  peripheral  flowers  of  the  peripheral  umbellules;  seventh, 
that  the  flowers  on  the  outer  sides  of  these  peripheral 
umbellules  are  those  in  which  the  bilateralness  reaches  a 
maximum;  and  eighth,  that  where  the  outer  umbellules 

touch  one  another,  the  flow- 
ers, being  unsymmetrically 
placed,  are  unsymmetrically 
bilateral.*  The  like  modifi- 
cations are  displayed,  though 
not  in  so  clearly-traceable  a 
way,  in  an  umbel  of  Tordy- 
Uum,  Fig.  252.  Considering 
how  obviously  these  various 
forms  are  related  to  the  vari- 
ous conditions,  we  should  be 
scarcely  able,  even  in  the 

*  I  had  intended  here  to  insert  a  figure  exhibiting  these  differences ;  but 
as  the  Cow -parsnip  does  not  flower  till  July,  and  as  I  can  find  no  drawing 
of  the  umbel  which  adequately  represents  its  details,  I  am  obliged  to  take 
another  instance. 


THE  SHAPES  OF  FLOWERS.  173 

absence  of  all  other  facts,  to  resist  the  conclusion  that  the 
differences  in  the  conditions  are  the  causes  of  the  differences 
in  the  forms. 

Composite  flowers  furnish  evidence  so  nearly  allied  to  that 
which  clustered  flowers  furnish,  that  we  may  fitly  glance 
at  them  under  the  same  head.  Such  a  common  type  of 
this  order  as  the  Sun-flower,  exempli- 
fies the  extremely  marked  difference 
which  arises  in  many  of  these  plants 
between  the  closely-packed  internal 
florets,  each  similarly  circumstanced  on 
all  sides,  and  the  external  florets,  not 
similarly  circumstanced  on  all  sides. 
In  Fig.  253,  representing  the  inner  and 
outer  florets  of  a  Daisy,  the  contrast  is 
marked  between  the  small  radial  corolla  of  the  one  and  the 
larger  bilateral  corolla  of  the  other.  In  many  cases,  how- 
ever, this  contrast  is  less  marked:  the  inner  florets  also 
having  their  outward-growing  prolongations — a  difference 
possibly  related  to  some  difference  in  the  habits  of  the  insects 
that  fertilize  them.  Nevertheless,  these  composite  flowers 
which  have  inner  florets  with  strap-shaped  corollas  out- 
wardly directed,  equally  conform  to  the  general  principle; 
both  in  the  radial  arrangement  of  the  assemblage  of  florets, 
and  in  the  bilateral  shape  of  each  floret;  which  has  its 
parts  alike  on  the  two  sides  of  a  line  passing  from  the  centre 
of  the  assemblage  to  the  circumference.  Certain 

other  members  of  this  order  fulfil  the  law  somewhat  differ- 
ently. In  Centaurea,  for  instance,  the  inner  florets  are  small 
and  vertical  in  direction,  while  the  outer  florets  are  large  and 
lateral  in  direction.  And  here  may  be  remarked,  in  passing, 
a  clear  indication  of  the  effect  which  great  flexibility  of  the 
petals  has  in  preventing  a  flower  from  losing  its  original 
radiate  form ;  for  while  in  C.  cyanus,  the  large  outward-grow- 
ing florets,  having  short,  stiff  divisions  of  the  corolla,  are 
decidedly  bilateral,  in  C.  scabiosa,  where  the  divisions  of  the 


174:       MORPHOLOGICAL  DEVELOPMENT. 

corolla  are  long  and  flexible,  the  radial  form  is  scarcely  at 
all  modified.  On  bearing  in  mind  the  probable  relations  of 
the  forms  to  insect-agency,  the  meaning  of  this  difference 
will  not  be  difficult  to  understand.* 

§  236.  In  extremely-varied  ways  there  are  thus  re-illus- 
trated among  flowers,  the  general  laws  of  form  which  leaves 
and  branches  and  entire  plants  disclose  to  us.  Composed  as 
each  cluster  of  flowers  is  of  individuals  that  are  originally 
similar;  and  composed  as  each  flower  is  of  homologous  foliar 
organs;  we  see  both  that  the  like  flowers  become  unlike  and 
the  like  parts  of  each  flower  become  unlike,  where  the  posi- 
tions involve  unlike  incidence  of  forces.  The  symmetry 
remains  radial  where  the  conditions  are  equal  all  round; 
shows  deviation  towards  two-sidedness  where  there  is  slight 
two-sidedness  of  conditions;  becomes  decidedly  bilateral 
where  the  conditions  are  decidedly  bilateral;  and  passes  into 
an  unsymmetrical  form  where  the  relations  to  the  environ- 
ment are  unsymmetrical. 

*  It  has  been  pointed  out  to  me  that  "  the  extreme  development  of  the 
corolla  so  often  found  in  the  outer  flowers  or  on  the  outer  side  of  the  outer 
flowers  in  closely  packed  inflorescences,  associated  as  it  often  is  with  disap- 
pearance of  stamens  or  carpels  or  both,  is  usually  put  down  to  specialization 
of  these  outer  flowers  for  attractive  purposes.  Since  the  whole  inflorescence 
is  increased  in  conspicuousness  by  such  a  modification,  it  is  supposed  that 
natural  selection  favoured  those  plants  which  sacrificed  a  portion  of  their 
seed-bearing  capacity  for  the  supposed  greater  advantage  of  securing  more 
insect  visits."  But  granting  this  interpretation,  it  may  still  be  held  that 
increase  of  attractiveness  due  to  increase  of  area  must  be  achieved  by  florets 
at  the  periphery,  and  that  their  ability  to  achieve  it  depends  on  their  having 
an  outer,  unoccupied,  space  which  the  inner  florets  have  not;  so  that,  though 
in  a  more  indirect  way,  their  different  development  is  determined  by  different 
exposure  to  conditions. 


CHAPTER  XI. 

THE    SHAPES   OF   VEGETAL   CELLS. 

§  237.  WE  come  now  to  aggregates  of  the  lowest  order. 
Already  something  has  been  said  (§  217)  concerning  the 
forms  of  those  morphological  units  which  exist  as  indepen- 
dent plants.  But  it  is  here  requisite  briefly  to  note  the 
modifications  undergone  by  them  where  they  become  compo- 
nents of  larger  plants. 

Of  the  numerous  cell-forms  which  are  found  in  the  tissues 
of  the  higher  plants,  it  will  suffice  to  give,  in  Fig.  254,  re- 
presenting a  section  of 
a  leaf,  a  single  example.  a 
In  this  it  will  be  seen 
that  the  cells  forming 
the  upper  and  lower  sur-  e 
faces,  a  and  &,  have  dif- 
ferences of  shape  related  d 
to  differences  in  the  inci-  j 
dence  of  forces:  they  are 
more  or  less  flattened  in 

relation  to  the  environment.  The  underneath  cells  at  c, 
form  a  class  which,  similarly  exposed  to  light  at  their 
outer  ends,  and,  as  we  may  assume,  largely  developed  in 
adjustment  to  their  active  assimilative  functions,  are,  by 
mutual  pressure,  made  to  grow  more  in  the  direction  of  their 
lengths  than  in  the  direction  of  their  breadths.  Then  on 
the  other  side  we  see  that  the  cells  d,  next  above  the  outer 
layer,  while  approximately  similar,  become  more  and  more 
dissimilar  as  they  diverge  from  the  surface,  ard  are  quite 

175 


176 


MORPHOLOGICAL  DEVELOPMENT. 


irregular  in  the  interior  e,  where  there  is  no  definiteness  in 
the  conditions  to  which  they  are  exposed.  Thus  the  diver- 
gences of  these  cells  from  primordial  sphericity  are  such  as 
correspond  with  unlikenesses  in  their  circumstances.  And 
throughout  the  more  complex  modifications  which  the  cells 
of  other  tissues  exhibit,  the  like  correspondences  hold. 

Among  plants  of  a  lower  order  of  aggregation,  we  have 
already  seen  how  cells  become  metamorphosed  as  they  become 
integrated  into  masses  having  definite  organizations.  The 
higher  Algce,  exemplified  in  Figs.  32,  34,  35,  show  this  very 
clearly.  Here  the  departure  from 
the  simple  cell-form  to  the  form 
of  an  elongated  prism,  is  mani- 
festly subordinated  to  the  con- 
trasts in  the  .relations  of  the 
parts.  And  it  is  interesting  to  ob- 
serve how,  in  one  of  the  branches 
of  Fig.  32,  we  pass  from  the  small, 
almost-spherical  cells  which  ter- 
minate the  branchlets,  to  the 
large,  much-modified  cells  which 
join  the  main  stem,  through  gra- 
dations obviously  related  in  their  changed  forms  to  the 
altered  actions  their  positions  expose  them  to. 

More  simply,  but  quite  as  conclusively,  do  the  inferior 
Algce,  of  which  Figs.   19 — 23  are  examples,  show  us  how 
J9 


cells  pass  from  their  original  spherical  symmetry  into  radial 
symmetry,  as  they  pass  from  a  state  in  which  they  are  simi- 


THE  SHAPES  OF  VEGETAL  CELLS. 


177 


larly-conditioned  on  all  sides,  to  a  state  in  which  two  of  their 
opposite  sides  or  ends  are  conditioned  in  ways  that  are  like 
one  another,  but  unlike  the  ways  in  which  all  other  sides  are 
conditioned. 

Still  more  instructive  are  the  morphological  differentia- 
tions of  those  protophytes  in  which  the  first  steps  towards  a 
higher  degree  of  integration  are  shown.  In  Fig.  10,  represent- 
ing one  of  the  transitional  forms  of  Desmidiacece,  it  is  to  be 
noted  that  besides  the  difference  between  the  transverse  and 
longitudinal  dimensions,  which  the  component  units  display 
in  common,  the  two  end-units  differ  from  the  rest :  they  have 


appendages  which  the  rest  have  not.  Once  more,  where  the 
integration  is  carried  on  in  such  ways  as  to  produce  not  strings 
but  clusters,  there  arise  contrasts  and  correspondences  just 
such  as  might  be  looked  for.  All  the  four  members  of  the 
group  shown  in  Fig.  12,  are  similarly  conditioned;  and  each 
of  them  has  a  bilateral  shape  answering  to  its  bilateral  rela- 
tions. In  Fig.  14  we  have  a  number  of  similarly-bilateral 
individuals  on  the  circumference,  including  a  central  in- 
dividual differing  from  the  rest  by  having  the  bilateral 
character  nearly  obliterated.  And  then,  in  Fig.  15,  we  have 
two  central  components  of  the  group,  deviating  more  deci- 
dedly from  those  that  surround  them.* 

*  One  of  my  critics  writes : — "  This  chapter  might  of  course  be  enormously 
extended,  not  only  as  in  the  preceding  ones  by  citation  of  quite  similar  cases, 
but  by  the  introduction  of  fresh  groups  of  cases." 


CHAPTER  XII. 


CHANGES   OF    SHAPE   OTHERWISE   CAUSED. 

§  238.  BESIDES  the  more  special  causes  of  modification  in 
the  shapes  of  plants  and  of  their  parts,  certain  more  general 
causes  must  be  briefly  noticed.  These  may  be  described  as 
consequences  of  variations  in  the  total  quantities  of  the 
matters  and  forces  furnished  to  plants  by  their  environments. 
Some  of  the  changes  of  form  so  produced  are  displayed  by 
plants  as  wholes,  and  others  only  by  their  parts.  We  will 
glance  at  them  in  this  order. 

§  239.  It  is  a  familiar  fact  that  luxuriant  shoots  have 
relatively-long  internodes;  and,  conversely,  that  a  shoot 
dwarfed  from  lack  of  sap,  has  its  nodes  closely  clustered:  a 
concomitant  result  being  that  the  lateral  axes,  where  these 
are  devek>ped,  become  in  the  one  case  far  apart  and  in  the 
other  case  near  together.  Fig.  255  represents  a  branch  to 
the  parts  of  which  the  longer  and 
shorter  internodes  so  resulting  give 
differential  characters.  A  whole  tree 
being  in  many  cases  simultaneously 
thus  affected  by  states  of  the  earth  or 
the  air,  all  parts  of  it  may  have  such 
variations  impressed  on  them;  and, 
indeed,  such  variations,  following  more 
or  less  regularly  the  changes  of  the 
seasons,  give  to  many  trees  manifest 


zxr 


CHANGES  OP  SHAPE  OTHERWISE  CAUSED.         179 

traits  of  structure.  In  Fig.  256,  a  shoot  of  PTiyllo cactus 
crenatus,  we  have  an  interesting  example  of  a  variation 
essentially  of  the  same  nature,  little  as  it  appears  to  be  so. 
For  each  of  the  lateral  indentations  is  here  the  seat  of  an 
axillary  bud;  and  these  we  see  are  separated  by  internpdes 
which,  becoming  broader  as  they  become  longer,  and  narrower 
as  they  become  shorter,  produce  changes  of  form  that  corre- 
spond with  changes  in  the  luxuriance  of  growth. 

To  complete  the  statement  it  must  be  added  that  these 
variations  of  nutrition  often  determine  the  development  or 
non-development  of  lateral  axes;  and  by  so  doing  cause  still 
more  marked  structural  differences.  The  Fox-glove  may  be 
named  as  a  plant  which  illustrates  this  truth.* 

§  240.  From  the  morphological  differentiations  caused  by 
unlikenesses  of  nutrition  felt  by  the  whole  plant,  we  pass 
now  to  those  which  are  thus  caused  in  some  of  its  parts  and 
not  in  others.  Among  such  are  the  contrasts  between 
flowering  axes,  and  the  axes  that  bear  leaves  only.  It  has 
already  been  shown  in  §  78,  that  the  belief  expressed  by 
Wolff  in  a  direct  connexion  between  fructification  and  innu- 
trition, is  justified  inductively  by  many  facts  of  many  kinds. 
Deductively  too,  in  §  79,  we  saw  reason  to  conclude  that  such 
a  relation  would  be  established  by  survival  of  the  fittest; 
seeing  that  it  would  profit  a  species  for  its  members  to  begin 
sending  off  migrating  germs  from  the  ends  of  those  axes 
which  innutrition  prevented  from  further  agamogenetic  mul- 
tiplication. Once  more,  when  considering  the  nature  of  the 
phasnogamic  axis,  we  found  support  for  this  belief  in  the  fact 

*  Natural  selection  may  have  operated  in  establishing  a  constitutional 
tendency  to  other  sudden  abridgments.  Mr.  Tansley  alleges  that  this  is  a 
part-cause  of  the  varying  distribution  of  leaves.  He  says : — "  I  have  myself 
made  some  observations  on  the  lencrth  of  internodes  in  the  Beech,  and  am 
satisfied  that  it  follows  quite  other  laws,  connected  with  the  suitable  dis- 
position of  the  leaves  on  the  branch.  Although  I  have  not  had  the  oppor- 
tunity of  following  up  this  line  of  work  so  as  in  any  way  to  generalize  the 
results,  I  suspect  that  '  indirect  equilibration '  is  a  widespread  cause  of  such 
variation." 


180  MORPHOLOGICAL   DEVELOPMENT. 

that  the  components  of  a  flower  exhibit  a  reversion  to  that 
type  from  which  the  phaenogamic  type  has  probably  arisen — 
a  reversion  which  the  laws  of  embryology  would  lead  us  to 
look  for  where  innutrition  had  arrested  development. 

Hence,  then,  we  may  properly  count  those  deviations  of 
structure  which  constitute  inflorescence,  as  among  the  mor- 
phological differentiations  produced  by  local  innutrition.  I  do 
not  mean  that  the  detailed  modifications  which  the  essential 
and  subservient  organs  of  fructification  display,  are  thus 
accounted  for:  we  have  seen  reason  to  think  them  otherwise 
caused.  But  I  mean  that  the  morphological  characters  which 
distinguish  gamogenetic  axes  in  general  from  agamogenetic 
axes,  such  as  non-development  of  the  internodes  and  dwarf- 
ing of  the  foliar  organs,  are  primarily  results  of  failure  in 
the  supply  of  some  material  required  for  further  growth.* 

§  241.  Another  trait  which  has  to  be  noticed  under  this 
head,  is  the  spiral,  or  rather  the  helical,  arrangement  of 
parts.  The  successive  nodes  of  a  phasnogam  habitually  bear 
their  appendages  in  ways  implying  more  or  less  twist  in  the 
substance  of  the  axis ;  and  in  climbing  plants  the  twist  is  such 

*  It  is  but  just  to  the  memory  of  Wolff,  here  to  point  out  that  he  was 
immensely  in  advance  of  Goethe  in  his  rationale  of  these  metamorphoses. 
Whatever  greater  elaboration  Goethe  gave  to  the  theory  considered  as  an 
induction,  seems  to  me  more  than  counter-balanced  by  the  irrationality  of  his 
deductive  interpreta1  ion ;  which  unites  mediaeval  physiology  with  Platonic 
philosophy.  A  dominant  idea  with  him  is  that  leaves  exist  for  the  purpose  of 
carrying  off  crude  juices— that  "as  long  as  there  are  crude  juices  to  be  carried 
oT,  the  plant  must  be  provided  with  organs  competent  to  effect  the  task  " ; 
that  while  "  the  less  pure  fluids  are  got  rid  of,  purer  ones  are  introduced  " 
and  that  "  if  nourishment  is  withheld,  that  operation  of  nature  (flowering)  is 
facilitated  and  hastened;  the  organs  of  the  nodes  (leaves)  become  more 
refined  in  texture,  the  action  of  the  purified  juices  becomes  stronger,  and  the 
transformation  of  parts  having  now  become  possible,  takes  place  without 
delay."  This  being  the  proximate  explanation,  the  ultimate  explanation  is, 
that  Nature  wishes  to  form  flowers— that  when  a  plant  flowers  it  "attains  the 
end  prescribed  to  it  by  nature";  and  that  so  "Nature  at  length  attains  her 
object."  Instead  of  vitiating  his  induction  by  a  teleology  that  is  as  unwar- 
ranted in  its  assigned  object  as  in  its  assigned  means,  Wolff  ascribes  the 
phenomena  to  a  cause  which,  whether  sufficient  or  not,  is  strictly  scientific  in 


CHANGED  OF  SHAPE  OTHERWISE  CAUSED.         181 

as  to  produce  a  corkscrew  shape.  This  structure  is  ascrib- 
able  to  differences  of  interstitial  nutrition.  Take  a  shoot 
which  is  growing  vertically.  It  is  clear  that  if  the  molecules 
are  added  with  perfect  equality  on  all  sides,  there  will  be  no 
tendency  towards  any  kind  of  lateral  deviation;  and  the 
successively-produced  parts  will  be  perpendicularly  over  one 
another.  But  any  inequality  in  the  rate  of  growth  on  the 
different  sides  of  the  shoot,  will  destroy  this  straightness  in 
the  lines  of  growth.  If  the  greatest  and  least  rates  of  mole- 
cular increase  happen  to  be  on  opposite  sides,  the  shoot  must 
assume  a  curve  of  single  curvature;  but  in  every  other  case 
of  unequal  molecular  increase,  a  curve  of  double  curvature 
must  result.  Now  it  is  a  corollary  from  the  instability  of  the 
homogeneous,  that  the  rates  of  growth  on  all  sides  of  a  shoot 
can  never  be  exactly  alike;  and  it  is  also  to  be  inferred  from 
the  same  general  law,  that  the  greatest  and  least  rates  of 
growth  will  not  occur  on  exactly  opposite  sides  of  the  shoot, 
at  the  same  time  that  equal  rates  of  growth  are  preserved  by 
the  two  other  sides.  Hence,  there  must  almost  inevitably 
arise  more  or  less  of  twist;  and  the  appendages  of  the  inter- 
nodes  will  so  be  prevented  from  occurring  perpendicularly 
one  over  another. 

A  deviation  of  this  kind,  necessarily  initiated  by  physical 
causes  in  conformity  with  the  general  laws  of  evolution,  is 
likely  to  be  made  regular  and  decided  by  natural  selection. 
For  under  ordinary  circumstances,  a  plant  profits  by  having 
its  axis  so  twisted  as  to  bring  the  appended  leaves  into  posi- 
tions which  prevent  them  from  shading  one  another.  And, 
manifestly,  modifications  in  the  forms,  sizes,  and  insertions  of 
the  leaves,  may,  under  the  same  agency,  lead  to  adapted 
modifications  of  the  twist.  We  must  therefore  ascribe  this 
common  characteristic  of  phaenogams,  primarily  to  local  differ- 
ences of  nutrition,  and  secondarily  to  survival  of  the  fittest. 

its  character.  Variation  of  nutrition  is  unquestionably  a  "true  cause"  of 
variation  in  plant-structure.  We  have  here  no  imaginary  action  of  a  fictitious 
agency ;  but  an  ascertained  action  of  a  known  agency. 


182        MORPHOLOGICAL  DEVELOPMENT. 

It  is  proper  to  add  that  there  are  some  Monocotyledons, 
as  Ravenala  madagascariensis,  in  which  this  character  does 
not  occur.  What  conditions  of  existence  they  are  that  here 
hold  this  natural  tendency  in  check,  it  is  not  easy  to  see.* 

*  The  Natural  History  Review  for  July,  1865,  contained  an  article  on 
the  doctrine  of  morphological  composition  set  forth  in  the  foregoing  Chaps. 
I.  to  III.  In  this  article,  which  unites  exposition  and  criticism  in  a  way  that 
is  unhappily  not  common  with  reviewers,  it  is  suggested  that  the  spiral  struc- 
ture may  be  caused  by  natural  selection.  When  this  article  appeared,  the 
foregoing  five  pages  were  standing  over  in  type,  as  surplus  from  No.  14,  issued 
in  June,  1865. 


CHAPTEE  XIII. 

MORPHOLOGICAL   DIFFERENTIATION    IN   ANIMALS. 

§  242.  THE  general  considerations  which  preluded  our  in- 
quiry into  the  shapes  of  plants  and  their  parts,  equally  serve, 
so  far  as  they  go,  to  prelude  an  inquiry  into  the  shapes  of 
animals  and  their  parts.  Among  animals,  as  among  plants, 
the  formation  of  aggregates  greater  in  bulk  or  higher  in  de- 
gree of  composition,  or  both;  is  accompanied  by  changes  of 
form  in  the  aggregates  as  wholes  as  well  as  by  changes  of 
form  in  their  parts;  and  the  processes  of  morphological 
differentiation  conform  to  the  same  general  laws  in  the  one 
kingdom  as  in  the  other. 

It  is  needless  to  recapitulate  the  several  kinds  of  modifi- 
cation to  be  explained,  and  the  several  factors  that  co- 
operate in  working  them.  In  so  far  as  these  are  common 
to  plants  and  animals,  the  preceding  chapters  have  suf- 
ficiently familiarized  them.  Nor  is  it  needful  to  specify 
afresh  the  several  types  of  symmetry  and  their  descriptive 
names;  for  what  is  true  of  them  in  the  one  case  is  true  of 
them  in  the  other.  There  is,  however,  one  new  and  all- 
important  factor  which  we  shall  have  now  to  take  into 
account;  and  about  this  a  few  preliminary  remarks  are 
requisite. 

§  243.  This  new  factor  is  motion — motion  of  the  organism 
in  relation  to  surrounding  objects,  or  of  the  parts  of  the 

183 


184:  MORPHOLOGICAL  DEVELOPMENT. 

organism  in  relation  to  one  another,  or  both.  Though  there 
are  plants,  especially  of  the  simpler  kinds,  which  move,  and 
though  a  few  of  the  simpler  animals  do  not  move ;  yet  move- 
ments are  so  exceptional  and  unobtrusive  in  the  one  king- 
dom, while  they  are  so  general  and  conspicuous  in  the  other, 
that  the  broad  distinction  commonly  made  is  well  warranted. 
What,  among  plants,  is  an  inappreciable  cause  of  morpho- 
logical differentiation,  becomes,  among  animals,  the  chief 
cause  of  morphological  differentiation. 

Eooted  animals  or  animals  otherwise  fixed,  of  course  pre- 
sent traits  of  structure  nearest  akin  to  those  we  have  lately 
been  studying.  The  motions  of  parts  in  relation  to  one  another 
and  to  the  environment,  being  governed  by  the  mode  of  aggre- 
gation and  mode  of  fixing,  we  are  presented  with  morpho- 
logical differentiations  similar  in  their  general  characters  to 
those  of  plants,  and  showing  us  parallel  kinds  of  symmetry 
under  parallel  conditions.  But  animals  which  move  from 
place  to  place  are  subject  to  an  additional  class  of  actions 
and  reactions.  These  actions  and  reactions  affect  them  in 
various  ways  according  to  their  various  modes  of  movement. 
Let  us  glance  at  the  several  leading  relations  between  shape 
and  motion  which  we  may  expect  to  find. 

If  an  organism  advances  through  a  homogeneous  medium 
with  one  end  always  foremost,  that  end,  being  exposed  to 
forces  unlike  those  to  which  the  other  end  is  exposed,  may 
be  expected  to  become  unlike  it;  and  supposing  this  to  be 
the  only  constant  contrast  of  conditions,  we  may  expect  an 
equal  distribution  of  the  parts  round  the  axis  of  movement — 
a  radial  symmetry.  If,  in  addition  to  this  habitual 

attitude  of  the  ends,  one  surface  of  the  body  is  always  upper- 
most and  another  always  lowermost,  there  arise  between  the 
top  and  bottom  dissimilarities  of  conditions,  while  the  two 
sides  remain  similarly  conditioned.  Hence  it  is  inferable 
that  such  an  organism  will  be  divisible  into  similar  halves 
by  a  vertical  plane  passing  through  its  axis  of  motion — will 
have  a  bilateral  symmetry.  We  may  presume  that  this 


MORPHOLOGICAL  DIFFERENTIATION  IN  ANIMALS.     185 

symmetry  will  deviate  but  little  from  double  bilateralness 
where  the  upper  and  under  parts  are  not  exposed  to  strongly- 
contrasted  influences;  while  we  may  rationally  look  for 
single  bilateral  symmetry  of  a  decided  kind,  in  creatures 
having  dorsal  and  ventral  parts  conversant  with  very  unlike 
regions  of  the  environment:  as  in  all  cases  where  the  move- 
ment is  over  a  solid  surface.  If  the  movement, 
though  over  a  solid  surface,  is  not  constant  in  direction,  but 
takes  place  as  often  on  one  side  as  on  another,  radial  sym- 
metry may  be  again  looked  for;  and  if  the  motions  are  still 
more  variously  directed — if  they  are  not  limited  to  approxi- 
mately-plane surfaces,  but  extend  to  surfaces  that  are  dis- 
tributed all  around  with  a  regular  irregularity — an  approach 
of  the  radial  towards  the  spherical  symmetry  is  to  be  antici- 
pated. Where  the  habits  are  such  that  the  inter- 
course between  the  organism  and  its  environment,  does  not 
involve  an  average  equality  of  actions  and  reactions  on  any 
two  or  more  sides,  there  may  be  expected  either  total  irregu- 
larity or  some  divergence  from  regularity. 

The  like  general  relations  between  forms  and  incident 
forces  are  inferable  in  the  component  parts  of  animals,  as 
well  as  in  the  animals  as  wholes.  It  is  needless,  however,  to 
occupy  space  by  descriptions  of  these.  Let  us  now  pass  to 
the  facts,  and  see  how  they  confirm,  a  posteriori,  the  conclu- 
sions here  reached  a  priori. 


CHAPTEE  XIV. 

THE   GENERAL   SHAPES   OF   ANIMALS. 

§  244.  CERTAIN  of  the  Protozoa  are  quite  indefinite  in  their 
shapes,  and  quite  inconstant  in  those  indefinite  shapes  which 
they  have — the  relations  of  their  parts  are  indeterminate 
both  in  space  and  time.  In  one  of  the  simpler  Khizopods,  at 
least  during  the  active  stage  of  its  existence,  no  permanent 
distinction  of  inside  and  outside  is  established;  and  hence 
there  can  arise  no  established  correspondence  between  the 
shape  of  the  outside  and  the  distribution  of  environing 
actions.  But  when  the  relation  of  inner  and  outer  becomes 
fixed,  either  over  part  of  the  mass  or  over  the  whole  of  it,  we 
have  kinds  of  symmetry  that  correspond  with  the  habitual 
incidence  of  forces.  An  Amceba  in  becoming  encysted, 
passes  from  an  indefinite,  ever-changing  form  into  a  spherical 
form;  and  the  order  of  symmetry  which  it  thus  assumes,  is 
in  harmony  with  the  average  equality  of  the  actions  on  all 
its  sides.  In  Difflugia,  Fig.  134,  and  still  better  in  Arcella, 
we  have  an  indefinitely-radial  symmetry  occurring  where  the 
conditions  are  different  above  and  below  but  alike  all  around. 
Among  the  Gregarinida  the  spherical  symmetry  and  sym- 
metry passing  from  that  into  the  radial,  are  such  as  appear 
to  be  congruous  with  the  simple  circumstances  of  these 
creatures  in  the  intestines  of  insects.  But  the  relations  of 
these  lowest  types  to  their  environments  are  comparatively 
so  indeterminate,  and  our  knowledge  of  their  actions  so 
188 


THE  GENERAL  SHAPES  OF  ANIMALS.  187 

scanty,  that  little  beyond  negative  evidence  can  be  expected 
from  the  study  of  them. 


137 


The  like  may  be  said  of  the  Infusoria.  These  are  more  or 
less  irregular.  In  some  cases,  where  the  line  of  movement 
through  the  water  is  tolerably  definite  and  constant,  we  have 
a  form  that  is  approximately  radial — externally  at  least. 
But  usually,  as  shown  in  Figs.  137,  138,  139,  there  is  either 
an  unsymmetrical  or  an  asymmetrical  shape.  And  when  one 
of  these  creatures  is  watched  under  the  microscope,  the  con- 
gruity  of  this  shape  with  the  incidence  of  forces  is  manifest. 
For  the  movements  are  conspicuously  varied  and  indetermi- 
nate— movements  which  do  not  expose  any  two  or  more  sides 
of  the  mass  to  approximately  equal  sets  of  actions.* 

§  245.  Among  aggregates  of  the  second  order,  as  among 
aggregates  of  the  first  order,  we  find  that  of  those  possessing 
any  definite  shapes  the  lowest  are  spherical  or  spheroidal. 
Such  are  some  of  the  Eadiolaria,  as  Collozoum  inerme.  These 
bodies  which  float  passively  in  the  sea,  and  present  in  turn 
all  their  sides  to  the  same  influences,  have  their  parts  dis- 
posed with  approximate  regularity  round  a  centre — approxi- 
mate, because  in  the  absence  of  locomotion  a  slight  irregu- 
larity of  growth,  almost  certain  to  take  place,  may  cause  a 
fixed  attitude  and  a  resulting  deviation  from  spherical  sym- 
metry. The  best  cases  in  illustration  of  the  truth  here 
named,  are  furnished  by  rotating  and  locomotive  organisms 
respecting  which  there  is  a  dispute  whether  they  are  animal 
or  vegetal — the  Volvocinece.  These,  already  instanced  under 

*  A  verifying  comment  on  this  paragraph  runs  as  follows : — "  In  the 
Hypotricha  Infusoria,  which  creep  over  solid  surfaces,  there  is  a  differen- 
tiation between  ventral  and  dorsal  surface  and  an  approach  to  bilateral  sym- 
metry. The  ventral  surface  is  provided  with  movable  cilia,  the  dorsal  with 
immobile  setae." 


188        MORPHOLOGICAL  DEVELOPMENT. 

the  one  head  in  §  218,  may  here  be  instanced  afresh  under 
the  other.  Further,  among  these  secondary  aggregates  in 
which  the  units,  only  physically  integrated,  have  not  had  their 
individualities  merged  into  an  individuality  of  a  higher 
order,  must  be  named  the  compound  Infusoria.  The  cluster 
of  Vorticellce  in  Fig.  144,  will  sufficiently  exemplify  them; 
and  the  striking  resemblance  borne  by  its  individuals  to 
those  of  a  radially-arranged  cluster  of  flowers,  will  show  how, 
under  analogous  conditions,  the  general  principles  of  mor- 
phological differentiation  are  similarly  illustrated  in  the  two 
kingdoms. 

§  246.  Eadial  symmetry  is  usual  in  low  aggregates  of 
the  second  order  which  have  their  parts  sufficiently  differen- 
tiated and  integrated  to  give  individualities  to  them  as  wholes. 
The  C  eel  enter  ata  offer  numerous  examples  of  this.  Solitary 
polypes — hydroid  or  helianthoid — mostly  stationary,  and 
when  they  move,  moving  with  any  side  foremost,  do  not  by 
locomotion  subject  their  bodies  to  habitual  contrasts  of  con- 
ditions. Seated  with  their  mouths  upwards  or  downwards, 
or  else  at  all  degrees  of  inclination,  the  individuals  of  a 
species  taken  together,  are  subject  to  no  mechanical  actions 
affecting  some  parts  of  their  discs  more  than  other  parts. 
And  this  indeterminateness  of  attitude  similarly  prevents 
their  relations  to  prey  from  being  such  as  subject  some  of 
their  prehensile  organs  to  forces  unlike  those  to  which  the 
rest  are  subject.  The  fixed  end  is  differently  conditioned 
from  the  free  end,  and  the  two  are  therefore  different;  but 
around  the  axis  running  from  the  fixed  to  the  free  end  the 
conditions  are  alike  in  all  directions,  and  the  form  therefore 
is  radial.  Again,  among  many  of  the  simple  free- 

swimming  Ilydrozoa,  the  same  general  truth  is  exemplified 
under  other  circumstances.  In  a  common  Medusa,  advanc- 
ing through  the  water  by  the  rhythmical  contractions  of  its 
disc,  the  mechanical  reactions  are  the  same  on  all  sides;  and 
as,  from  accidental  causes,  every  part  of  the  edge  of  the  disc 


THE  GENERAL  SHAPES  OF  ANIMALS. 


189 


comes  uppermost  in  its  turn,  no  part  is  permanently  affected 
in  a  different  way  from  the  rest.  Hence  the  radial  form  con- 
tinues. 

In  others  of  this  same  group,  however,  there  occur  forms 
which  show  us  an  incipient  bilateralness ;  and  help  us  to  see 
how  a  more  decided  bilateralness  may  arise.  Sundry  of  the 
Medusidce  are  proliferous,  giving  origin  to  gemma?  from  the 
body  of  the  central  polypite  or  from  certain  points  on  the 
edge  of  the  disc;  and  this  budding,  unless  it  occurs  equally 
on  all  sides,  which  it  does  not  and  is  unlikely  to  do,  must 
tend  to  destroy  the  balance  of  the  disc,  and  to  make  its 
attitude  less  changeable.  In  other  cases  the  growth  of  a 
large  process  [a  much-developed  tentacle]  from  the  edge  of 
the  disc  on  one  side,  as  in  Steenstrupia,  Fig.  257,  constitutes 
a  similar  modification,  and  a  cause  of  further  modification. 
The  animal  is  no  longer  divisible  into  any  two  quite  similar 
halves,  except  those  formed  by  a  plane  passing  through  the 
process ;  and  unless  the  process  is  of  the  same  specific  gravity 
as  the  disc,  it  must  tend  towards  either  the  lowest  or  the 
highest  point,  and  must  so  serve  to  increase  the  bilateralness, 
by  keeping  the  two  sides  of  the  disc  similarly  conditioned 
while  the  top  and  bottom  are  differently  conditioned.  Fig.  258 
represents  the  underside  of  another  Medusa,  in  which  a  more 
decided  bilateralness  is  produced  by  the  presence  of  two  such 
processes.  Among  the 

simple  free-swimming  Acti- 
nozoa,  occur  like  deviations 
from  radial  symmetry,  along 
with  like  motions  through  the 
water  in  bilateral  attitudes. 
Of  this  a  Cydippe  is  a  familiar  _ 
example.  Though  radial  in 
some  of  its  characters,  as  in 
the  distribution  of  its  meridi- 
onal bands  of  locomotive  paddles  with  their  accompanying 
canals,  this  creature  has  a  two-sided  distribution  of  tentacles 


190        MORPHOLOGICAL  DEVELOPMENT. 

and  various  other  parts,  corresponding  with  its  two-sided 
attitude  in  moving  through  the  water.  And  in  other  genera 
of  this  group,  as  in  Cesium,  Eurhamphcea,  and  Callianira, 
that  almost  equal  distribution  of  parts  which  characterizes 
the  Beroe  is  quite  lost. 

Here  seems  a  fit  place  to  meet  the  objection  which  some 
may  feel  to  this  and  other  such  illustrations,  that  they  amount 
very  much  to  physical  truisms.  If  the  parts  of  a  Medusa  are 
disposed  in  radial  symmetry  round  the  axis  of  motion  through 
the  water,  there  will  of  course  be  no  means  of  maintaining 
one  part  of  its  edge  uppermost  more  than  another;  and  the 
equality  of  conditions  may  be  ascribed  to  the  radiateness,  as 
much  as  the  radiateness  to  the  equality  of  conditions.  Con- 
versely, when  the  parts  are  not  radially  arranged  around  the 
axis  of  motion,  they  must  gravitate  towards  some  one  atti- 
tude, implying  a  balance  on  the  two  sides  of  a  vertical  plane 
— a  bilateralness ;  and  the  two-sided  conditions  so  necessi- 
tated, may  be  as  much  ascribed  to  the  bilateralness  as  the 
bilateralness  to  the  two-sided  conditions.  Doubt- 

less the  form  and  the  conditions  are,  in  the  way  alleged, 
necessary  correlates;  and  in  so  far  as  it  asserts  this,  the  ob- 
jection harmonizes  with  the  argument.  To  the  difficulty 
which  it  at  the  same  time  raises  by  the  implied  question — 
Why  make  the  form  the  result  of  the  conditions,  rather  than 
the  conditions  the  result  .of  the  form  ?  the  reply  is  this : — 
The  radial  type,  both  as  being  the  least  differentiated  type 
and  as  being  the  most  obviously  related  to  lower  types,  must 
be  taken  as  antecedent  to  the  bilateral  type.  The  indi- 
vidual variations  which  incidental  circumstances  produce  in 
the  radial  type,  will  not  cause  divergence  of  a  species  from 
the  radial  type,  unless  such  variations  give  advantages  to  the 
individuals  displaying  them ;  which  there  is  no  reason  to  sup- 
pose they  will  always  do.  Those  occasional  deviations  from 
the  radial  type,  which  the  law  of  the  instability  of  the  homo- 
geneous warrants  us  in  expecting  to  take  place,  will,  however, 
in  some  cases  be  beneficial;  and  will  then  be  likely  to  estab- 


THE  GENERAL  SHAPES  OF  ANIMALS.  191 

lish  themselves.  Such  deviations  must  tend  to  destroy  the 
original  indefiniteness  and  variability  of  attitude — must  cause 
gravitation  towards  an  habitual  attitude.  And  gravitation 
towards  an  habitual  attitude  having  once  commenced,  will 
continually  increase,  where  increase  of  it  is  not  negatived  by 
adverse  agencies :  each  further  degree  of  bilateralness  render- 
ing more  decided  the  actions  that  conduce  to  bilateralness.  If 
this  reply  be  thought  insufficient,  it  may  be  enforced  by  the 
further  one,  that  as,  among  plants,  the  incident  forces  are  the 
antecedents  and  the  forms  the  consequents  (changes  of  forces 
being  in  many  cases  visibly  followed  by  changes  of  forms)  we 
are  warranted  in  concluding  that  the  like  order  of  cause  and 
effect  holds  among  animals.* 

§  247.  Keeping  to  the  same  type  but  passing  to  a  higher 
degree  of  composition,  we  meet  more  complex  and  varied 
illustrations  of  the  same  general  laws.  In  the  compound 
*  Criticisms  on  the  above  passage  have  shown  the  need  for  naming  sun- 
dry complications.  These  complications  chiefly,  if  not  wholly,  arise  from 
changes  in  modes  of  life — changes  from  the  locomotive  to  the  stationary,  and 
from  the  stationary  to  the  locomotive.  Referring  to  my  statement  that  (ignor- 
ing the  spherical)  the  radial  type  is  the  lowest  and  must  be  taken  as  ante- 
cedent to  the  bilateral  type,  it  is  alleged  that  all  existing  "radial  animals 
above  Protozoa  are  probably  derived  from  free  swimming,  bilaterally-sym- 
metrical animals."  If  this  is  intended  to  include  the  planulse  of  the  hydroid 
polyps,  then  it  seems  rather  a  straining  of  the  evidence.  These  locomotive 
embryos,  described  as  severally  having  the  structure  of  a  gastrula  with  a 
closed  mouth,  can  be  said  to  show  bilateralness  only  because  the  first  two  ten- 
tacle'' make  their  appearance  on  opposite  sides  of  the  mouth — a  bilateralness 
which  lasts  only  till  two  other  tentacles  make  their  appearance  in  a  plane  at 
right  angles,  so  giving  the  radial  structure.  I  think  the  criticism  applies  only 
to  cases  furnished  by  Echinoderms.  The  larvae  of  these  creatures  have  bilat- 
erally-symmetrical structures,  which  they  retain  as  long  as  they  swim  about 
and  which  such  of  them  as  fix  themselves  lose  by  becoming  similarly  related 
to  conditions  all  round :  the  radial  structure  being  retained  by  those  types 
which,  becoming  subsequently  detached,  move  about  miscellaneously.  But,  as 
happens  in  some  of  the  Sea-urchins  and  still  more  among  the  Holothurians, 
the  structure  is  again  made  bilaterally-symmetrical  by  a  locomotive  life  pur- 
sued with  one  end  foremost.  Should  it  be  contended  that  the  conditions  and 
the  forms  are  reciprocally  influential— that  either  may  initiate  the  other,  it 
still  remains  unquestionable  that  ordinarily  the  conditions  are  the  antecedents, 
as  is  so  abundantly  shown  by  plants. 


192       MORPHOLOGICAL  DEVELOPMENT. 

Ccelenterata,  presenting  clusters  of  individuals  which  are 
severally  homologous  with  the  solitary  individuals  last  dealt 
with,  we  have  to  note  both  the  shapes  of  the  individuals  thus 
united,  and  the  shapes  of  the  aggregates  made  up  of  them. 

Such  of  the  fixed. Hydrozoa  andActinozoa  as  form  branched 
societies,  continue  radial;  both  because  their  varied  attitudes 
do  not  expose  them  to  appreciable  differences  in  their  rela- 
tions to  those  surrounding  actions  which  chiefly  concern 
them  (the  actions  of  prey),  and  because  such  differences,  even 
if  they  were  appreciable,  would  be  so  averaged  in  their  effects 
on  the  dissimilarly-placed  members  of  each  group  as  to  be 
neutralized  in  the  race.  Among  the  tree- 
like coral-polypedoms,  as  well  as  in  such 
ramified  assemblages  of  simpler  polypes 
as  are  shown  in  Figs.  149,  150,  we  have, 
indeed,  cases  in  many  respects  parallel 
to  the  cases  of  scattered  flowers  (§  233), 
which  though  placed  laterally  remain 
radial,  because  no  differentiating  agency 
can  act  uniformly  on  all  of  them.  Meanwhile,  in  the 

groups  which  these  united  individuals  compose,  we  see  the 
shapes  of  plants  further  simulated  under  a  further  parallelism 
of  conditions.  The  attached  ends  differ  from  the  free  ends 
as  they  do  in  plants;  and  the  regular  or  irregular  branches 
obviously  stand  to  environing  actions  in  relations  analogous 
to  those  in  which  the  branches  of  plants  stand. 

The  members  of  those  compound  Ccelenterata  which  move 
through  the  water  by  their  own  actions,  in  attitudes  that  are 
approximately  constant,  show  us  a  more  or  less  distinct  two- 
sidedness.  Diphyes,  Fig.  259,  furnishes  an  example.  Each 
of  the  largely-developed  and  modified  polypites  forming  its 
swimming  sacs  is  bilateral,  in  correspondence  with  the  bi- 
lateralness  of  its  conditions;  and  in  each  of  the  appended 
polypites  the  insertion  of  the  solitary  tentacle  produces  a 
kindred  divergence  from  the  primitive  radial  type.  The 

aggregate,  too,  which  here  very  much  subordinates  its  mem- 


THE  GENERAL  SHAPES  OF  ANIMALS. 


193 


bers,  exhibits  the  same  conformity  of  structure  to  circum- 
stances. It  admits  of  symmetrical  bisection  by  a  plane  pass- 
ing through  its  two  contractile  sacs,  or  nectocalyces,  but  not 


by  any  other  plane;  and  the  plane  which  thus  symmetrically 
bisects  it,  is  the  vertical  plane  on  the  two  sides  of  which  its 
parts  are  similarly  conditioned  as  it  propels  itself  through 
the  water. 

Another  group  of  the  oceanic  Hydrozoa,  the  Pliysophoridce, 
furnishes  interesting  evidence — not  so  much  in  respect  of  the 
forms  of  the  united  individuals,  which  we  may  pass  over,  as 
in  respect  of  the  forms  of  the  aggregates.  Some  of  these 
are  without  swimming  organs,  and  have  their  parts  sus- 
pended from  air-vessels  which  habitually  float  on  the  surface 
of  the  water.  Hence  the  distribution  of  their  parts  is  asym- 
metrical. The  Pliysalia,  Fig.  152,  is  an  example.  Here  the 
relations  of  the  integrated  group  of 
individuals  to  the  environment  are  in- 
definite; and  there  is  thus  no  agency 
tending  to  change  that  comparatively 
irregular  mode  of  growth  which  is  pro- 
bably derived  from  a  primordial  type 
of  the  branched  Hydrozoa. 

So  various  are  the  modes  of  union 
among  the  compound  CceUnterata,  that 
it  is  out  of  the  question  to  deal  with 
them  all.  Even  did  space  permit,  it 
would  be  impracticable  for  any  one  but 
a  professed  naturalist,  to  trace  through- 
59 


194  MORPHOLOGICAL  DEVELOPMENT. 

out  this  group  the  relations  between  shapes  and  conditions  of 
existence.  The  above  must  be  taken  simply  as  a  few  of  the 
most  significant  and  easily-interpretable  cases. 

§  248.  In  the  sub-kingdoms  Polyzoa  and  Tunicata  we  meet 
with  examples  not  wholly  unlike  the  foregoing.  Among  the 
types  assembled  under  these  names  there  are  simple  indivi- 
duals or  aggregates  of  the  second  order,  and  societies  or 
tertiary  aggregates  produced  by  their  union.  The  relations 
of  forms  to  forces  have  to  be  traced  in  both. 

Solitary  Ascidians,  fixed  or  floating,  carry  on  an  inactive 
and  indefinite  converse  with  the  actions  in  the  environment. 
Without  power  to  move  about  vivaciously,  and  unable  to 
catch  any  prey  but  that  contained  in  the  currents  of  water 
they  absorb  and  expel,  these  creatures  are  not  exposed  to 
sets  of  forces  which  are  equal  on  two  or  more  sides ;  and  their 
shapes  consequently  remain  vague.  Though  internally  their 
parts  have  a  partially-symmetrical  arrangement,  due  to  their 
derivation,  yet  they  are  substantially  unsymmetrical  in  that 
part  of  the  body  which  is  concerned  with  the  environment. 
Fig.  156  is  an  example.*  Among  the  composite 

Ascidians,  floating  and  fixed,  the  shape  of  the  aggregate, 
partly  determined  by  the  habitual  mode  of  gemmation  and 
partly  by  the  surrounding  conditions  in  each  case,  is  in 
great  measure  indefinite.  We  can  say  no  more  about  it  than 
that  it  is  not  obviously  at  variance  with  the  laws  alleged. 

Evidence  of  a  more  positive  kind  occurs  among  those  com- 
pound Molluscoida  which  are  most  like  the  compound 
Coslenterata  in  their  modes  of  union — the  Polyzoa.  Many  of 
these  form  groups  that  are  more  or  less  irregular — spread- 
ing as  films  over  solid  surfaces,  combining  into  sea-weed- 
like  fronds,  budding  out  from  creeping  stolons,  or  growing 
up  into  tree-shaped  societies;  and  besides  aggregating 

*  Should  it  be  proved  that  the  Ascidian  is  a  degraded  vertebrate,  then  the 
argument  will  be  strengthened;  since  loss  of  bilateral  symmetry  has  gone 
along  with  change  to  asymmetrical  conditions. 


THE  GENERAL  SHAPES  OF  ANIMALS.  195 

irregularly  they  are  irregularly  placed  on  surfaces  inclined 
in  all  directions.  Merely  noting  that  this  asymmetrical 
distribution  of  the  united  individuals  is  explained  by  the 
absence  of  definiteness  in  the  relations  of  the  aggregate  to 
incident  forces,  it  concerns  us  chiefly  to  observe  that  the 
united  individuals  severally  exemplify  the  same  truth  as  do 
similarly-united  individuals  among  the  Ccelenterata.  Averag- 
ing the  members  of  each  society,  the  ciliated  tentacles  they 
protrude  are  similarly  related  to  prey  on  all  sides;  and 
therefore  remain  the  same  on  all  sides.  This  distribution  of 
tentacles  is  not,  however,  witEout  exception.  Among  the 
fresh-water  Polyzoa  there  are  some  genera,  as  Plumatella  and 
Crystatella,  in  which  the  arrangement  of  these  parts  is  very 
decidedly  bilateral.  Some  species  of  them  show  us  such 
relations  of  the  individuals  to  one  another  and  to  their  sur- 
face of  attachment,  as  give  a  clue  to  the  modification;  but 
in  other  species  the  meaning  of  this  deviation  from  the 
radial  type  is  not  obvious. 

§  249.  In  the  Platyhelminthes  good  examples  of  the  con- 
nexions between  forms  and  forces  occur.  The  Planaria 
exemplifies  the  single  bilateral  symmetry  which,  even  in 
very  inferior  forms,  accompanies  the  habit  of  moving  in  one 
direction  over  a  solid  surface.  Humbly  organized  as  are 
these  creatures  and  their  allies  the  Nemertidce,  we  see  in 
them,  just  as  clearly  as  in  the  highest  animals,  that  where 
the  movements  subject  the  body  to  different  forces  at  its  two 
ends,  different  forces  on  its  under  and  upper  surfaces,  and 
like  forces  along  its  two  sides,  there  arises  a  corresponding 
form,  unlike  at  its  extremities,  unlike  above  and  below,  but 
having  its  two  sides  alike. 

The  Echinodermata  furnish  us  with  instructive  illustra- 
tions— instructive  because  among  types  that  are  nearly  allied, 
we  meet  with  wide  deviations  of  form  answering  to  marked 
contrasts  in  the  relations  to  the  environment.  The  facts  fall 
into  four  groups.  The  Crinoidea,  once  so  abundant 


196        MORPHOLOGICAL  DEVELOPMENT. 

and  now  so  rare,  present  a  radial  symmetry  answering  to 
an  incidence  of  forces  that  are  equal  on  all  sides.  In  the 
general  attitudes  of  their  parts  towards  surrounding  actions, 
they  are  like  uniaxial  plants  or  like  polypes;  and  show,  as 
those  do,  marked  differences  between  the  attached  ends  and 
the  free  ends,  along  with  even  distributions  of  parts  all  round 
their  axes.  In  the  OpJiiuridea,  and  in  the  Star- 

fishes, we  have  radial  symmetry  co-existing  with  very  differ- 
ent habits;  but  habits  which  nevertheless  account  for  the 
maintenance  of  the  form.  Holding  on  to  rocks  and  weeds 
by  its  simple  or  branched  arms,  or  by  the  suckers  borne  on 
the  unfler  surface  of  its  rays,  one  of  these  creatures  moves 
about  not  always  with  one  side  foremost,  but  with  any  side 
foremost.  Consequently,  averaging  its  movements,  its  arms 
or  rays  are  equally  affected,  and  therefore  remain  the  same 
on  all  sides.  On  watching  the  ways  of  the  com- 

mon Sea-urchin,  we  are  similarly  furnished  with  an  ex- 
planation of  its  spherical,  or  rather  its  spheroidal,  figure. 
Here  the  habit  is  not  to  move  over  any  one  approximately- 
flat  surface;  but  the  habit  is  to  hold  on  by  several  surfaces 
on  different  sides  at  the  same  time.  Frequenting  crevices 
and  the  interstices  among  stones  and  weeds,  the  Sea-urchin 
protrudes  the  suckers  arranged  in  meridional  bands  over  its 
shell,  laying  hold  of  objects  now  on  this  side  and  now  on  that, 
now  above  and  now  below:  the  result  being  that  it  does  not 
move  in  all  directions  over  one  plane  but  in  all  directions 
through  space.  Hence  the  approach  in  general  form  towards 
spherical  symmetry — an  approach  which  is,  however,  re- 
strained by  the  relations  of  the  parts  to  the  mouth  and  vent : 
the  conditions  not  being  exactly  the  same  at  the  two  poles  as 
at  other  parts  of  the  surface.  Still  more  significant  is 

that  deviation  from  this  shape  which  occurs  among  such  of 
the  Echinidea  as  have  habitats  of  a  different  kind,  and  con- 
sequently, different  habits.  The  genera  Echinocyamns,  Spa- 
tangus,  Brissus,  and  Amphidotus,  diverge  markedly  towards 
a  bilateral  structure.  These  creatures  are  found  not  on  rocky 


THE  GENERAL  SHAPES  OP  ANIMALS.  197 

shores  but  on  flat  sea-bottoms,  and  some  of  them  only  on 
bottoms  of  sand  or  mud.  Here,  there  is  none  of  that  distri- 
bution of  surfaces  on  all  sides  which  makes  the  spheroidal 
form  congruous  with  the  conditions.  Having  to  move  about 
over  an  approximately-horizontal  plane,  any  deviation  of 
structure  arising  accidentally  which  leads  to  one  side  being 
kept  always  foremost,  will  be  an  advantage:  greater  fitness 
to  function  becoming  possible  in  proportion  as  function 
becomes  fixed.  Survival  of  the  fittest  will  therefore  tend  to 
establish,  under  such  conditions,  a  form  that  keeps  the  same 
part  in  advance — a  form  in  which,  consequently,  the  original 
radial  symmetry  diverges  more  and  more  towards  bilateral 
symmetry. 

§  250.  Very  definite  and  comparatively  uniform,  are  the 
relations  between  shapes  and  circumstances  among  the 
Annulosa:  including  under  that  title  the  Annelida  and  the 
Arthropoda.  The  agreements  and  the  disagreements  are 
equally  instructive. 

At  one  time  or  other  of  its  life,  if  not  throughout  its  life, 
every  annulose  animal  is  locomotive;  and  its  temporary  or 
permanent  locomotion,  being  carried  on  with  one  end  habitu- 
ally foremost  and  one  surface  habitually  uppermost,  it  fulfils 
those  conditions  under  which  bilateral  symmetry  arises. 
Accordingly,  bilateral  symmetry  is  traceable  throughout  the 
whole  of  this  sub-kingdom.  Traceable,  we  must  say, 
because,  though  it  is  extremely  conspicuous  in  the  immense 
majority  of  annulose  types,  it  is  to  a  considerable  extent 
obscured  where  obscuration  is  to  be  expected.  The  embryos 
of  the  Tubicolw,  after  swimming  about  a  while,  settle  down 
and  build  themselves  tubes,  from  which  they  protrude 
their  heads;  and  in  them,  or  in  some  of  them,  the  bilateral 
symmetry  is  disguised  by  the  development  of  head-append- 
ages in  an  all-sided  manner.  The  tentacles  of  Terebella  are 
distributed  much  in  the  same  way  as  those  of  a  polype.  The 
breathing  organs  in  Sabella  unispira,  Fig.  2GO,  do  not  corre- 


198       MORPHOLOGICAL  DEVELOPMENT. 

spond  on  opposite  sides  of  a  median  plane.  Even  here,  how- 
ever, the  body  retains  its  primitive  bilateralness ;  and  it  is 
further  to  be  remarked  that  this  loss  of  bilateralness  in  the 
external  appendages,  does  not  occur  where  the  relations  to 
external  conditions  continue  bilateral:  witness  the  Serpula, 
Fig.  261,  which  has  its  respiratory  tufts  arranged  in  a  two- 

A 


sided  way,  under  the  two-sided  conditions  involved  by  the 
habitual  position  of  its  tube. 

The  community  of  symmetry  among  the  higher  Annulosa, 
has  an  unobserved  significance.  That  Flies,  Beetles,  Lob- 
sters, Centipedes,  Spiders,  Mites,  have  in  common  the  cha- 
racters, that  the  end  which  moves  in  advance  differs  from 
the  hinder  end,  that  the  upper  surface  differs  from  the  under 
surface,  and  that  the  two  sides  are  alike,  is  a  truth  received 
as  a  matter  of  course.  After  all  that  has  been  said  above, 
however,  it  will  be  seen  to  have  a  meaning  not  to  be  over- 
looked ;  since  it  supplies  a  million- fold  illustration  of  the  laws 
which  have  been  set  forth.  It  is  needless  to  give  diagrams. 
Every  reader  can  call  to  mind  the  unity  indicated. 

While,  however,  annulose  animals  repeat  so  uniformly 
these  traits  of  structure,  there  are  certain  other  traits  in 
which  they  are  variously  contrasted;  and  their  contrasts 
have  to  be  here  noted,  as  serving  further  to  build  up  the 
general  argument.  In  them  we  see  the  stages  through  which 


THE  GENERAL  SHAPES  OF  ANIMALS. 


199 


bilateral  symmetry  becomes  gradually  more  marked,  as  the 
conditions  it  responds  to  become  more  decided.  A 

common  Earth-worm  may  be  instanced  as  a  member  of 
this  sub-kingdom  that  is  among  the  least-conspicuously 
bilateral.  Though  internally  its  parts  have  a  two-sided 
arrangement;  and  though  the  positions  of  its  orifices  give  it 
an  external  two-sidedness,  at  the  same  time  that  they  estab- 
lish a  difference  between  the  two  ends;  yet  its  two-sidedness 
is  not  strongly-marked.  The  form  deviates  but  little  from 
what  we  have  distinguished  as  triple  bilateral  symmetry:  if 
the  creature  is  cut  across  the  middle,  the  head  and  tail  ends 
are  very  much  alike;  if  cut  in  two  along  its  axis  by  a  hori- 
zontal plane,  the  under  and  upper  halves  are  very  much 
alike,  externally  if  not  internally;  and  if  cut  in  two  along  its 
axis  by  a  vertical  plane,  the  two  sides  are  quite  alike. 
Figs.  263  and  264  will  make  this  clear.  Such  creatures 

as  the  Julus  and  the  Centipede,  may  be  taken  as  showing 
a  transition  to  double  bilateral  symmetry.  Besides  being 
divisible  into  exactly  similar  halves  by  a  vertical  plane  pass- 
ing through  its  axis,  one  of  these  animals  may  be  bisected 
transversely  into  parts  that  differ  only  slightly;  but  if  cut  in 


-    2 

rh 
•  uj 

^^  A[\ 


two  by  a  horizontal  plane  passing  through  its  axis,  the  under 
and  upper  halves  are  decidedly  unlike.  Figs.  265,  266, 
exhibit  these  traits.  Among  the  isopodous  crustaceans, 

the  departure  from  these  low  types  of  symmetry  is  more 


200       MORPHOLOGICAL  DEVELOPMENT. 

marked.  As  shown  in  Figs.  267  and  268,  the  contrast 
between  the  upper  and  under  parts  is  greater,  and  the  head 
and  tail  ends  differ  more  obviously.  In  all  the  higher 

Arthropoda,  the  unlikeness  between  the  front  half  and  the 
hind  half  has  become  conspicuous.  There  is  in  them  single 
bilateral  symmetry  of  so  pronounced  a  kind,  that  no  other 
resemblance  is  suggested  than  that  between  the  two  sides. 
By  Figs.  269  and  270,  representing  a  decapodous  crustacean 
divided  longitudinally  and  transversely,  this  truth  is  made 
manifest.  On  calling  to  mind  the  habits  of  the 

creatures  here  drawn  and  described,  it  will  be  seen  that 
they  explain  these  forms.  The  incidence  of  forces  is  the 
same  all  around  the  Earth-worm  as  it  burrows  through  the 
compact  ground.  The  Centipede,  creeping  amid  loose  soil  or 
debris  or  beneath  stones,  insinuates  itself  between  solid  sur- 
faces— the  interstices  being  mostly  greater  in  one  dimension 
than  in  others.  And  all  the  higher  Annulosa,  moving  about 
as  they  do  over  exposed  objects,  have  their  dorsal  and  ventral 
parts  as  dissimilarly  acted  upon  as  are  their  two  ends. 

One  other  fact  only  respecting  annulose  animals  needs  to 
be  noticed  under  this  head  —  the  fact,  namely,  that  they 
become  unsymmetrical  where  their  parts  are  unsymmetric- 
ally  related  to  the  environment.  The  common  Hermit-crab 
serves  as  an  instance.  Here,  in  addition  to  the  unlikeness  of 
the  two  sides  implied  by  that  curvature  of  the  body  which 
fits  the  creature  to  the  shell  it  inhabits,  there  is  an  unlikeness 
due  to  the  greater  development  of  the  limbs,  and  especially 
the  claws,  on  the  outer  side.  As  in  the  embryo  of  the 
Hermit-crab  the  two  sides  are  alike;  and  as  both  the  embryo 
and  the  ancestor  lived  in  such  a  way,  being  free, 
that  the  conditions  were  alike  on  the  two  sides; 
and  as  the  embryo  may  be  taken  to  repre- 
sent the  type  from  which  the  Hermit-crab  has 
been  derived;  we  have  in  this  case  evidence 
that  a  symmetrically-bilateral  form  has  been 
moulded  into  an  unsymmetrically-bilateral 


THE  GENERAL  SHAPES  OP  ANIMALS.  201 

form,  by  the  action  of  unsymmetrically-bilateral  conditions. 
A  further  illustration  is  supplied  by  Bopyrus,  Fig.  271 :  a 
parasite  which  lives  in  the  branchial  chamber  of  prawns,  and 
whose  habits  similarly  account  for  its  distorted  shape. 

§  251.  Among  the  Mollusca  we  find  more  varied  relations 
between  shapes  and  circumstances.  Some  of  these  relations 
are  highly  instructive. 

Mollusks  of  one  order,  the  Pteropoda,  swim  in  the  sea 
much  in  the  same  way  that  butterflies  fly  in 
the  air,  and  have  shapes  not  altogether  unlike 
those  of  butterflies.  Fig.  272  represents  one 
of  these  creatures.  That  its  bilaterally-sym- 
metrical shape  harmonizes  with  its  bilater- 
ally-symmetrical conditions  is  sufficiently 
obvious. 

Among  the  Lamellibranchiata,  we  have 
diverse  forms  accompanying  diverse  modes  of  life.  Such 
of  them  as  frequently  move  about,  like  the  fresh-water 
Mussel,  have  their  two  valves  and  the  contained  parts 
alike  on  the  opposite  sides  of  a  vertical  plane:  they  are 
bilaterally  symmetrical  in  conformity  with  their  mode  of 
movement.  The  marine  Mussel,  too,  though  habitually 
fixed,  and  though  not  usually  so  fixed  that  its  two  valves  are 
similarly  conditioned,  still  retains  that  bilateral  symmetry 
which  is  characteristic  of  the  order;  and  it  does  this  because 
in  the  species  considered  as  a  whole,  the  two  valves  are  not 
dissimilarly  conditioned.  If  the  positions  of  the  various 
individuals  are  averaged,  it  will  be  seen  that  the  differen- 
tiating actions  neutralize  one  another.  In  certain 
other  fixed  Lamellibranchs,  however,  there  is  a  considerable 
deviation  from  bilateral  symmetry;  and  it  is  a  deviation  of 
the  kind  to  be  anticipated  under  the  circumstances.  Where 
one  valve  is  always  downwards,  or  next  to  the  surface  of 
attachment,  while  the  other  valve  is  always  upwards,  or  next 
to  the  environing  water,  we  may  expect  to  find  the  two 


202  MORPHOLOGICAL  DEVELOPMENT. 

valves  become  unlike.  This  we  do  find :  witness  the  Oyster. 
In  the  Oyster,  too,  we  see  a  further  irregularity.  There  is  a 
great  indefiniteness  of  outline,  both  in  the  shell  and  in  the 
animal — an  indefiniteness  made  manifest  by  comparing  dif- 
ferent individuals.  We  have  but  to  remember  that  growing 
clustered  together,  as  Oysters  do,  they  must  interfere  with 
one  another  in  various  ways  and  degrees,  to  see  how  the 
indeterminateness  of  form  and  the  variety  of  form  are 
accounted  for. 

Among  the  Gasteropods  modifications  of  a  more  definite 
kind  occur.  "In  all  Mollusks,"  says  Professor  Huxley, 
"  the  axis  of  the  body  is  at  first  straight,  and  its  parts  are 
arranged  symmetrically  with  regard  to  a  longitudinal  vertical 
plane,  just  as  in  a  vertebrate  or  an  articulate  embryo."  In 
some  Gasteropods,  as  the  Chiton,  this  bilateral  symmetry 
is  retained — the  relations  of  the  body  to  surrounding  actions 
not  being  such  as  to  disturb  it.  But  in  those  more  numerous 
types  which  have  spiral  shells,  there  is  a  marked  deviation 
from  bilateral  symmetry,  as  might  be  expected.  "  This 
asymmetrical  over-development  never  affects  the  head  or 
foot  of  the  mollusk  " :  only  those  parts  which,  by  inclosure 
in  a  shell,  are  protected  from  environing  actions,  lose  their 
bilateralness ;  while  the  external  parts,  subjected  by  the 
movements  of  the  creatures  to  bilateral  conditions,  remain 
bilateral.  Here,  however,  a  difficulty  meets  us.  Why  is  it 
that  the  naked  Gasteropods,  such*  as  our  common  slugs, 
deviate  from  bilateral  symmetry,  though  their  modes  of 
movement  are  those  along  with  which  complete  bilateral 
symmetry  usually  occurs?  The  reply  is  that  their  devia- 
tions from  bilateral  symmetry  are  probably  inherited,  and 
that  they  are  maintained  in  such  parts  of  their  organization 
as  are  not  exposed  to  bilaterally-symmetrical  conditions. 
There  is  reason  to  believe  that  the  naked  Gasteropods  are 
descended  from  Gasteropods  which  had  shells:  the  evidence 
being  that  the  naked  Gasteropods  have  shells  during  the 
early  stages  of  their  development,  and  that  some  of  them 


THE  GENERAL  SHAPES  OF  ANIMALS.  203 

retain  rudimentary  shells  throughout  life.  Now  the  shelled 
Gasteropods  deviate  from  bilateral  symmetry  in  the  disposi- 
tion of  both  the  alimentary  system  and  the  reproductive 
system.  The  naked  Gasteropods,  in  losing  their  shells,  have 
lost  that  immense  one-sided  development  of  the  alimentary 
system  which  fitted  them  to  their  shells,  and  have  acquired 
that  bilateral  symmetry  of  external  figure  which  fits  them 
for  their  habits  of  locomotion;  but  the  reproductive  system 
remains  one-sided,  because,  in  respect  to  it,  the  relations  to 
external  conditions  remain  one-sided. 

The  Cephalopods  show  us  bilaterally-symmetrical  external 
forms  along  with  habits  of  movement  through  the  water  in 
two-sided  attitudes.  At  the  same  time,  in  the  radial  distri- 
bution of  the  arms,  enabling  one  of  these  creatures  to  take 
an  all-sided  grasp  of  its  prey,  we  see  how  readily  upon  one 
kind  of  symmetry  there  may  be  partially  developed  another 
kind  of  symmetry,  where  the  relations  to  conditions  favour  it. 

§  252.  The  Vertebrata  illustrate  afresh  the  truths  which 
we  have  already  traced  among  the  Annulosa.  Flying  through 
the  air,  swimming  through  the  water,  and  running  over  the 
earth  as  vertebrate  animals  do,  in  common  with  annulose 
animals,  they  are,  in  common  with  annulose  animals,  different 
at  their  anterior  and  posterior  ends,  different  at  their  dorsal 
and  ventral  surfaces,  but  alike  along  their  two  sides.  This 
single  bilateral  symmetry  remains  constant  under  the  ex- 
tremest  modifications  of  form.  Among  fish  we  see  it  alike 
in  the  horizontally-flattened  Skate,  in  the  vertically-flattened 
Bream,  in  the  almost  spherical  Diodon,  and  in  the  greatly- 
elongated  Syngnathus.  Among  reptiles  the  Turtle,  the  Snake, 
and  the  Crocodile  all  display  it.  And  under  the  countless 
modifications  of  structure  displayed  by  birds  and  mammals, 
it  remains  conspicuous. 

A  less  obvious  fact  which  it  concerns  us  to  note  among  the 
Vertebrate,  parallel  to  one  which  we  noted  among  the  An- 
nulosa, is  that  whereas  the  lower  vertebrate  forms  deviate 


204 


MORPHOLOGICAL  DEVELOPMENT. 


but  little  from  triple  bilateral  symmetry,  the  deviation  be- 
comes great  as  we  ascend.  Figs.  273  and  274  show  how, 
besides  being  divisible  into  similar  halves  by  a  vertical  plane 
passing  through  its  axis,  a  Fish  is  divisible  into  halves  that 
are  not  very  dissimilar  by  a  horizontal  plane  passing  through 
its  axis,  and  also  into  other  not  very  dissimilar  halves  by  a 
plane  cutting  it  transversely.  If,  as  shown  in  Figs.  275  and 
276,  analogous  sections  be  made  of  a  superior  Keptile,  the 
divided  parts  differ  more  decidedly.  When  a  Mammal  and  a 


Bird  are  treated  in  the  same  way,  as  shown  in  Figs.  277, 
278,  and  Figs.  279,  280,  the  parts  marked  off  by  the  dividing 
planes  are  unlike  in  far  greater  degrees.  On  considering 


THE   GENERAL   SHAPES  OF  ANIMALS.  £05 

the  mechanical  converse  between  organisms  of  these  several 
types  and  their  environments  —  on  remembering  that  the 
fish  habitually  moves  through  a  homogeneous  medium  of 
nearly  the  same  specific  gravity  as  itself,  that  the  terrestrial 
reptile  either  crawls  on  the  surface  or  raises  itself  very  in- 
completely above  it,  that  the  more  active  mammal,  having 
its  supporting  parts  more  fully  developed,  thereby  has  the 
under  half  of  its  body  made  more  different  from  the  upper 
half,  and  that  the  bird  is  subject  by  its  mode  of  life  to  yet 
another  set  of  actions  and  reactions;  we  shall  see  that  these 
facts  are  quite  congruous  with  the  general  doctrine,  and 
furnish  further  support  to  it. 

One  other  significant  piece  of  evidence  must  be  named. 
Among  the  Annulosa  we  found  unsymmetrical  bilateralness 
in  creatures  having  habits  exposing  them  to  unlike  conditions 
on  their  two  sides ;  and  among  the  Vertebrata  we  find  parallel 
cases.  They  are  presented  by  the  Pleuronectidce — the  order 
of  distorted  flat  fishes  to  which  the  Sole  and  the  Flounder 
belong.  On  the  hypothesis  of  evolution,  we  must  conclude 
that  fishes  of  this  order  have  arisen  from  an  ordinary  bila- 
terally-symmetrical type  of  fish,  which,  feeding  at  the  bottom 
of  the  sea,  gained  some  advantage  by  placing  itself  with  one 
of  its  sides  4°wnwards,  instead  of  maintaining  the  vertical 
attitude.  Besides  the  general  reason  there  are  special 
reasons  for  concluding  this.  In  the  first  place,  the  young 
Sole  or  Flounder  is  bilaterally  symmetrical — has  its  eyes  on 
opposite  sides  of  its  head  and  swims  in  the  usual  way.  In 
the  second  place,  the  metamorphosis  which  produces  the  un- 
symmetrical structure  sometimes  does  not  take  place — there 
are  abnormal  Flounders  that  swim  vertically,  like  other  fishes. 
In  the  third  place,  the  transition  from  the  symmetrical  struc- 
ture to  the  unsymmetrical  structure  may  be  traced.  Almost 
incredible  though  it  seems,  one  of  the  eyes  is  transferred 
from  the  under-side  of  the  head  to  the  upper-side:  the 
transfer  being  effected  by  a  distorted  development  of  the 
cranial  bones — atrophy  of  some  and  hypertrophy  of  others, 


206  MORPHOLOGICAL  DEVELOPMENT. 

along  with  a  general  twist.  This  metamorphosis  furnishes 
several  remarkable  illustrations  of  the  way  in  which  forms 
become  moulded  into  harmony  with  incident  forces.  For 
besides  the  divergence  from  bilateral  symmetry  involved  by 
presence  of  both  eyes  upon  the  upper  side,  there  is  a  further 
divergence  from  bilateral  symmetry  involved  by  differentiation 
of  the  two  sides  in  respect  to  the  contours  of  their  surfaces 
and  the  sizes  of  their  fins.  And  then,  what  is  still  more 
significant,  there  is  a  near  approach  to  likeness  between  the 
halves  that  were  originally  unlike,  but  are,  under  the  new 
circumstances,  exposed  to  like  conditions.  The  body  is 
divisible  into  similarly-shaped  parts  by  a  plane  cutting  it 
along  the  side  from  head  to  tail :  "  the  dorsal  and  ventral 
instead  of  the  lateral  halves  become  symmetrical  in  outline 
and  are  equipoised." 

§  253.  Thus,  little  as  there  seems  in  common  between  the 
shapes  of  plants  and  the  shapes  of  animals,  we  yet  find,  on 
analysis,  that  the  same  general  truths  are  displayed  by 
both.  The  one  ultimate  principle  that  in  any  organism  equal 
amounts  of  growth  take  place  in  those  directions  in  which 
the  incident  forces  are  equal,  serves  as  a  key  to  the  phenomena 
of  morphological  differentiation.  By  it  we  are  furnished 
with  interpretations  of  those  likenesses  and  unlikenesses  of 
parts,  which  are  exhibited  in  the  several  kinds  of  symmetry; 
and  when  we  take  into  account  inherited  effects,  wrought 
under  ancestral  conditions  contrasted  in  various  ways  with 
present  conditions,  we  are  enabled  to  comprehend,  in  a  gen- 
eral way,  the  actions  by  which  animals  have  been  moulded 
into  the  shapes  they  possess. 

To  fill  up  the  outline  of  the  argument,  so  as  to  make  it 
correspond  throughout  with  the  argument  respecting  vegetal 
forms,  it  would  be  proper  here  to  devote  a  chapter  to  the 
differentiations  of  those  homologous  segments  out  of  which 
animals  of  certain  types  are  composed.  Though,  among  most 
animals  of  the  third  degree  of  composition,  such  as  the 


THE  GENERAL  SHAPES  OF  ANIMALS.  207 

rooted  Hydrozoa,  the  Polyzoa,  and  the  Ascidioida,  the  united 
individuals  are  not  reduced  to  the  condition  of  segments  of  a 
composite  individual,  and  do  not  display  any  marked  differ- 
entiations; yet  there  are  some  animals  in  which  such 
subordinations,  and  consequent  heterogeneities,  occur.  The 
oceanic  Hydrozoa  form  one  group  of  them ;  and  we  have  seen 
reason  to  conclude  that  the  Anmdosa  form  another  group. 
It  is  not  worth  while,  however,  to.  occupy  space  in  detailing 
these  unlikenesses  of  homologous  segments,  and  seeking 
specific  explanations  of  them.  Among  the  oceanic  Hydrozoa 
they  are  extremely  varied;  and  the  habits  and  derivations  of 
these  creatures  are  so  little  known,  that  there  are  no  ade- 
quate data  for  interpreting  the  forms  of  the  parts  in  terms 
of  their  relations  to  the  environment.  Conversely,  among 
the  Annulosa  those  differentiations  of  the  homologous  seg- 
ments which  accompany  their  progressing  integration,  have 
so  much  in  common,  and  have  general  causes  which  are  so 
obvious,  that  it  is  needless  to  deal  with  them  at  any  length. 
They  are  all  explicable  as  due  to  the  exposure  of  different 
parts  of  the  chain  of  segments  to  different  sets  of  actions  and 
reactions:  the  most  general  contrast  being  that  between  the 
anterior  segments  and  the  posterior  segments,  answering  to 
the  most  general  contrast  of  conditions  to  which  annul  ose 
animals  subject  their  segments;  and  the  more  special  con- 
trasts answering  to  the  contrasts  of  conditions  entailed  by 
their  more  special  habits. 

Were  an  exhaustive  treatment  of  the  subject  practicable, 
there  should  here,  also,  come  a  chapter  devoted  to  the  in- 
ternal structures  of  animals — meaning,  more  especially,  the 
shapes  and  arrangements  of  the  viscera.  The  relations 
between  forms  and  forces  among  these  inclosed  parts  are, 
however,  mostly  too  obscure  to  allow  of  interpretation. 
Protected  as  the  viscera  are  in  great  measure  from  the  inci- 
dence of  external  forces,  we  are  not  likely  to  find  much 
correspondence  between  their  distribution  and  the  distribu- 
tion of  external  forces.  In  this  case  the  influences,  partly 


208  MORPHOLOGICAL  DEVELOPMENT. 

mechanical,  partly  physiological,  which  the  organs  exercise 
on  one  another,  become  the  chief  causes  of  their  changes  of 
figure  and  arrangement;  and  these  influences  are  complex 
and  indefinite.  One  general  fact  may,  indeed,  be  noted — the 
fact,  namely,  that  the  divergence  towards  asymmetry  which 
generally  characterizes  the  viscera,  is  marked  among  those 
of  them  which  are  most  removed  from  mechanical  converse 
with  the  environment,  but  not  so  marked  among  those  of 
them  which  are  less  removed  from  such  converse.  Thus 
while,  throughout  tthe  Vertebrata,  the  alimentary  system, 
with  the  exception  of  its  two  extremities,  is  asymmetrically 
arranged,  the  respiratory  system,  which  occupies  one  end  of 
the  body,  generally  deviates  but  little  from  bilateral  sym- 
metry, and  the  reproductive  system,  partly  occupying  the 
other  end  of  the  body,  is  in  the  main  bilaterally  symmetrical : 
such  deviation  from  bilateral  symmetry  as  occurs,  being 
found  in  its  most  interiorly-placed  parts,  the  ovaries.  Just 
indicating  these  facts  as  having  a  certain  significance,  it  will 
be  best  to  leave  this  part  of  the  subject  as  too  involved  for 
detailed  treatment. 

Internal  structures  of  one  class,  however,  not  included 
among  the  viscera,  admit  of  general  interpretation — struc- 
tures which,  though  internal,  are  brought  into  tolerably- 
direct  relations  with  environing  forces,  and  are  therefore 
subordinate  in  their  forms  to  the  distribution  of  those  forces. 
These  internal  structures  it  will  be  desirable  to  deal  with 
at  some  length ;  both  because  they  furnish  important  illustra- 
tions enforcing  the  general  argument,  and  because  an  inter- 
pretation of  them  which  we  have  seen  reason  to  reject,  can- 
not be  rejected  without  raising  the  demand  for  some  other 
interpretation. 


CHAPTER  XV. 

THE  SHAPES  OF  VERTEBKATE  SKELETONS. 

§  254.  WHEN  an  elongated  mass  of  any  substance  is 
transversely  strained,  different  parts  of  the  mass  are  ex- 
posed to  forces  of  opposite  kinds.  If,  for  example,  a  bar 
of  metal  or  wood  is  supported  at  its  two  ends,  as  shown  in 
Fig.  281,  and  has  to  bear  a  weight  on  its  centre,  its  lower 


/\  fj 

part  is  thrown  into  a  state  of  tension,  while  its  upper  part  is 
thrown  into  a  state  of  compression.  As  will  be  manifest  to 
any  one  who  observes  what  happens  on  breaking  a  stick 
across  his  knee,  the  greatest  degree  of  tension  falls  on  the 
fibres  forming  the  convex  surface,  while  the  fibres  forming 
the  concave  surface  are  subject  to  the  greatest  degree  of 
compression.  Between  these  extremes  the  fibres  at  different 
depths  are  subject  to  different  forces.  Progressing  upwards 
from  the  under  surface  of  the  bar  shown  in  Fig.  281,  the 
tension  of  the  fibres  becomes  less;  and  progressing  down- 
wards from  the  upper  surface,  the  compression  of  the  fibres 
becomes  less;  until,  at  a  certain  distance  between  the  two 
surfaces,  there  is  a  place  at  which  the  fibres  are  neither  ex- 
tended nor  compressed.  This,  shown  by  the  dotted  line  in 
60  o09 


210  MORPHOLOGICAL  DEVELOPMENT. 

the  figure,  is  called  in  mechanical  language  the  "neutral 
axis."  It  varies  in  position  with  the  nature  of  the  substance 
strained :  being,  in  common  pine-wood,  at  a  distance  of  about 
five-eighths  of  the  depth  from  the  upper  surface,  or  three- 
eighths  from  the  under  surface.  Clearly,  if  such  a  piece  of 
wood,  instead  of  being  subject  to  a  downward  force,  is  secured 
at  its  ends  and  subject  to  an  upward  force,  the  distribution 
of  the  compressions  and  tensions  will  be  reversed,  and  the 
neutral  axis  will  be  nearest  to  the  upper  surface.  Fig.  282 
represents  these  opposite  attitudes  of  the  bar  and  the  changed 


position  of  its  neutral  axis:  the  arrow  indicating  the  direc- 
tion of  the  force  producing  the  upward  bend,  and  the  faint 
dotted  line  a,  showing  the  previous  position  of  the  neutral  axis. 
Between  the  two  neutral  axes  will  be  seen  a  central  space; 
and  it  is  obvious  that  when  the  bar  has  its  strain  from  time 
to  time  reversed,  the  repeated  changes  of  its  molecular  con- 
dition must  affect  the  central  space  in  a  way  different  from 
that  in  which  they  affect  the  two  outer  spaces.  Fig.  283  is  a 
diagram  conveying  some  idea  of  these  contrasts  in  molecular 
condition.  If  A  B  C  D  be  the  middle  part  of  a  bar  thus 
treated,  while  G  H  and  K  L  are  the  alternating  neutral 
axes ;  then  the  forces  to  which  the  bar  is  in  each  case  subject, 
may  be  readily  shown.  Supposing  the  deflecting  force  to 
be  acting  in  the  direction  of  the  arrow  E,  then  the  tensions 
to  which  the  fibres  between  G  and  F  are  exposed,  will  be 
represented  by  a  series  of  lines  increasing  in  length  as  the 
distance  from  G  increases;  so  that  the  triangle  G  F  M,  will 
express  the  amount  and  distribution  of  all  the  molecular 
tensions.  But  the  molecular  compressions  throughout  the 
space  from  G  to  E,  must  balance  the  molecular  tensions; 
and  hence,  if  the  triangle  G  E  N  be  made  equal  to  the  tri- 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.   211 

angle  G  F  M,  the  parallel  lines  of  which  it  is  composed  (here 
dotted  for  the  sake  of  distinction)  will  express  the  amount 
and  distribution  of  the  compressions  between  E  and  G. 


Similarly,  when  the  deflecting  force  is  in  the  direction  of  the 
arrow  F,  the  compressions  and  tensions  will  be  quantitatively 
symbolized  by  the  triangles  K  F  0,  and  K  E  P.  And 
thus  the  several  spaces  occupied  by  full  lines  and  by  dotted 
lines  and  by  the  two  together,  will  represent  the  different 
actions  to  which  different  parts  of  the  transverse  section  are 
subject  by  alternating  transverse  strains.  Here,  then,  it  is 
made  manifest  to  the  eye  that  the  central  space  between  G 
and  K,  is  differently  conditioned  from  the  spaces  above  and 
below  it;  and  that  the  difference  of  condition  is  sharply 
marked  off.  The  fibres  forming  the  outer  surface  C  D,  are 
subject  to  violent  tensions  and  violent  compressions.  Pro- 
gressing inwards  the  tensions  and  compressions  decrease — 
the  tensions  the  more  rapidly.  As  we  approach  the  point  G, 
the  tensions  to  which  the  fibres  are  alternately  subject,  bear 
smaller  and  smaller  ratios  to  the  compressions,  and  disappear 
at  the  point  G.  Thence  to  the  centre  occur  compressions 
only,  of  alternating  intensities,  becoming  at  the  centre  small 


212       MORPHOLOGICAL  DEVELOPMENT. 

and  equal ;  and  from  the  centre  we  advance,  through  a  reverse 
series  of  changes,  to  the  other  side. 

Thus  it  is  demonstrable  that  any  substance  in  which  the 
power  of  resisting  compression  is  unequal  to  the  power  of 
resisting  tension,  cannot  be  subject  to  alternating  transverse 
strains,  without  having  a  central  portion  differentiated  in  its 
conditions  from  the  outer  portions,  and  consequently  dif- 
ferentiated in  its  structure.  This  conclusion  may  easily  be 
verified  by  experiment.  If  something-  having  a  certain 
toughness  but  not  difficult  to  break,  as  a  thick  piece  of  sheet 
lead,  be  bent  from  side  to  side  till  it  is  broken,  the  surface  of 
fracture  will  exhibit  an  unlikeness  of  texture  between  the 
inner  and  outer  parts. 

§  255.  And  now  for  the  application  of  this  seemingly- 
irrelevant  truth.  Though  it  has  no  obvious  connection  with 
the  interpretation  of  vertebral  structure,  we  shall  soon  see 
that  it  fundamentally  concerns  us. 

The  simplest  type  of  vertebrate  animal,  the  fish,  has  a 
mode  of  locomotion  which  involves  alternating  transverse 
strains.  It  is  not,  indeed,  subjected  to  alternating  transverse 
strains  by  some  outer  agency,  as  in  the  case  we  have  been 
investigating:  it  subjects  itself  to  them.  But  though  the 
strains  are  here  internally  produced  instead  of  externally 
produced,  the  case  is  not  therefore  removed  into  a  wholly 
384  different  category.  For  sup- 
posing Fig.  284  to  represent 
the  outline  of  a  fish  when 
bent  on  one  side  (the  dotted  lines  representing  its  outline 
when  the  bend  is  reversed),  it  is  clear  that  part  of  the  sub- 
stance forming  the  convex  half  must  be  in  a  state  of  tension. 
This  state  of  tension  implies  the  existence  in  the  other  half 
of  some  counter-balancing  compression.  And  between  the 
two  there  must  be  a  neutral  axis.  The  way  in  which  this 
conclusion  is  reconcilable  with  the  fact  that  there  is  tension 
somewhere  in  the  concave  side  of  a  fish,  since  the  curve  is 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.   213 

caused  by  muscular  contractions  on  the  concave  side,  will  be 
made  clear  by  the  rude  illustration  which  a  bow  supplies. 
A  bow  may  be  bent  by  a  thrust  against  its  middle  (the  two 
ends  being  held  back),  or  it  may  be  bent  by  contracting 
a  string  that  unites  its  ends;  but  the  distributions  of  me- 
chanical forces  within  the  wood  of  the  bow,  though  not  quite 
alike  in  the  two  cases,  will  be  very  similar.  Now  while  the 
muscular  action  on  the  concave  side  of  a  fish  differs  from  that 
represented  by  the  tightened  string  of  a  bow,  the  difference 
is  not  such  as  to  destroy  the  applicability  of  the  illustration: 
the  parallel  holds  so  far  as  this,  that  within  that  portion  of 
the  fish's  body  which  is  passively  bent  by  the  contracting 
muscles,  there  must  be,  as  in  a  strung  bow,  a  part  in  com- 
pression, a  part  in  tension,  and  an  intermediate  part  which 
is  neutral. 

After  thus  seeing  that  even  in  the  developed  fish  with 
its  complex  locomotive  apparatus,  this  law  of  the  transverse 
strain  holds  in  a  qualified  way,  we  shall  understand  how 
much  more  it  must  hold  in  any  form  that  may  be  supposed 
to  initiate  the  vertebrate  type — a  form  devoid  of  that 
segmentation  by  which  the  vertebrate  type  is  more  or  less 
characterized.  We  shall  see  that  assuming  a  rudimentary 
animal,  still  simpler  than  the  AmpJiioxus,  to  have  a  feeble 
power  of  moving  itself  through  the  water  by  the  undulations 
of  its  body,  or  some  part  of  its  body,  there  will  necessarily 
come  into  play  certain  reactions  which  must  affect  the  median 
portion  of  the  undulating  mass  in  a  way  unlike  that  in 
which  they  affect  its  lateral  portions.  And  if  there  exists  in 
this  median  portion  a  tissue  which  keeps  its  place  with  any 
constancy,  we  may  expect  that  the  differential  conditions 
produced  in  it  by  the  transverse  strain,  will  initiate  a  dif- 
ferentiation. It  is  true  that  the  distribution  of  the  viscera 
in  the  AmpJiioxus,  Fig.  191,  and  in  the  type  from  which  we 
may  suppose  it  to  have  arisen,  is  such  as  to  interfere  with  this 
process.  It  is  also  true  that  the  actions  and  reactions  de- 
scribed would  not  of  themselves  give  to  the  median  portion 


214:  MORPHOLOGICAL  DEVELOPMENT. 

a  cylindrical  shape,  like  that  of  the  cartilaginous  rod  running 
along  the  back  of  the  Amphioxus.  But  what  we  have  here 
to  note  in  the  first  place  is,  that  these  habitual  alternate 


flexions  have  a  tendency  to  mark  off  from  the  outer  parts 
an  unlike  inner  part,  which  may  be  seized  hold  of,  main- 
tained, and  further  modified,  by  natural  selection,  should 
any  advantage  thereby  result.  And  we  have  to  note  in  the 
second  place,  that  an  advantage  is  likely  to  result.  The 
contractions  cannot  be  effective  in  producing  undulations, 
unless  the  general  shape  of  the  body  is  maintained.  External 
muscular  fibres  unopposed  by  an  internal  resistant  mass, 
would  cause  collapse  of  the  body.  To  meet  the  require- 
ments there  must  be  a  means  of  maintaining  longitudinal 
rigidity  without  preventing  bends  from  side  to  side ;  and  such 
a  means  is  presented  by  a  structure  initiated  as  described. 
In  brief,  whether  we  have  or  have  not  the  actual  cause,  we 
have  here  at  any  rate  "  a  true  cause."  Though  there  are 
difficulties  in  tracing  out  the  process  in  a  definite  way,  it 
may  at  least  be  said  that  the  mechanical  genesis  of  this  rudi- 
mentary vertebrate  axis  is  quite  conceivable.  And  even  the 
difficulties  may,  I  think,  be  more  fully  met  than  at  first 
sight  seems  possible. 

What  is  to  be  said  of  the  other  leading  trait  which  the 
simplest  vertebrate  animal  has  in  common  with  all  higher 
vertebrate  animals — the  segmentation  of  its  lateral  muscular 
masses?  Is  this,  too,  explicable  on  the  mechanical  hypo- 
thesis? Have  we,  in  the  alternating  transverse  strains,  a 
cause  for  the  fact  that  while  the  rudimentary  vertebrate  axis 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.   215 

is  without  any  divisions,  there  are  definite  divisions  of  the 
substance  forming  the  animal's  sides?  I  think  we  have.  A 
glance  at  the  distribution  of  forces  under  the  transverse 
strain,  as  represented  in  the  foregoing  diagrams,  will  show 
how  much  more  severe  is  the  strain  on  the  outer  parts  than 
on  the  inner  parts ;  and  how,  consequently,  any  modifications 
of  structure  eventually  necessitated,  will  arise  peripherally 
before  they  arise  centrally.  The  perception  of  this  may  be 
enforced  by  a  simple  experiment.  Take  a  stick  of  sealing- 
wax  and  warm  it  slowly  and  moderately  before  the  fire,  so  as 
to  give  it  a  little  flexibility.  Then  bend  it  gently  until  it  is 
curved  into  a  semi-circle.  On  the  convex  surface  small 
cracks  will  be  seen,  and  on  the  concave  surface  wrinkles; 
while  between  the  two  the  substance  remains  undistorted. 
If  the  bend  be  reversed  and  re-reversed,  time  after  time, 
these  cracks  and  wrinkles  will  become  fissures  which  gradu- 
ally deepen.  But  now,  if  changes  of  this  class,  entailed  by 
alternating  transverse  strains,  commence  superficially,  as  they 
manifestly  must;  there  arise  the  further  questions — What 
will  be  the  special  modifications  produced  under  these  special 
conditions?  and  through  what  stages  will  these  modifica- 
tions progress?  Every  one  has  literally  at  hand  an  example 
of  the  way  in  which  a  flexible  external  layer  that  is  now 
extended  and  now  compressed,  by  the  bending  of  the  mass  it 
covers,  becomes  creased;  and  a  glance  at  the  palms  and  the 
fingers  will  show  that  the  creases  are  near  one  another 
where  the  skin  is  thin,  and  far  apart  where  the  skin  is  thick. 
Between  this  familiar  case  and  the  case  of  the  rhinoceros- 
hide,  in  which  there  are  but  a  few  large  folds,  various  grada- 
tions may  be  traced.  Now  the  like  must  happen  with  the 
increasing  layers  of  contractile  fibres  forming  the  sides  of 
the  muscular  tunic  in  such  a  type  as  that  supposed.  The 
bendings  will  produce  in  them  small  wrinkles  while  they  are 
thin,  but  more  decided  and  comparatively  distant  fissures 
as  they  become  thick.  Fig.  289,  which  is  a  horizontal  longi- 
tudinal section,  shows  how  these  thickening  layers  will 


216  MORPHOLOGICAL  DEVELOPMENT. 

adjust  themselves  on  the  convex  and  the  concave  surfaces, 
supposing  the  fibres  of  which  they  are  composed  to  be  ob- 
lique, as  their  function  requires;  and  it 
is   not   difficult  to   see  that  when   once 
definite  divisions  have  been  established, 
they  will  advance  inwards  as  the  layers 
develop;  and  will  so  produce  a  series  of 
muscular  bundles.      Here  then  we  have 
something  like  the  myocommata  [or  myotomes  as  now  called] 
which  are  traceable  in  the  Ampliioxus,  and  are  conspicuous 
in  all  superior  fishes. 

§  256.  These  are  highly  speculative  conceptions.  I  have 
ventured  to  present  them  with  the  view  of  implying  that 
the  hypothesis  of  the  mechanical  genesis  of  vertebrate  struc- 
ture is  not  wholly  at  fault  when  applied  to  the  most  rudi- 
mentary vertebrate  animal.  Lest  it  should  be  alleged  that 
the  question  is  begged  if  we  set  out  with  a  type  which,  like 
the  Amphioxus,  already  displays  segmentation  throughout 
its  muscular  system,  it  seemed  needful  to  indicate  conceiv- 
able modes  in  which  there  may  have  been  mechanically  pro- 
duced those  leading  traits  that  distinguish  the  Amphioxus. 
All  I  intend  to  suggest  is  that  mechanical  actions  have  been 
at  work,  and  that  probably  they  have  operated  in  the  manner 
alleged :  so  preparing  the  way  for  natural  selection. 

But  now  let  us  return  to  the  region  of  established  fact,  and 
consider  whether  such  actions  and  reactions  as  we  actually 
witness,  are  adequate  causes  of  those  observed  differentiations 
and  integrations  which  distinguish  the  more-developed  verte- 
brate animals.  Let  us  see  whether  the  theory  of  mechanical 
genesis  affords  us  a  deductive  interpretation  of  the  inductive 
generalizations. 

Before  proceeding,  we  must  note  a  process  of  functional 
adaptation  which  here  co-operates  with  natural  selection. 
I  refer  to  the  usual  formation  of  denser  tissues  at  those 
parts  of  an  organism  which  are  exposed  to  the  greatest 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.   217 

strains — either  compressions  or  tensions.  Instances  of  hard- 
ening under  compression  are  made  familiar  to  us  by  the 
skin.  We  have  the  general  contrast  between  the  soft  skin 
covering  the  body  at  large,  and  the  indurated  skin  covering 
the  inner  surfaces  of  the  hands  and  the  soles  of  the  feet. 
We  have  the  fact  that  even  within  these  areas  the  parts 
on  which  the  pressure  is  habitually  greatest  have  the  skin 
always  thickest;  and  that  in  each  person  special  points 
exposed  to  special  pressures  become  specially  dense — often 
as  dense  as  horn.  Further,  we  have  the  converse  fact  that 
the  skin  of  little-used  hands  becomes  abnormally  thin — even 
losing,  in  places,  that  ribbed  structure  which  distinguishes 
skin  subject  to  rough  usage.  Of  increased  density  directly 
following  increased  tension,  the  skeletons,  whether  of  men 
or  animals,  furnish  abundant  evidence.  Anatomists  easily 
discriminate  between  the  bones  of  a  strong  man  and  those  of 
a  weak  man,  by  the  greater  development  of  those  ridges  and 
crests  to  which  the  muscles  are  attached ;  and  naturalists,  on 
comparing  the  remains  of  domesticated  animals  with  those 
of  wild  animals  of  the  same  species,  find  kindred  differences. 
The  first  of  these  facts  shows  unmistakably  the  immediate 
effect  of  function  on  structure,  and  by  obvious  alliance  with 
it  the  second  may  be  held  to  do  the  same:  both  implying 
that  the  deposit  of  dense  substance  capable  of  great  resist- 
ance, constantly  takes  place  at  points  where  the  tension  is 
excessive. 

Taking  into  account,  then,  this  adaptive  process,  con- 
tinually aided  by  the  survival  of  individuals  in  which  it 
has  taken  place  most  rapidly,  we  may  expect,  on  tracing  up 
the  evolution  of  the  vertebrate  axis,  to  find  that  as  the  mus- 
cular power  becomes  greater  there  arise  larger  and  harder 
masses  of  tissue,  serving  the  muscles  as  points  d'appui;  and 
that  these  arise  first  in  those  places  where  the  strains  are 
greatest.  Now  this  is  just  what  we  do  find.  The  myocom- 
mata  are  so  placed  that  their  actions  are  likely  to  affect  first 
that  upper  coat  of  the  notochord,  where  there  are  found 


218     •   MORPHOLOGICAL  DEVELOPMENT. 

"  quadrate  masses  of  somewhat  denser  tissue,"  which  "  seem 
faintly  to  represent  neural  spines,"  even  in  the  Amphioxus. 
It  is  by  the  development  of  the  neural  spines,  and  after  them 
of  the.  hffimal  spines,  that  the  segments  of  the  vertebral 
column  are  first  marked  out ;  and  under  the  increasing  strains 
of  more-developed  myocommata,  it  is  just  these  peripheral 
appendages  of  the  vertebral  segments  that  must  be  most 
subject  to  the  forces  which  cause  the  formation  of  denser 
tissue.  It  follows  from  the  mechanical  hypothesis  that  as 
the  muscular  segmentation  must  begin  externally  and  pro- 
gress inwards,  so,  too,  must  the  vertebral  segmentation. 
Besides  thus  finding  reason  for  the  fact  that  in  fishes  with 
wholly  cartilaginous  skeletons,  the  vertebral  segments  are 
indicated  by  these  processes,  while  yet  the  notochord  is 
unsegmented;  we  find  a  like  reason  for  the  fact  that  the 
transition  from  the  less-dense  cartilaginous  skeleton  to  the 
more-dense  osseous  skeleton,  pursues  a  parallel  course.  In 
the  existing  Lepidosiren,  which  by  uniting  certain  piscine  and 
amphibian  characters  betrays  its  close  alliance  with  primitive 
types,  the  axial  part  of  the  vertebral  column  is  unossified, 
while  there  is  ossification  of  the  peripheral  parts.  Similarly 
with  numerous  genera  of  fishes  classed  as  palaeozoic.  The 
fossil  remains  of  them  show  that  while  the  neural  and  hremal 
spines  consisted  of  bone,  the  central  parts  of  the  vertebra 
were  not  bony.  It  may  in  some  cases  be  noted,  too,  both  in 
extant  and  in  fossil  forms,  that  while  the  ossification  is  com- 
plete at  the  outer  extremities  of  the  spines  it  is  incomplete 
at  their  inner  extremities — thus  similarly  implying  centri- 
petal development. 

§  257.  After  these  explanations  the  process  of  eventual 
segmentation  in  the  spinal  axis  itself,  will  be  readily  under- 
stood. The  original  cartilaginous  rod  has  to  maintain  longi- 
tudinal rigidity  while  permitting  lateral  flexion.  As  fast  as 
it  becomes  definitely  marked  out,  it  will  begin  to  concentrate 
within  itself  a  great  part  of  those  pressures  and  tensions 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.    219 

caused  by  transverse  strains.  As  already  said,  it  must  be 
acted  upon  much  in  the  same  manner  as  a  bow,  though  it  is 
bent  by  forces  acting  in  a  more  indirect  way ;  and  like  a  bow, 
it  must,  at  each  bend,  have  the  substance  of  its  convex  side 
extended  and  the  substance  of  its  concave  side  compressed. 
So  long  as  the  vertebrate  animal  is  small  or  inert,  such  a 
cartilaginous  rod  may  have  sufficient  strength  to  withstand 
the  muscular  strains;  but,  other  things  equal,  the  evolution 
of  an  animal  that  is  large,  or  active,  or  both,  implies  mus- 
cular strains  which  must  tend  to  cause  modification  in  such  a 
cartilaginous  rod.  ¥  he  results  of  greater  bulk  and  of  greater 
vivacity  may  be  best  dealt  with  separately.  As  the 

animal  increases  in  size,  the  rod  will  grow  both  longer  and 
thicker.  On  looking  back  at  the  diagrams  of  forces  caused 
by  transverse  strains,  it  will  be  seen  that  as  the  rod  grows 
thicker,  its  outer  parts  must  be  exposed  to  more  severe  ten- 
sions and  pressures  if  the  degree  of  bend  is  the  same.  It  is 
doubtless  true  that  when  the  fish,  advancing  by  lateral 
undulations,  becomes  longer,  the  curvature  assumed  by  the 
body  at  each  movement  becomes  less;  and  that  from  this 
cause  the  outer  parts  of  the  notochord  are,  other  things 
equal,  less  strained — the  two  changes  thus  partially  neutrali- 
zing one  another.  But  other  things  are  not  equal.  For 
while,  supposing  the  shape  of  the  body  to  remain  constant, 
the  force  exerted  in  moving  the  body  increases  as  the  cubes 
of  its  dimensions,  the  sectional  area  of  the  notochord,  on 
which  fall  the  reactions  of  this  exerted  force,  increases  only 
as  the  squares  of  the  dimensions :  whence  results  a  greater 
stress  upon  its  substance.  This,  however,  will  not  be  very 
decided  where  there  is  no  considerable  activity.  It  is  clear 
that  augmenting  bulk,  taken  alone,  involves  but  a  moderate 
residuary  increase  of  strain  on  each  portion  of  the  notochord ; 
and  this  is  probably  the  reason  why  it  is  possible  for  a  large 
sluggish  fish  like  the  Sturgeon,  to  retain  the  notochordal 
structure.  But  now,  passing  to  the  effects  of  greater 

activity,  a  like  dynamical  inquiry  at  once  shows  us  how  rapid- 


220        MORPHOLOGICAL  DEVELOPMENT. 

ly  the  violence  of  the  actions  and  reactions  rises  as  the  move- 
ments become  more  vivacious.  In  the  first  place,  the  resist- 
ance of  a  medium  such  as  water  increases  as  the  square  of 
the  velocity  of  the  body  moving  through  it ;  so  that  to  main- 
tain double  the  speed,  a  fish  has  to  expend  four  times  the 
energy.  But  the  fish  has  to  do  more  than  this — it  has  to 
initiate  this  speed,  or  to  impress  on  its  mass  the  force  implied 
by  this  speed.  Now  the  vis  viva  of  a  moving  body  varies  as 
the  square  of  the  velocity;  whence  it  follows  that  the  energy 
required  to  generate  that  vis  viva  is  measured  by  the  square 
of  the  velocity  it  produces.  Consequently,  did  the  fish  put 
itself  in  motion  instantaneously)  the  expenditure  of  energy  in 
generating  its  own  vis  viva  and  simultaneously  overcoming 
the  resistance  of  the  water,  would  vary  as  the  fourth  power 
of  the  velocity.  But  the  fish  cannot  put  itself  in  motion 
instantaneously — it  must  do  it  by  increments ;  and  thus  it 
results  that  the  amounts  of  the  forces  expended  to  give  itself 
different  velocities  must  be  represented  by  some  series  of 
numbers  falling  between  the  squares  and  the  fourth  powers 
of  those  velocities.  Were  the  increments  slowly  accumulated, 
the  ratios  of  increasing  effort  would  but  little  exceed  the  ratios 
of  the  squares;  but  whoever  observes  the  sudden,  convulsive 
action  with  which  an  alarmed  fish  darts  out  of  a  shallow  into 
deep  water,  will  see  that  the  velocity  is  rapidly  generated, 
and  that  therefore  the  ratios  of  increasing  effort  probably 
exceed  the  ratios  of  the  squares  very  considerably.  At  any 
rate  it  will  be  clear  that  the  efforts  made  by  fishes  in  rushing 
upon  prey  or  escaping  enemies  (and  it  is  these  extreme  efforts 
which  here  concern  us)  must,  as  fishes  become  more  active, 
rapidly  exalt  the  strains  to  be  borne  by  their  motor  organs; 
and  that  of  these  strains,  those  which  fall  upon  the  noto- 
chord  must  be  exalted  in  proportion  to  the  rest.  Thus  the 
development  of  locomotive  power,  which  survival  of  the 
fittest  must  tend  in  most  cases  to  favour,  involves  such  in- 
crease of  stress  on  the  primitive  cartilaginous  rod  as  will 
tend,  other  things  equal,  to  cause  its  modification. 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.   221 

What  must  its  modification  be  ?  Considering  the  compli- 
cation of  the  influences  at  work,  conspiring,  as  above  indi- 
cated, in  various  ways  and  degrees,  we  cannot  expect  to  do 
more  than  form  an  idea  of  its  average  character.  The  nature 
of  the  changes  which  the  notochord  is  likely  to  undergo,  where 
greater  bulk  is  accompanied  by  higher  activity,  is  rudely 
indicated  by  Figs.  291,  292,  and  293.  The  successively 


*91 


thicker  lines  represent  the  successively  greater  strains  to 
which  the  outer  layers  of  tissue  are  exposed;  and  the  widen- 
ing inter-spaces  represent  the  greater  extensions  which  they 
have  to  bear  when  they  become  convex,  or  else  the  greater 
gaps  that  must  be  formed  in  them.  Had  these  outer  layers 
to  undergo  extension  only,  as  on  the  convex  side,  continued 
natural  selection  might  result  in  the  formation  of  a  tissue 
elastic  enough  to  admit  of  the  requisite  stretching.  But  at 
each  alternate  bend  these  outer  layers,  becoming  concave, 
are  subject  to  increased  compression — a  compression  which 
they  cannot  withstand  if  they  have  become  simply  more 
extensible.  To  withstand  this  greater  compression  they  must 
become  harder  as  well  as  more  extensible.  How  are  these 
two  requirements  to  be  reconciled?  If,  as  facts  warrant 
us  in  supposing,  a  formation  of  denser  substance  occurs  at 
those  parts  of  the  notochord  where  the  strain  is  greatest; 
it  is  clear  that  this  formation  cannot  so  go  on  as  to  produce 
a  continuous  mass :  the  perpetual  flexions  must  prevent  this. 
If  matter  that  will  not  yield  at  each  bend,  is  deposited  while 
the  bendings  are  continually  taking  place,  the  bendings  will 
maintain  certain  places  of  discontinuity  in  the  deposit — 
places  at  which  the  whole  of  the  stretching  consequent  on 
each  bend  will  be  concentrated.  And  thus  the  tendency  will 
be  to  form  segments  of  hard  tissue  capable  of  great  resistance 


222       MORPHOLOGICAL  DEVELOPMENT. 

to  compression,  with  intervals  filled  by  elastic  tissue  capable 
of  great  resistance  to  extension — a  vertebral  column. 

And  now  observe  how  the  progress  of  ossification  is  just 
such  as  conforms  to  this  view.  That  centripetal  develop- 
ment of  segments  which  holds  of  the  vertebrate  animal  as  a 
whole,  as,  if  caused  by  transverse  strains,  it  ought  to  do,  and 
which  holds  of  the  vertebral  column  as  a  whole,  as  it  ought 
to  do,  holds  also  of  the  central  axis.  On  the  mechanical 
hypothesis,  the  outer  surface  of  the  notochord  should  be  the 
first  part  to  undergo  induration,  and  that  division  into  seg- 
ments which  must  accompany  induration.  And  accordingly, 
in  a  vertebral  column  of  which  the  axis  is  beginning  to 
ossify,  the  centrums  consist  of  bony  rings  inclosing  a  still- 
continuous  rod  of  cartilage. 

§  258.  Sundry  other  general-  facts  disclosed  by  the  com- 
parative morphology  of  the  Vertebrata,  supply  further  con- 
firmation. Let  us  take  first  the  structure  of  the  skull. 

On  considering  the  arrangement  of  the  muscular  flakes,  or 
myocommata,  in  any  ordinary  fish  which  comes  to  table — an 
arrangement  already  sketched  out  in  the  Amphioxus — it  is 
not  difficult  to  see  that  that  portion  of  the  body  out  of  which 
the  head  of  the  vertebrate  animal  becomes  developed,  is  a 
portion  which  cannot  subject  itself  to  bendings  in  the  same 
degree  as  the  rest  of  the  body.  The  muscles  developed  there 
must  be  comparatively  short,  and  much  interfered  with  by 
the  pre-existing  orifices.  Hence  the  cephalic  part  will  not 
partake  in  any  considerable  degree  of  the  lateral  undula- 
tions ;  and  there  will  not  tend  to  arise  in  it  any  such  distinct 
segmentation  as  arises  elsewhere.  We  have  here,  then,  an 
explanation  of  the  fact,  that  from  the  beginning  the  develop- 
ment of  the  head  follows  a  course  unlike  that  of  the  spinal 
column;  and  of  the  fact  that  the  segmentation,  so  far  as  it 
can  be  traced  in  the  head,  is  most  readily  to  be  traced  in  the 
occipital  region  and  becomes  lost  in  the  region  of  the  face. 
For  if,  a's  we  have  seen,  the  segmentation  consequent  on 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.   223 

mechanical  actions  and  reactions  must  progress  from  without 
inwards,  affecting  last  of  all  the  axis;  and  if,  as  we  have 
seen,  the  region  of  the  head  is  so  circumstanced  that  the 
causes  of  segmentation  act  but  feebly  even  on  its  periphery ; 
then  that  terminal  portion  of  the  primitive  notochord  which 
is  included  in  the  head,  having  to  undergo  no  lateral  bend- 
ings,  may  ossify  without  division  into  segments. 

Of  other  incidental  evidences  supplied  by  comparative 
morphology,  let  me  next  refer  to  the  supernumerary  bones, 
which  the  theory  of  Goethe  and  Oken  as  elaborated  by  Prof. 
Owen,  has  to  get  rid  of  by  gratuitous  suppositions.  In  many 
fishes,  for  example,  there  are  what  have  been  called  inter- 
neural  spines  and  inter-hasmal  spines.  These  cannot  by  any 
ingenuity  be  affiliated  upon  the  archetypal  vertebra,  and 
they  are  therefore  arbitrarily  rejected  as  bones  belonging  to 
the  exo-skeleton ;  though  in  shape  and  texture  they  are 
similar  to  the  spines  between  which  they  are  placed.  On  the 
hypothesis  of  evolution,  however,  these  additional  bones  are 
accounted  for  as  arising  under  actions  like  those  that  gave 
origin  to  the  bones  adjacent  to  them.  And  similarly  with 
such  bones  as  those  called  sesamoid;  together  with  others 
too  numerous  to  name. 

§'259.  Of  course  the  foregoing  synthesis  is  to  be  taken 
simply  as  an  adumbration  of  the  process  by  which  the  verte- 
brate structure  may  have  arisen  through  the  continued 
actions  of  known  agencies.  The  motive  for  attempting  it  has 
been  two-fold.  Having,  as  before  said,  given  reasons  for  con- 
cluding that  the  segments  of  a  vertebrate  animal  are  not 
homologous  in  the  same  sense  as  are  those  of  an  annulose 
animal,  it  seemed  needful  to  do  something  towards  showing 
how  they  are  otherwise  to  be  accounted  for ;  and  having  here, 
for  our  general  subject,  the  likenesses  and  differences  among 
the  parts  of  organisms,  as  determined  by  incident  forces,  it 
seemed  out  of  the  question  to  pass  by  the  problem  presented 
by  the  vertebrate  skeleton. 


224       MORPHOLOGICAL  DEVELOPMENT. 

Leaving  out  all  that  is  hypothetical,  the  general  argument 
may  be  briefly  presented  thus: — The  evolution  from  the 
simplest  known  vertebrate  animal  of  a  powerful  and  active 
vertebrate  animal,  implies  the  development  of  a  stronger 
internal  fulcrum.  The  internal  fulcrum  cannot  be  made 
stronger  without  becoming  more  dense.  And  it  cannot  be- 
come more  dense  while  retaining  its  lateral  flexibility,  with- 
out becoming  divided  into  segments.  Further,  in  conformity 
with  the  general  principles  thus  far  traced,  these  segments 
must  be  alike  in  proportion  as  the  forces  to  which  they  are 
exposed  are  alike,  and  unlike  in  proportion  as  these  forces 
are  unlike;  and  so  there  necessarily  results  that  unity  in 
variety  by  which  the  vertebral  column  is  from  the  beginning 
characterized.  Once  more,  we  see  that  the  explanation  ex- 
tends to  those  innumerable  and  more  marked  divergences 
from  homogeneity,  which  vertebrae  undergo  in  the  various 
higher  animals.  Thus,  the  production  of  vertebrae,  the  pro- 
duction of  likenesses  among  vertebrae,  the  production  of  un- 
likenesses  among  vertebrae,  and  the  production  of  unlike- 
nesses  among  vertebral  columns,  are  interpretable  as  parts  of 
one  general  process,  and  as  harmonizing  with  one  general 
principle. 

Whether  sufficient  or  insufficient,  the  explanation  here 
given  assigns  causes  of  known  kinds  producing  effects  such 
as  they  are  known  to  produce.  It  does  not,  as  a  solution  of 
one  mystery,  offer  another  mystery  of  which  no  solution  is 
to  be  asked.  It  does  not  allege  a  Platonic  iSco,  or  fictitious 
entity,  which  explains  the  vertebrate  skeleton  by  absorbing 
into  itself  all  the  inexplicability.  On  the  contrary,  it  assumes 
nothing  beyond  agencies  by  which  structures  in  general  are 
moulded — agencies  by  which  these  particular  structures  are, 
indeed,  notoriously  modifiable.  An  ascertained  cause  of  cer- 
tain traits  in  vertebras  and  other  bones,  it  extends  to  all 
other  traits  of  vertebrae;  and  at  the  same  time  assimilates 
the  morphological  phenomena  they  present  to  much  wider 
classes  of  morphological  phenomena. 


THE  SHAPES  OP  VERTEBRATE  SKELETONS.   225 

[NOTE. — The  theory  set  forth  in  the  foregoing  chapter,  is 
an  elaboration  of  one  suggested  at  the  close  of  a  criticism 
of  Prof.  Owen's  Archetype  and  Homologies  of  the  Vertebrate 
Skeleton,  already  referred  to  in  §  210  as  having  been  pub- 
lished in  the  Medico-Chirurgical  Review  for  October,  1858. 
It  is  now  reproduced  in  Appendix  B.  Since  the  issue  of  this 
elaborated  exposition,  in  No.  15  of  my  serial  in  December, 
1865,  verifications  of  it  have  from  time  to  time  been  published. 
In  his  work  The  Primary  Factors  of  Organic  Evolution, 
Prof.  Cope  of  Philadelphia  writes : — 

"  Mr.  Herbert  Spencer  has  endeavoured  to  account  for  the 
origin  of  the  segmentation  of  muscles  into  myotomes,  and 
the  division  of  the  sheath  of  the  notochord  into  vertebrae, 
by  supposing  it  to  be  due  to  the  lateral  swimming  movements 
of  the  fishes,  which  first  exhibit  these  structures.  With  this 
view  various  later  authors  have  agreed,  and  I  have  offered 
some  additional  evidence  of  the  soundness  of  this  position 
with  respect  to  the  vertebral  axis  of  Batrachia,  and  the 
origin  of  limb  articulations.  It  is  true  that  the  origin  of 
segmentation  in  the  vertebral  column  of  the  true  fishes  and 
the  Batrachia  turns  out  to  have  been  less  simple  in  its  pro- 
cess than  was  suggested  by  Mr.  Spencer,  but  his  general 
principle  holds  good,  now  that  paleontology  has  cleared  up 
the  subject"  (pp.  367-8). 

An  allusion  in  the  foregoing  extract  is  made  by  Prof.  Cope 
to  certain  observations  set  forth  in  his  work  entitled  The 
Origin  of  the  Fittest.  On  pp.  305-6  of  it  will  be  found  the 
following  sentences : — 

"  Now,  all  the  Permian  land-animals,  reptiles  and  batra- 
chians,  retain  this  notochord  with  the  elements  of  osseous 
vertebrae,  in  a  greater  or  less  degree  of  completeness.  There 
are  some  in  South  Africa,  I  believe,  in  which  the  ossification 
has  come  clear  through  the  notochord;  but  they  are  few. 
.  .  .  There  is  something  to  be  said  as  to  the  condition  of 
the  column  from  a  mechanical  standpoint,  and  it  is  this: 
that  the  chorda  exists,  with  its  osseous  elements  disposed 
61 


223        MORPHOLOGICAL  DEVELOPMENT. 

about  it;  and  in  the  Permian  batrachians,  equally  related  to 
salamanders  and  frogs,  these  osseous  elements  are  arranged  in 
the  sheath  or  skin  of  the  chorda ;  and  they  are  in  the  form  of 
regular  concave  segments,  very  much  like  such  segments  as 
you  can  take  from  the  skin  of  an  orange — but  parts  of  a 
cylinder,  and  having  greater  or  less  dimensions  according  to 
the  group  or  species.  Now,  the  point  of  divergence  of  these 
segments  is  on  the  side  of  the  column.  The  contacts  are 
placed  on  the  side  of  the  column  where  the  segments  separ- 
ate— the  upper  segments  rising  and  the  lower  segments 
coming  downward.  To  the  upper  segments  are  attached  the 
arches  and  their  articulations,  and  the  lower  segments  are 
like  the  segments  of  a  cylinder.  If  you  take  a  flexible 
cylinder,  and  cover  it  with  a  more  or  less  inflexible  skin  or 
sheath,  and  bend  that  cylinder  sidewise,  you  of  course  will 
find  that  the  wrinkles  or  fractures  of  that  part  of  the  surface 
will  take  place  along  the  line  of  the  shortest  curve,  which 
is  on  the  side;  and,  as  a  matter  of  fact,  you  have  breaks 
of  very  much  the  character  of  the  segments  of  the  Permian 
Batrachia.  ...  In  the  cylinder  bending  both  ways,  of 
course  the  shortest  line  of  curve  is  right  at  the  centre  of  the 
side  of  that  cylinder,  and  the  longest  curve  is  of  course  at 
the  summit  and  base,  and  the  shortest  curve  will  be  the  point 
of  fracture.  And  that  is  exactly  what  I  presume  has 
happened  in  the  case  of  the  construction  of  the  segments  of 
the  sheath  of  the  vertebral  column,  by  the  lateral  motion  of 
the  animal  in  swimming,  and  which  has  been  the  actual  cause 
of  the  disposition  of  the  osseous  material  in  its  form.  .  .  . 
That  is  the  state  of  the  vertebral  column  of  many  of  the 
Vertebrata  of  the  Permian  period." 

In  his  essay  on  "  The  Mechanical  Causes  of  the  Develop- 
ment of  the  Hard  Parts  of  the  Mammalia,"  published  in  the 
American  Journal  of  Morphology  (Vol.  Ill),  Prof.  Cope  has 
carried  the  interpretation  further,  by  showing  that  in  kindred 
ways  the  genesis  of  articulations  and  limb-bones  may  be  ex- 


THE  SHAPES  OF  VERTEBRATE  SKELETONS.   227 

plained.  On  p.  163  he  enunciates  the  general  principle  of 
his  interpretation  as  follows : — 

"  It  cannot  have  been  otherwise  than  that,  since  the 
motions  of  animals  continued  during  the  evolution  of  their 
hard  parts,  these  hard  parts  grew  in  exact  adaptation  to 
these  movements.  Thus  at  the  points  of  greatest  flexure 
joints  would  be  formed,  and  between  these  joints  the  deposit 
would  be  continuous." 

Evidently  if  osseous  structures  are  produced  by  deposits 
of  calcareous  matters  in  pre-existing  cartilaginous  structures, 
or  other  structures  of  flexible  materials,  the  deposits  must  be 
so  carried  on  that  while  dense  resistant  masses  are  produced 
these  must  admit  of  such  free  movements  as  the  creature's 
life  necessitates,  and  must  so  form  adapted  joints. 

Let  it  be  understood,  however,  that  the  hypothesis  set 
forth  in  the  foregoing  chapter  and  extended  by  Prof.  Cope, 
which  serves  to  interpret  a  large  part  of  the  phenomena 
of  osseous  structures  in  the  Vertebrata,  does  not  serve  to 
interpret  them  all.  While  the  formation  of  hard  parts  has 
been  in  large  measure  initiated  and  regulated  by  tensions  and 
pressures,  there  are  hard  parts  the  formation  of  which  cannot 
be  thus  explained.  The  bones  of  the  skull  are  the  most 
obvious  instances.  These  are  apparently  referable  to  no 
other  cause  than  the  survival  of  the  fittest — the  survival  of 
individual  animals  in  which  greater  density  of  the  brain- 
covering  yielded  better  protection  against  external  injuries. 
Without  enumerating  other  instances  which  might  be  given, 
it  will  suffice  to  recognize  the  truth  that  natural  selection  of 
favourable  variations  and  the  inheritance  of  functionally- 
produced  changes  have  all  along  co-operated:  each  of  them 
in  some  cases  acting  alone,  but  in  other  cases  both  acting 
together.] 


CHAPTER  XVI. 

THE    SHAPES   OF   ANIMAL-CELLS. 

§  260.  AMONG  animals  as  among  plants,  the  laws  of  mor- 
phological differentiation  must  be  conformed  to  by  the  mor- 
phological units,  as  well  as  by  the  larger  parts  and  by  the 
wholes  formed  of  them.  It  remains  here  to  point  out  that 
the  conformity  is  traceable  where  the  conditions  are  simple. 

In  the  shapes  assumed  by  those  rapidly-multiplying  cells 
out  of  which  each  animal  is  developed,  there  is  a  conspicuous 
subordination  to  the  surrounding  actions. 
Fig.  294  represents  the  cellular  embryonic 
mass  that  arises  by  repeated  spontaneous 
fissions.  In  it  we  see  how  the  cells,  origin- 
ally spherical,  are  changed  by  pressure 
against  one  another  and  against  the  limit- 
ing membrane;  and  how  their  likenesses 
and  unlikenesses  are  determined  by  the  likenesses  and  un- 
likenesses  of  the  forces  to  which  they  are  exposed.  This  fact 
may  be  thought  scarcely  worth  pointing  out.  But  it  is 
worth  pointing  out,  because  what  is  here  so  obvious  a  con- 
sequence of  mechanical  actions,  is  in  other  cases  a  conse- 
quence of  actions  composite  in  their  kinds  and  involved  in 
their  distribution.  Just  as  the  equalities  and  inequalities  of 
dimensions  among  aggregated  cells,  are  here  caused  by  the 
equalities  and  inequalities  among  their  mutual  pressures  in 
different  directions;  so,  though  less  manifestly,  the  equalities 
228 


THE  SHAPES  OP  ANIMAL  CELLS. 


229 


and  inequalities  of  dimensions  among  other  aggregated  cells, 
are  caused  by  the  equalities  and  inequalities  of  the  osmotic, 
chemical,  thermal,  and  other  forces  besides  the  mechanical, 
to  which  their  different  positions  subject  them. 

§  261.  This  we  shall  readily  see  on  observing  the  ordinary 
structures  of  limiting  membranes,  internal  and  external. 
In  Fig.  295,  is  shown  a  29s 

much-magnified  section 
of  a  papilla  from  the 
gum.  The  cells  of  which 
it  is  composed  originate 
in  its  deeper  part;  and 
are  at  first  approximately 
spherical.  Those  of  them 
which,  as  they  develop,  are  thrust  outwards  by  the  new 
cells  that  continually  take  their  places,  have  their  shapes 
gradually  changed.  As  they  grow  and  successively  advance 
to  replace  the  superficial  cells,  when  these  exfoliate,  they 
become  exposed  to  forces  which  are  more  and  more  different 
in  the  direction  of  the  surface  from  what  they  are  in  lateral 
directions;  and  their  dimensions  gradually  assume  corre- 
sponding differences. 

Another  species  of  limiting  membrane,  called  cylinder- 
epithelium,  is  represented 
in  Fig.  296.  Though  its 
mode  of  development  is 
such  as  to  render  the 
shapes  of  its  cells  quite 
unlike  those  of  pavement- 
epithelium,  as  the  above-described  kind  is  sometimes  called, 
its  cells  equally  exemplify  the  same  general  truth.  For  the 
chief  contrast  which  each  of  them  presents,  is  the  contrast 
between  its  dimension  at  right  angles  to  the  surface  of  the 
membrane,  and  its  dimension  parallel  to  that  surface. 

It  is  needless  for  our  present  purpose  to  examine  further 


230       MORPHOLOGICAL  DEVELOPMENT. 

the  evidence  furnished  by  Histology;  nor,  indeed,  would 
further  examination  of  this  evidence  be  likely  to  yield 
definite  results.  In  the  cases  given  above  we  have  marked 
differences  among  the  incident  forces;  and  therefore  have  a 
chance  of  finding,  as  we  do  find,  relations  between  these  and 
differences  of  form.  But  the  cells  composing  masses  of 
tissue  are  severally  subject  to  forces  which  are  indeterminate; 
and  therefore  the  interpretation  of  their  shapes  is  imprac- 
ticable. It  must  suffice  to  observe  that  so  far  as  the  facts  go 
they  are  congruous  with  the  hypothesis. 


CHAPTER  XVII. 

SUMMARY   OF    MORPHOLOGICAL   DEVELOPMENT. 

§  262.  THAT  any  formula  should  be  capable  of  expressing 
a  common  character  in  the  shapes  of  things  so  unlike  as  a 
tree  and  a  cow,  a  flower  and  a  centipede,  is  a  remarkable 
fact ;  and  is  a  fact  which  affords  strong  primd  facie  evidence 
of  truth.  For  in  proportion  to  the  diversity  and  multiplicity 
of  the  cases  to  which  any  statement  applies,  is  the  probability 
that  it  sets  forth  the  essential  relations.  Those  connexions 
which  remain  constant  under  all  varieties  of  manifestation, 
are  most  likely  to  be  the  causal  connexions. 

Still  higher  will  appear  the  likelihood  of  an  alleged  law  of 
organic  form  possessing  so  great  a  comprehensiveness,  when 
we  remember  that  on  the  hypothesis  of  Evolution,  there  must 
exist  between  all  organisms  and  their  environments,  certain 
congruities  expressible  in  terms  of  their  actions  and  reactions. 
The  forces  being,  on  this  hypothesis,  the  causes  of  the  forms, 
it  is  inferable,  a  priori,  that  the  forms  must  admit  of  generali- 
zation in  terms  of  the  forces ;  and  hence,  such  a  generalization 
arrived  at  a  posteriori,  gains  the  further  probability  due  to 
fulfilment  of  anticipation. 

Nearer  yet  to  certainty  seems  the  conclusion  thus  reached, 
on  finding  that  it  does  but  assert  in  their  special  manifesta- 
tions, the  laws  of  Evolution  in  general — the  laws  of  that 
universal  re-distribution  of  matter  and  motion  which  hold 

231 


232        MORPHOLOGICAL  DEVELOPMENT. 

throughout  the  totality  of  things,  as  well  as  in  each  of  its 
parts. 

It  will  be  .useful  to  glance  back  over  the  various  minor 
inferences  arrived  at,  and  contemplate  them  in  their  ensemble 
from  these  higher  points  of  view. 

§  263.  That  process  of  integration  which  every  plant  dis- 
plays during  its  life,  we  found  reason  to  think  has  gone  on 
during  the  life  of  the  vegetal  kingdom  as  a  whole.  Proto- 
plasm into  cells,  cells  into  folia,  folia  into  axes,  axes  into 
branched  combinations — such,  in  brief,  are  the  stages  passed 
through  by  every  shrub;  and  such  appear  to  have  been  the 
stages  through  which  plants  of  successively-higher  kinds 
have  been  evolved  from  lower  kinds.  Even  among  certain 
groups  of  plants  now  existing,  we  find  aggregates  of  the  first 
order  passing  through  various  gradations  into  aggregates  of 
the  second  order — here  forming  small,  incoherent,  indefinite 
assemblages,  and  there  forming  large,  definite,  coherent 
fronds.  Similar  transitions  are  traceable  through  which 
these  integrated  aggregates  of  the  second  order  pass  into 
aggregates  of  the  third  order:  in  one  species  the  unions  of 
parent-fronds  with  the  fronds  that  bud  out  from  them,  being 
temporary,  and  in  another  species  such  unions  being  longer 
continued;  until,  in  species  still  higher,  by  a  gemmation 
which  is  habitual  and  regular,  there  is  produced  a  definitely- 
integrated  aggregate  of  the  third  order — an  axis  bearing 
fronds  or  leaves.  And  even  between  this  type  and  a  type 
further  compounded,  a  link  occurs  in  the  plants  which  cast 
off,  in  the  shape  of  bulbils,  some  of  the  young  axes  they 
produce.  As  among  plants,  so  among  animals.  A 

like  spontaneous  fission  of  cells  ends  here  in  separation,  there 
in  partial  aggregation,  while  elsewhere,  by  closer  combina- 
tion of  the  multiplying  units,  there  arises  a  coherent  and 
tolerably  definite  individual  of  the  second  order.  By  the 
budding  of  individuals  of  the  second  order,  there  are  in  some 
cases  produced  other  separate  individuals  like  them;  in  some 


SUMMARY  OF  MORPHOLOGICAL  DEVELOPMENT.    233 

cases  temporary  aggregates  of  such  like  individuals ;  and  in 
other  cases  permanent  aggregates  of  them:  certain  of  which 
become  so  definitely  integrated  that  the  individualities  of 
their  component  members  are  almost  lost  in  a  tertiary  indi- 
viduality. 

Along  with  this  progressive  integration  there  has  gone  on 
a  progressive  differentiation.  Vegetal  units  of  whatever 
order,  originally  homogeneous,  have  become  heterogeneous 
while  they  have  become  united.  Spherical  cells  aggregating 
into  threads,  into  laminae,  into  masses,  and  into  special  tis- 
sues, lose  their  sphericity;  and  instead  of  remaining  all 
alike  assume  innumerable  unlikenesses — from  uniformity 
pass  into  multiformity.  Fronds  combining  to  form  axes, 
severally  acquire  definite  differences  between  their  attached 
ends  and  their  free  ends;  while  they  also  diverge  from  one 
another  in  their  shapes  at  different  parts  of  the  axes  they 
compose.  And  axes,  uniting  into  aggregates  of  a  still  higher 
order,  become  contrasted  in  their  sizes,  curvatures,  and  the 
arrangements  of  their  appendages.  Similarly  among 

animals.  Those  components  of  them  which,  with  a  certain 
license,  we  class  as  morphological  units,  while  losing  their 
minor  individualities  in  the  major  individualities  formed  of 
them,  grow  definitely  unlike  as  they  grow  definitely  com- 
bined. And  where  the  aggregates  so  produced  become,  by 
coalescence,  segments  of  aggregates  of  a  still  higher  order, 
they,  too,  diverge  from  one  another  in  their  shapes. 

The  morphological  differentiation  which  thus  goes  hand 
in  hand  with  morphological  integration,  is  clearly  what  the 
perpetually-complicating  conditions  would  lead  us  to  antici- 
pate. Every  addition  of  a  new  unit  to  an  aggregate  of  such 
units,  must  affect  the  circumstances  of  the  other  units  in  all 
varieties  of  ways  and  degrees,  according  to  their  relative 
positions — must  alter  the  distribution  of  mechanical  strains 
throughout  the  mass,  must  modify  the  process  of  nutrition, 
must  affect  the  relations  of  neighbouring  parts  to  surround- 
ing diffused  actions;  that  is,  must  initiate  a  changed  inci- 


234       MORPHOLOGICAL  DEVELOPMENT. 

dence  of  forces  tending  ever  to  produce  changed  structural 
arrangements. 

§  264.  This  broad  statement  of  the  correspondence  be- 
tween the  general  facts  of  Morphological  Development  and 
the  principles  of  Evolution  at  large,  may  be  reduced  to 
statements  of  a  much  more  specific  kind.  The  phenomena 
of  symmetry  and  unsymmetry  and  asymmetry,  which  we 
have  traced  out  among  organic  forms,  are  demonstrably  in 
harmony  with  those  laws  of  the  re-distribution  of  matter  and 
motion  to  which  Evolution  conforms.  Besides  the  myriad- 
fold  illustrations  of  the  instability  of  the  homogeneous, 
afforded  by  these  aggregates  of  units  of  each  order,  which, 
at  first  alike,  lapse  gradually  into  unlikeness;  and  besides 
the  myriad-fold  illustrations  of  the  multiplication  of  effects, 
which  these  ever-complicating  differentiations  exhibit  to  us; 
we  have  also  myriad-fold  illustrations  of  the  definite  equali- 
ties and  inequalities  of  structures,  produced  by  definite  equali- 
ties and  inequalities  of  forces. 

The  proposition  arrived  at  when  dealing  with  the  causes 
of  Evolution,  "  that  in  the  actions  and  reactions  of  force  and 
matter,  an  unlikeness  in  either  of  the  factors  necessitates  an 
unlikeness  in  the  effects;  and  that  in  the  absence  of  unlike- 
ness in  either  of  the  factors  the  effects  must  be  alike  "  (First 
Principles,  §  169),  is  a  proposition  which  implies  all  these 
particular  likenesses  and  unlikenesses  of  parts  which  we 
have  been  tracing.  For  have  we  not  everywhere  seen  that 
the  strongest  contrasts  are  between  the  parts  that  are  most 
contrasted  in  their  conditions ;  while  the  most  similar  parts 
are  those  most-similarly  conditioned?  In  every  plant  the 
leading  difference  is  between  the  attached  end  and  the  free 
end;  in  every  branch  it  is  the  same;  in  every  leaf  it  is 
the  same.  And  in  every  plant  the  leading  likenesses  are 
those  between  the  two  sides  of  the  branch,  the  two  sides  of 
the  leaf,  and  the  two  sides  of  the  flower,  where  these  parts 
are  two-sided  in  their  conditions ;  or  between  all  sides  of  the 


SUMMARY  OF  MORPHOLOGICAL  DEVELOPMENT.  235 

branch,  all  sides  of  the  leaf,  and  all  sides  of  the  flower,  where 
these  parts  are  similarly  conditioned  on  all  sides.  So,  too,  is  it 
with  animals  which  move  about.  The  most  marked  contrasts 
they  present  are  those  between  the  part  in  advance  and  the 
part  behind,  and  between  the  upper  part  and  the  under  part ; 
while  there  is  complete  correspondence  between  the  two  sides. 
Externally  the  likenesses  and  differences  among  limbs,  and 
internally  the  likenesses  and  differences  among  vertebrae,  are 
expressible  in  terms  of  this  same  law. 

And  here,  indeed,  we  may  see  clearly  that  these  truths  are 
corollaries  from  that  ultimate  truth  to  which  all  phenomena 
of  Evolution  are  referable.  It  is  an  inevitable  deduction 
from  the  persistence  of  force,  that  organic  forms  which  have 
been  progressively  evolved,  must  present  just  those  funda- 
mental traits  of  form  which  we  find  them  present.  It  cannot 
but  be  that  during  the  intercourse  between  an  organism  and 
its  environment,  equal  forces  acting  under  equal  conditions 
must  produce  equal  effects;  for  to  say  otherwise  is,  by  im- 
plication, to  say  that  some  force  can  produce  more  or  less 
than  its  equivalent  effect,  which  is  to  deny  the  persistence  of 
force.  Hence  those  parts  of  an  organism  which  are,  by  its 
habits  of  life,  exposed  to  like  amounts  and  like  combina- 
tions of  actions  and  reactions,  must  develop  alike;  while 
unlikenesses  of  development  must  as  unavoidably  follow 
unlikenesses  among  these  agencies.  And  this  being  so,  all 
the  specialities  of  symmetry  and  unsymmetry  and  asymmetry 
which  we  have  traced,  are  necessary  consequences. 


PART  Y. 

PHYSIOLOGICAL   DEVELOPMENT 


CHAPTER  I. 

THE   PROBLEMS   OF   PHYSIOLOGY. 

§  265.  THE  questions  to  be  treated  under  the  above  title 
are  widely  different  from  those  which  it  ordinarily  expresses. 
We  have  no  alternative,  however,  but  to  use  Physiology  in 
a  sense  co-extensive  with  that  in  which  we  have  used 
Morphology.  We  must  here  consider  the  facts  of  function 
in  a  manner  parallel  to  that  in  which  we  have,  in  the  fore- 
going Part,  considered  the  facts  of  structure.  As,  hitherto, 
we  have  concerned  ourselves  with  those  most  general  pheno- 
mena of  organic  form  which,  holding  irrespective  of  class 
and  order  and  sub-kingdom,  illustrate  the  processes  of 
integration  and  differentiation  characterizing  Evolution  at 
large;  so,  now,  we  have  to  concern  ourselves  with  the  evi- 
dences of  those  differentiations  and  integrations  of  organic 
functions  which  have  simultaneously  arisen,  and  which 
similarly  transcend  the  limits  of  zoological  and  botanical 
divisions.  How  heterogeneities  of  action  have  progressed 
along  with  heterogeneities  of  structure — that  is  the  inquiry 
before  us;  and  obviously,  in  pursuing  it,  all  the  specialities 
with  which  Physiology  usually  deals  can  serve  us  only  as 
materials. 

Before  entering  on  the  study  of  Morphological  Develop- 
ment, it  was  pointed  out  that  while  facts  of  structure  may 
be  empirically  generalized  apart  from  facts  of  function,  they 
cannot  be  rationally  interpreted  apart;  and  throughout  the 

239 


240  PHYSIOLOGICAL  DEVELOPMENT. 

foregoing  pages  this  truth  has  been  made  abundantly  mani- 
fest. Here  we  are  obliged  to  recognize  the  inter-dependence 
still  more  distinctly;  for  the  phenomena  of  function  cannot 
even  be  conceived  without  direct  and  perpetual  consciousness 
of  the  phenomena  of  structure.  Though  the  subject-matter 
of  Physiology  is  as  broadly  distinguished  from  the  subject- 
matter  of  Morphology  as  motion  is  from  matter;  yet,  just  as 
the  laws  of  motion  cannot  be  known  apart  from  some  matter 
moved,  so  there  can  be  no  knowledge  of  function  without  a 
knowledge  of  some  structure  as  performing  function. 

Much  more  than  this  is  obvious.  The  study  of  functions, 
considered  from  our  present  point  of  view  as  arising  by 
Evolution,  must  be  carried  on  mainly  by  the  study  of  the 
correlative  structures.  Doubtless,  by  experimenting  on  the 
organisms  which  are  growing  and  moving  around  us,  we  may 
ascertain  the  connexions  existing  among  certain  of  their 
actions,  while  we  have  little  or  no  knowledge  of  the  special 
parts  concerned  in  those  actions.  In  a  living  animal  that 
can  be  conveniently  kept  under  observation,  we  may  learn 
the  way  in  which  conspicuous  functions  vary  together — how 
the  rate  of  a  man's  pulse  increases  with  the  amount  of 
muscular  exertion  he  is  undergoing;  or  how  a  horse's 
rapidity  of  breathing  is  in  part  dependent  on  his  speed. 
But  though  observations  of  this  order  are  indispensable — 
though  by  accumulation  and  comparison  of  such  observations 
we  learn  which  parts  perform  which  functions — though  such 
observations,  prosecuted  so  as  to  disclose  the  actions  of  all 
parts  under  all  circumstances,  constitute,  when  properly 
generalized  and  co-ordinated,  what  is  commonly  understood 
as  Physiology;  yet  such  observations  help  us  but  a  little 
way  towards  learning  how  functions  came  to  be  established 
and  specialized.  We  have  next  to  no  power  of  tracing  up 
the  genesis  of  a  function  considered  purely  as  a  function — no 
opportunity  of  observing  the  progressively-increasing  quan- 
tities of  a  given  action  that  have  arisen  in  any  order  of 
organisms.  In  nearly  all  cases  we  are  able  only  to  show 


THE  PROBLEMS  OP  PHYSIOLOGY.  241 

the  greater  growth  of  the  part  which  we  have  found  performs 
the  action,  and  to  infer  that  greater  action  of  the  part  has 
accompanied  greater  growth  of  it.  The  tracing  out  of 
Physiological  Development,  then,  becomes  substantially  a 
tracing  out  of  the  development  of  the  organs  by  which  the 
functions  are  known  to  be  discharged — the  differentiation 
and  integration  of  the  functions  being  presumed  to  have 
progressed  hand  in  hand  with  the  differentiation  and  integra- 
tion of  the  organs.  Between  the  inquiry  pursued  in  Part  IV, 
and  the  inquiry  to  be  pursued  in  this  Part,  the  contrast  is 
that,  in  the  first  place,  facts  of  structure  are  now  to  be  used 
to  interpret  facts  of  function,  instead  of  conversely;  and,  in 
the  second  place,  the  facts  of  structure  to  be  so  used  are  not 
those  of  conspicuous  shape  so  much  as  those  of  minute  texture 
and  chemical  composition. 

§  266.  The  problems  of  Physiology,  in  the  wide  sense 
above  described,  are,  like  the  problems  of  Morphology,  to  be 
considered  as  problems  to  which  answers  must  be  given  in 
terms  of  incident  forces.  On  the  hypothesis  of  Evolution 
these  specializations  of  tissues  and  accompanying  concentra- 
tions of  functions,  must,  like  the  specializations  of  shape  in 
an  organism  and  its  component  divisions,  be  due  to  the  ac- 
tions and  reactions  which  its  intercourse  with  the  environment 
involves ;  and  the  task  before  us  is  to  explain  how  they  are 
wrought — how  they  are  to  be  comprehended  as  results  of 
such  actions  and  reactions. 

Or,  to  define  these  problems  still  more  specifically : — Those 
extremely  unstable  substances  composing  the  protoplasm 
of  which  organisms  are  mainly  built,  have  to  be  traced 
through  the  various  modifications  in  their  properties  and 
powers,  that  are  entailed  on  them  by  changes  of  relation  to 
agencies  of  all  kinds.  Those  organic  colloids  which  pass  from 
liquid  to  solid  and  from  soluble  to  insoluble  on  the  slightest 
molecular  disturbance — those  albumenoid  matters  which,  as 
we  see  in  clotted  blood  or  the  coagulable  lymph  poured 


242  PHYSIOLOGICAL  DEVELOPMENT. 

out  on  abraded  surfaces  and  causing  adhesion  between 
inflamed  membranes,  assume  new  forms  with  the  greatest 
readiness — are  to  have  their  metamorphoses  studied  in  con- 
nexion with  the  influences  at  work.  Those  compounds  which, 
as  we  see  in  the  quickly-acquired  brownness  of  a  bitten  apple 
or  in  the  dark  stains  produced  by  the  milky  juice  of  a  Dande- 
lion, immediately  begin  to  alter  when  the  surrounding  actions 
alter,  are  to  be  everywhere  considered'  as  undergoing  modifi- 
cations by  modified  conditions.  Organic  bodies,  consisting 
of  substances  that,  as  I  here  purposely  remind  the  reader, 
are  prone  beyond  all  others  to  change  when  the  incident 
forces  are  changed,  we  must  contemplate  as  in  all  their  parts 
differently  changed  in  response  to  the  different  changes  of 
the  incident  forces.  And  then  we  have  to  regard  the  con- 
comitant differentiations  of  their  reactions  as  being  concomi- 
tant differentiations  of  their  functions. 

Here,  as  before,  we  must  take  into  account  two  classes 
of  factors.  We  have  to  bear  in  mind  the  inherited  results  of 
actions  to  which  antecedent  organisms  were  exposed,  and  to 
join  with  these  the  results  of  present  actions.  Each  organism 
is  to  be  considered  as  presenting  a  moving  equilibrium  of 
functions,  and  a  correlative  arrangement  of  structures,  pro- 
duced by  the  aggregate  of  actions  and  reactions  that  have 
taken  place  between  all  ancestral  organisms  and  their  envi- 
ronments. The  tendency  in  each  organism  to  repeat  this 
adjusted  arrangement  of  functions  and  structures,  must  be 
regarded  as  from  time  to  time  interfered  with  by  actions  to 
which  its  inherited  equilibrium  is  not  adjusted — actions  to 
which,  therefore,  its  equilibrium  has  to  be  re-adjusted.  And 
in  studying  physiological  development  we  have  in  all  cases 
to  contemplate  the  progressing  compromise  between  the  old 
and  the  new,  ending  in  a  restored  balance  or  adaptation. 

Manifestly  our  data  are  so  scanty  that  nothing  more 
than  very  general  and  approximate  interpretations  of  this 
kind  are  possible.  If  the  hypothesis  of  Evolution  fur- 
nishes us  with  a  rude  conception  of  the  way  in  which  the 


THE  PROBLEMS  OP  PHYSIOLOGY.  243 

more  conspicuous  and  important  differentiations  of  functions 
have  arisen,  it  is  as  much  as  can  be  expected. 

§  267.  It  will  be  best,  for  brevity  and  clearness,  to  deal 
with  these  physiological  problems  as  we  dealt  with  the 
morphological  ones — to  carry  on  the  inductive  statement  and 
the  deductive  interpretation  hand-in-hand:  so  disposing  of 
each  general  truth  before  passing  to  the  next.  Treating 
separately  vegetal  organisms  and  animal  organisms,  we  will 
in  each  kingdom  consider: — first,  the  physiological  differen- 
tiations and  accompanying  changes  of  structure  which  arise 
between  outer  tissues  and  inner  tissues;  next,  those  which 
arise  between  different  parts  of  the  outer  tissues ;  and,  finally, 
those  which  arise  between  different  parts  of  the  inner  tissues. 
What  little  has  to  be  said  concerning  physiological  integra- 
tion must  come  last.  For  though,  in  tracing  up  Morpho- 
logical Evolution,  we  have  to  study  those  processes  of  inte- 
gration by  which  organic  aggregates  are  formed,  before 
studying  the  differentiations  that  arise  among  their  parts; 
we  must,  contrariwise,  in  tracing  up  Physiological  Evolution, 
study  the  genesis  of  the  different  functions  before  we  study 
the  interdependence  that  eventually  arises  among  them  and 
constitutes  physiological  unity. 


CHAPTER  II. 

DIFFERENTIATIONS    BETWEEN     THE     OUTER    AND    INNER 
TISSUES   OF   PLANTS. 

§  268.  THE  simplest  plant  presents  a  contrast  between  its 
peripheral  substance  and  its  central  substance.  In  each  pro- 
tophyte,  be  it  a  spherical  cell  or  a  branched  tube,  or  such 
a  more-specialized  form  as  a  Desmid,  a  marked  unlikeness 
exists  between  the  limiting  layer  and  that  which  it  limits. 
These  vegetal  aggregates  of  the  first  order  may  differ  widely 
from  one  another  in  the  natures  of  their  outer  coats  and  in 
the  natures  of  their  contents.  As  in  the  Palmella-form  of 
one  of  the  lower  Algce,  there  may  exist  a  clothing  of  jelly; 
or,  as  in  Diatom,  the  walls  may  take  the  form  of  silicious 
valves  variously  sculptured.  The  contained  matter  may  be 
partly  or  wholly  here  green,  there  red,  and  in  other  cases 
brown.  But  amid  all  these  diversities  there  is  this  one 
uniformity — a  strong  distinction  between  the  parts  in  contact 
with  the  environment  and  the  parts  not  in  contact  with  the 
environment. 

When  we  remember  that  this  trait  is  one  which  these 
simple  living  bodies  have  in  common  with  bodies  that  are 
not  living  —  when  we  remember  that  each  inorganic  mass 
eventually  has  its  outer  part  more  or  less  differentiated  from 
its  inner  part,  here  by  oxidation,  there  by  drying,  and  else- 
where by  the  actions  of  light,  of  moisture,  of  frost;  we  can 
scarcely  resist  the  conclusion  that,  in  the  one  case  as  in  the 
other,  the  contrast  is  due  to  the  unlike  actions  to  which  the 
244 


THE  OUTER  AND  INNER  TISSUES  OF  PLANTS.     245 

parts  are  subject.  Given  an  originally-homogeneous  portion 
of  protoplasm,  and  it  follows  from  the  general  laws  of  Evolu- 
tion (First  Principles,  §§  149 — 155),  first,  that  it  must  lose 
its  homogeneity,  and,  second,  that  the  leading  dissimilarities 
must  arise  between  the  parts  most-dissimilarly  conditioned — 
that  is,  between  the  outside  and  the  inside.  The  exterior 
must  bear  amounts  and  kinds  of  force  unlike  the  amounts 
and  kinds  which  the  interior  bears ;  and  from  the  persistence 
of  force  it  follows  inevitably  that  unlike  effects  must  be 
wrought  on  them — they  must  be  differentiated. 

What  is  the  limit  towards  which  the  differentiation  tends  ? 
We  have  seen  that  the  re-distribution  of  matter  and  motion 
whence,  under  certain  conditions,  evolution  results,  can 
never  cease  until  equilibrium  is  reached  —  proximately  a 
moving  equilibrium,  and  finally  a  complete  equilibrium  (First 
Principles,  §§170 — 175).  Hence,  the  differentiation  must 
go  on  until  it  establishes  such  differences  in  the  parts  as 
shall  balance  the  differences  in  the  forces  acting  on  them. 
When  dealing  with  equilibration  in  general,  we  saw  that  this 
process  is  what  is  called  adaptation  (First  Principles,  §  173) ; 
and,  in  this  work,  we  saw  that  by  it  the  totality  of  func- 
tions of  an  organism  is  brought  into  correspondence  with  the 
totality  of  actions  affecting  it  (§§  159—163).  Manifestly  in 
this  case,  as  in  all  others,  either  death  or  adjustment  must 
eventually  result.  A  force  falling  on  one  of  these  minute 
aggregates  of  protoplasm,  must  expend  itself  in  working  its 
equivalent  of  change.  If  this  force  is  such  that  in  expend- 
ing itself  it  disturbs  beyond  rectification  the  balance  of  the 
organic  processes,  then  the  aggregate  is  disintegrated  or  de- 
composed. But  if  it  does  not  overthrow  that  moving  equi- 
librium constituting  the  life  of  the  aggregate,  then  the 
aggregate  continues  in  that  modified  form  produced  by  the 
expenditure  of  the  force.  Thus,  by  direct  equilibration,  con- 
tinually furthered  by  indirect  equilibration,  there  must  arise 
this  distinction  between  the  outer  part  adapted  to  meet  outer 
forces,  and  the  inner  part  adapted  to  meet  inner  forces.  And 


246  PHYSIOLOGICAL  DEVELOPMENT. 

their  respective  actions,  as  thus  meeting  outer  and  inner 
forces,  must  be  what  we  call  their  respective  functions. 

§  269.  Aggregates  of  the  second  order  exhibit  parallel 
traits,  admitting  of  parallel  interpretations.  Integrated 
masses  of  cells  or  units  homologous  with  protophytes, 
habitually  show  us  contrasts  between  the  characters  of  the 
superficial  tissues  and  the  central  tissues.  Such  among  these 
aggregates  of  the  second  order  as  have  their  component  units 
arranged  into  threads  or  laminae,  single  or  double,  cannot,  of 
course,  furnish  contrasts  of  this  kind ;  for  all  their  units  are 
as  much  external  as  internal.  We  must  turn  to  the  more  or 
less  massive  forms. 

Of  these,  among  Fungi,  the  common  Puff-ball  is  a  good 
example — good  because  it  presents  this  fundamental  differen- 
tiation but  little  complicated  by  others.  In  it  we  have  a 
cortical  layer  of  interwoven  hyphae  obviously  unlike  the 
mass  of  spores  which  it  incloses.  So  far  as  the  unlikeness 
between  external  and  internal  parts  is  concerned,  we  see  here 
a  relation  analogous  to  that  existing  in  the  simple  cell;  and 
we  see  in  it  a  similar  meaning:  there  is  a  physiological 
differentiation  corresponding  to  the  difference  in  the  incidence 
of  forces. 

Under  various  forms  the  Algce  show  just  the  same  rela- 
tion. Where,  as  in  Codium  Bursa,  we  have  the  ramified 
tubular  branches  of  the  thallus  aggregated  into  a  hollow 
globular  mass,  the  outer  and  inner  surfaces  are  contrasted 
both  in  colour  and  structure,  though  the  tubules  composing 
the  two  surfaces  are  continuous  with  one  another.  In  Rivu- 
laria,  again,  we  see  the  like,  both  in  the  radial  arrangement 
of  the  imbedded  threads  and  in  the  difference  of  colour 
between  the  exterior  of  the  imbedding  jelly  and  its  interior. 
The  more-developed  Algce  of  all  kinds  repeat  the  antithesis. 
In  branched  stems,  when  they  consist  of  more  than  single 
rows  of  cells,  the  outer  cells  become  unlike  the  inner,  as  shown 
in  Fig.  35.  Such  types  as  Chrysymenia  rosea  show  us  this 


THE  OUTER  AND  INNER  TISSUES  OF  PLANTS.     247 

unlikeness  very  conspicuously.  And  it  holds  even  with  rib- 
bon-shaped fronds.  Wherever  one  of  these  is  composed  of 
three,  four,  or  more  layers,  as  in  Laminaria  and  Punctaria, 
the  cells  of  the  external  layers  are  strongly  distinguished 
from  those  of  the  internal  layers,  both  by  their  comparative 
smallness  and  by  their  deep  colour. 

§  270.  The  higher  plants  variously  display  the  like 
fundamental  distinction  between  outer  and  inner  tissues. 
Each  leaf,  thin  as  it  is,  exemplifies  this  differentiation  of  the 
parts  immediately  in  contact  with  the  environment  from  the 
parts  not  in  immediate  contact  with  the  environment.  Its 
epidermal  cells,  forming  a  protecting  envelope,  diverge  physi- 
cally and  chemically  from  the  mesophyll  cells,  which  carry 
on  the  more  active  functions.  And  the  contrast  may  be 
observed  to  establish  itself  in  the  course  of  development.  At 
first  the  component  cells  of  the  leaf  are  all  alike;  and  this 
unlikeness  between  the  cells  of  the  outer  and  inner  la}rers, 
arises  simultaneously  with  the  rise  of  differences  in  their  con- 
ditions— differences  that  have  acted  on  all  ancestral  leaves 
as  they  act  on  the  individual  leaf. 

An  unlikeness  more  marked  in  kind  but  similar  in  mean- 
ing, exists  between  the  bark  of  every  branch  and  the  tissues 
it  clothes.  The  phaenogamic  axis,  especially  when  it  under- 
goes what  is  known  as  secondary  thickening,  is  commonly 
characterized  by  an  outer  zone  of  cells  (the  cork  layer)  differ- 
ing from  the  inner  layers  in  character  and  function,  as  it 
differs  from  them  in  position.  Subject  as  this  outer  layer  is 
to  the  unmitigated  actions  of  forces  around — to  abrasions,  to 
extremes  of  heat  and  cold,  to  evaporation  and  soaking  with 
water — its  units  have  to  be  brought  into  equilibrium  with 
these  more  violent  actions,  and  have  acquired  molecular 
constitutions  more  stable  than  those  of  the  interior  cells. 
That  is  to  say,  the  forces  which  differentiate  the  cortical  part 
from  the  rest  are  the  forces  which  it  has  to  resist,  and  from 
which  it  passively  protects  the  parts  within.  How 


248  PHYSIOLOGICAL  DEVELOPMENT. 

clearly  this  heterogeneity  of  structure  and  function  is  conse- 
quent upon'  intercourse  with  the  environment,  every  tree 
and  shrub  shows.  The  young  shoots,  alike  of  annuals  and 
perennials,  are  quite  green  and  soft  at  their  extremities. 
Among  plants  of  short  lives,  there  is  usually  but  a  slight 
development  of  bark  or  none  at  all:  such  traces  of  it  as  the 
surface  of  the  axis  acquires  being  seen  only  at  its  lowermost 
or  oldest  portion.  In  long-lived  plants,  however,  this  forma- 
tion of  a  tough  opaque  coating  takes  place  more  rapidly ;  and 
shows  us  distinctly  the  connexion  between  the  degree  of 
differentiation  and  the  length  of  exposure.  For,  in  a  growing 
twig,  we  see  that  the  bark,  invisible  at  the  bud,  thickens  by 
insensible  gradations  as  we  go  downwards  to  the  junction  of 
the  twig  with  the  branch ;  and  we  come  to  still  thicker  parts 
of  it  as  we  descend  along  the  branch  towards  the  main  stem. 
Moreover,  on  examining  main  stems  we  find  that  while  in 
some  trees  the  bark,  cracked  by  expansion  of  the  wood,  drops 
off  in  flakes,  leaving  exposed  patches  of  the  inner  tissue  which 
presently  become  green  and  finally  develop  new  bark;  in 
other  trees  the  exfoliated  flakes  continue  adherent,  and  in  the 
course  of  years  form  a  rugged  fissured  coat:  so  producing 
a  still  more  marked  contrast  between  outside  and  in- 
side. Of  course  the  establishment  of  this  hetero- 
geneity is  furthered  by  natural  selection,  which,  where  a 
protective  covering  is  needed,  gives  an  advantage  to  those 
individuals  in  which  it  has  become  strongest.  But  that  this 
divergence  of  structure  commences  as  a  direct  adaptation,  is 
clearly  shown  by  other  facts  than  the  foregoing.  There  is 
the  fact  that  many  of  the  plants  which  in  our  gardens 
develop  bark  with  considerable  rapidity,  do  not  develop  it 
with  the  same  rapidity  in  a  greenhouse.  And  there  is  the 
fact  that  plants  which,  in  some  climates,  have  their  stems 
covered  only  by  thin  semi-transparent  layers,  acquire  thick 
opaque  layers  when  taken  to  other  climates. 

Just  noting,  for  the  sake  of  completeness,  that  in  the 
roots  of  the  higher  plants  there  arises  a  contrast  between 


THE  OUTER  AND  INNER  TISSUES  OF  PLANTS.      249 

outer  and  inner  parts,  parallel  to  the  one  we  have  traced  in 
their  branches,  let  me  draw  attention  to  another  differentia- 
tion of  the  same  ultimate  nature,  which  the  higher  plants 
exhibit  to  us — a  differentiation  which,  familiar  though  it  is, 
gains  a  new  meaning  by  association  with  those  named  above, 
and  makes  their  meaning  still  more  manifest.  Each  great 
plant  shows  it.  When,  by  the  budding  of  axes  out  of  axes, 
there  is  produced  one  of  those  highly-compounded  Phsenogams 
which  we  call  a  tree,  the  central  part  of  the  aggregate  be- 
comes functionally  and  structurally  unlike  the  peripheral 
part.  On  looking  into  a  large  tree,  or  even  a  small  one 
which  has  thick  foliage,  like  the  Laurel,  we  see  that  the  in- 
ternal branches  are  almost  or  quite  bare  of  leaves,  while  the 
leaf-clad  branches  form  an  external  stratum;  and  all  our 
experience  unites  in  proving  that  this  contrast  arises  by 
degrees,  as  fast  as  the  growth  of  the  tree  entails  a  contrast  be- 
tween the  conditions  to  which  inner  and  outer  branches  are 
exposed.  Now  when,  in  these  most-composite  aggregates, 
we  see  a  differentiation  between  peripheral  and  central  parts 
demons  trably  caused  by  a  difference  in  the  relations  of  these 
parts  to  environing  forces,  we  get  support  for  the  conclusion 
otherv/ise  reached,  that  there  is  a  parallel  cause  for  the  paral- 
lel differentiations  exhibited  by  all  aggregates  of  lower  orders 
— branches,  leaves,  cells. 

§  271.  Before  leaving  this  most  general  physiological 
differentiation,  it  may  be  well  to  say  something  respecting 
certain  secondary  unlikenesses  which  usually  arise  between 
interior  and  exterior.  For  the  contrast  is  not,  as  might  be 
supposed  from  the  foregoing  descriptions,  a  simple  contrast : 
it  is  a  compound  contrast.  The  outer  structure  itself  is 
usually  divisible  into  concentric  structures.  This  is  equally 
true  of  a  protophyte  and  of  a  phasnogamic  axis.  Between 
the  centre  of  an  independent  vegetal  cell  and  its  surface, 
there  are  at  least  two  layers ;  and  the  bark  coating  the  sub- 
stance of  a  shoot,  besides  being  itself  compound,  includes 


250  PHYSIOLOGICAL  DEVELOPMENT. 

another  tissue  lying  between  it  and  the  wood.  What  is  the 
physical  interpretation  of  these  facts  ? 

When  a  mass  of  something  we  distinguish  as  inert  matter 
is  exposed  to  external  agencies  capable  of  working  changes  in 
it — when  it  is  chemically  acted  upon,  or  when,  being  dry,  it 
is  allowed  to  soak,  or  when,  being  wet,  it  is  allowed  to  dry — 
the  changes  set  up  progress  in  an  equable  way  from  the 
surface  towards  the  centre.  At  any  time  during  the  process 
(supposing  no  other  action  supervenes)  the  modification 
wrought,  first  completed  at  the  outside,  either  gradually 
diminishes  as  we  approach  the  centre,  or  ceases  suddenly  at 
a  certain  distance  from  the  centre.  But  now  suppose  that 
the  mass,  instead  of  being  inert,  is  the  seat  of  active  changes 
— suppose  that  it  is  a  portion  of  complex  colloidal  substance, 
permeable  by  light  and  by  fluids  capable  of  affecting  its 
unstable  molecules — suppose  that  its  interior  is  a  source  of 
forces  continually  liberated  and  diffusing  themselves  out- 
wards. Is  it  not  likely  that  while  at  the  centre  the  action 
of  the  internally-liberated  forces  will  dominate,  and  while  at 
the  surface  the  action  of  the  environing  forces  will  dominate, 
there  will  be  between  the  two  a  certain  place  at  which  their 
actions  balance?  May  we  not  expect  that  this  will  be  the 
place  where  the  most  unstable  matter  exists — the  place  out- 
side of  which  the  matter  becomes  relatively  stable  in  the 
face  of  external  forces,  and  inside  of  which  the  matter  be- 
comes relatively  stable  in  the  face  of  internal  forces?  And 
must  we  not  conclude  that  though  part  of  the  adjustment  is 
due  to  indirect  equilibration,  the  initiation  of  it  is  due  to 
direct  equilibration? 

But  we  are  here  chiefly  concerned  with  the  more  general 
interpretation,  which  is  independent  of  any  such  speculation 
as  the  foregoing.  These  contrasted  tissues  and  the  contrasted 
functions  they  severally  perform  are,  beyond  question,  sub- 
ordinated to  the  relations  of  outside  and  inside.  And  the 
evidence  makes  it  tolerably  clear  that  the  unlike  actions  or 
forces  involved  by  the  relations  of  outside  and  inside,  deter- 
mine these  contrasts — partly  directly  and  partly  indirectly. 


CHAPTER  III. 

DIFFERENTIATIONS    AMONG    THE    OUTER    TISSUES    OF    PLANTS. 

§  272.  THE  motionless  protococcoid  forms  of  lower  Algce, 
which  do  not  permanently  expose  any  parts  of  their  surfaces 
to  actions  unlike  those  which  other  parts  are  exposed  to,  have 
no  parts  of  their  surfaces  unlike  the  rest  in  function  and 
composition.  This  is  what  the  hypothesis  prepares  us  for. 
If  physiological  differentiations  are  determined  by  differences 
in  the  incidence  of  forces,  then  there  will  be  no  such  differ- 
entiations where  there  are  no  such  differences.  Contrariwise, 
it  is  to  be  expected  that  the  most  conspicuous  unlikeness  of 
function  and  minute  structure  will  arise  between  the  most- 
dissimilarly  circumstanced  parts  of  the  surface.  We  find 
that  they  do.  The  upper  end  and  the  lower  end,  or,  more 
strictly  speaking,  the  free  end  and  the  attached  end,  habitu- 
ally present  the  strongest  physiological  contrasts. 

Even  aggregates  of  the  first  order  illustrate  this  truth. 
Such  so-called  unicellular  plants  as  those  delineated  in 
Figs.  4,  5,  and  6,  show  us,  on  comparing  the  contents  of 
their  fixed  ends  and  their  loose  ends,  that  different  processes 
are  going  on  in  them,  and  that  different  functions  are 
being  performed  by  their  limiting  membranes.  Caulerpa 
prolifera,  which  "consists  of  a  little  creeping  stem  with 
roots  below  and  leaves  above,"  originating  "  in  the  growth 
of  a  body  which  may  be  regarded  as  an  individual  cell," 
supplies  a  still-better  example.  Among  aggre- 

251 


252  PHYSIOLOGICAL  DEVELOPMENT. 

gates  of  the  second  order  a  like  connexion  is  displayed  in 
more  various  modes  but  with  equal  consistency.  As  before, 
the  Puff-ball  served  to  exemplify  the  primary  physiological 
differentiation  of  outer  parts  from  inner  parts;  so,  here,  it 
supplies  a  simple  illustration  of  the  way  in  which  the 
differentiated  outer  part  is  re-differentiated,  in  correspon- 
dence with  the  chief  contrast  in  its  relations  to  the  environ- 
ment. The  only  marked  unlikeness  which  the  cortical  layer 
of  the  Puff-ball  presents,  is  that  between  the  portion  next 
the  ground  and  the  opposite  portion.  The  better-developed 
Fungi  exhibit  a  more  decided  heterogeneity  of  parallel  kind. 
Such  incrusting  Algce  as  Ralfsia  verrucosa  furnish  a  kin- 
dred contrast;  and  in  the  higher  Algce  it  is  uniformly 
repeated.  Phsenogams  display  this  physiologi- 

cal differentiation  very  conspicuously.  That  earth  and  air 
are  unlike  portions  of  the  environment,  subjecting  roots  and 
leaves  to  unlike  physical  forces,  which  entail  on  them  unlike 
reactions,  and  that  the  unlike  functions  and  structures  of 
their  respective  surfaces  are  fitted  to  these  unlike  physical 
forces,  are  familiar  facts  which  it  would  be  needless  here 
to  name,  were  it  not  that  they  must  be  counted  as  coming 
within  a  wider  group  of  facts. 

Is  this  unlikeness  between  the  outer  tissues  of  the  attached 
ends  and  those  of  the  free  ends  in  plants,  determined  by 
their  converse  with  the  unlike  parts  of  the  environment? 
That  they  result  from  an  equilibration  partly  arising  in 
the  individual  and  partly  arising  by  the  survival  of  indivi- 
duals in  which  it  has  been  carried  furthest,  is  inferable 
a  priori;  and  this  a  priori  argument  may  be  adequately 
enforced  by  arguments  of  the  inductive  order.  A  few 
typical  ones  must  here  suffice.  The  gemmules 

of  the  Marchantia  are  little  disc-shaped  masses  of  cells 
composed  of  two  or  more  layers.  Their  sides  being  alike, 
there  is  nothing  to  determine  which  side  falls  lowermost 
when  one  of  them  is  detached.  Whichever  side  falls  lower- 
most, however,  presently  begins  to  send  out  rootlets,  while 


THE  OUTER  TISSUES  OF  PLANTS.  253 

the  uppermost  side  begins  to  assume  those  characters  which 
distinguish  the  face  of  the  frond.  When  this  differentiation 
has  commenced,  the  tendency  to  its  complete  establishment 
becomes  more  and  more  decided;  as  is  proved  by  the  fact 
that  if  the  positions  of  the  surfaces  be  altered,  the  gemmule 
bends  itself  so  as  to  re-adjust  them :  the  change  towards 
equilibrium  with  environing  forces  having  been  once  set  up, 
there  is  acquired,  as  it  were,  an  increasing  momentum  which 
resists  any  counter-change.  But  the  evidence  shows  that 
at  the  outset,  the  relations  to  earth  and  air  alone  deter- 
mine the  differentiation  of  the  under  surface  from  the 
upper.  The  experiences  of  the  gardener,  multi- 

plying his  plants  by  cuttings  and  layers,  constitute  another 
class  of  evidences  not  to  be  omitted:  they  are  commonplace 
but  instructive  examples  of  physiological  differentiation. 
While  circumstanced  as  it  usually  is,  the  meristematic  tissue 
of  each  branch  in  a  Phaonogam  continues  to  perform  its 
ordinary  function — regularly  producing  on  its  outer  side  the 
cortical  substances,  and  on  its  inner  side  the  vascular  and 
woody  tissues.  But  change  the  conditions  to  those  which 
the  underground  part  of  the  plant  is  exposed  to,  and  there 
begins  another  differentiation  resulting  in  underground  struc- 
tures. Contact  with  water  often  suffices  alone  to  produce 
this  result,  as  in  the  branches  of  some  trees  when  they  droop 
into  a  pool,  or  as  occasionally  with  a  cutting  placed  in  a 
bottle  of  water;  and  when  the  light  is  excluded  by  im- 
bedding the  end  of  the  cutting,  or  the  middle  of  the  still- 
attached  branch,  in  the  earth,  this  production  of  tissues 
adapted  to  the  function  of  absorbing  moisture  and  mineral 
constituents  proceeds  still  more  readily.  With  such  cases 
may  be  grouped  those  in  which  this  development  of  under- 
ground organs  by  an  above-ground  tissue,  is  not  excep- 
tional but  habitual.  Creeping  plants  furnish  good  illus- 
trations. From  the  shoots  of  the  Ground-Ivy,  rootlets  are 
put  out  into  the  soil  in  a  manner  differing  but  little  from 
that  in  which  they  are  put  out  by  an  imbedded  layer;  save 


254  PHYSIOLOGICAL  DEVELOPMENT. 

that  the  process  follows  naturally-induced  conditions  instead 
of  following  artificially-induced  conditions.  But  in  the 
common  Ivy  which,  instead  of  running  along  the  surface 
of  the  earth,  runs  up  inclined  or  vertical  surfaces,  we  see  the 
process  interestingly  modified  without  being  essentially 
changed.  The  rootlets,  here  differentiated  by  their  con- 
ditions into  organs  of  attachment  much  more  than  organs  of 
absorption,  still  develop  on  that  side  of  the  shoot  next  the 
supporting  surface,  and  do  not  develop  where  the  shoot, 
growing  away  from  the  tree  or  wall,  is  surrounded  equally 
on  all  sides  by  light  and  air:  thus  showing,  undeniably, 
that  the  production  of  the  rootlets  is  determined  by  the 
differential  incidence  of  forces.  Though  survival  of  the 
fittest  doubtless  furthered  this  transition  yet  it  clearly  did 
not  initiate  it.  That  greenness  which  may  be 

observed  in  these  Ivy-branch  rootlets  while  they  are  quite 
young,  soft,  and  unshaded,  introduces  us  to  facts  which  are 
the  converse  of  the  foregoing  facts ;  and  proves  that  the  parts 
ordinarily  imbedded  in  the  soil  and  adapted  to  its  actions, 
acquire,  often  in  very  marked  degrees,  the  superficial 
structures  of  the  aerial  parts,  when  they  are  exposed  to  light 
and  air.  This  may  be  witnessed  in  Maize,  which,  when 
luxuriant,  sends  out  from  its  nodes  near  the  ground,  clusters 
of  roots  that  are  thick,  succulent,  and  of  the  same  colour  as 
the  leaves.  Examples  more  familiar  to  us  in  England  occur 
in  every  field  of  turnips.  On  noting  how  green  is  the  un- 
covered part  of  a  turnip-root,  and  how  manifestly  the 
area  over  which  the  greenness  extends  varies  with  the  area 
exposed  to  light,  as  well  as  with  the  degree  of  the  exposure, 
it  will  be  seen  that  beyond  question,  root-tissue  assumes 
to  a  considerable  extent  the  appearances  and  function  of 
leaf-tissue,  when  subject  to  the  same  agencies.  Let  us  not 
forget,  too,  that  where  exposed  roots  do  not  approach  in 
superficial  character  towards  leaves,  they  approach  in 
superficial  character  towards  stems :  becoming  clothed 
with  a  thick,  fissured  bark,  like  that  of  the  trunk  and 


THE  OUTER  TISSUES  OF  PLANTS.  255 

branches.  But    the    most    conclusive    evidence    is 

furnished  by  the  actual  substitutions  of  surface-structures 
and  functions,  that  occur  in  aerial  organs  which  have  taken 
to  growing  permanently  under  ground,  and  in  under-ground 
organs  which  have  taken  to  growing  permanently  in  the 
air.  On  the  one  hand,  there  is  the  rhizome  exemplified  by 
Ginger — a  stem  which,  instead  of  shooting  up  vertically, 
runs  horizontally  below  the  surface  of  the  soil,  and  assumes 
the  character  of  a  root,  alike  in  colour,  texture,  and 
production  of  rootlets;  and  there  is  that  kind  of  swollen 
under-ground  axis,  bearing  axillary  buds,  which  the  Potato 
exemplifies — a  structure  which,  though  homologically  an 
axis,  simulates  a  tuberous  root  in  surface-character,  and 
when  exposed  to  the  air,  manifests  no  greater  readiness  to 
develop  chlorophyll  than  a  tuberous  root  does.  On  the  other 
hand,  there  are  the  aerial  roots  of  certain  Orchids  which, 
habitually  green  at  their  tips,  continue  green  throughout 
their  whole  lengths  when  kept  moist;  which  have  become 
leaf-like  not  only  by  this  development  of  chlorophyll,  but 
also  by  the  acquirement  of  stomata;  and  which  do  not  bury 
themselves  in  the  soil  when  they  have  the  opportunity.* 
Thus  we  have  aerial  organs  so  completely  changed  to  fit 
under-ground  actions,  that  they  will  not  resume  aerial  func- 
tions; and  under-ground  organs  so  completely  changed  to 
fit  aerial  actions,  that  they  will  not  resume  under-ground 
functions. 

That  the  physiological  differentiation  between  the  part  of 
a  plant's  surface  which  is  exposed  to  light  and  air  and  the 
part  which  is  exposed  to  darkness  and  moisture  and  solid 

*  A  critical  comment  made  on  this  sentence  runs  as  follows : — "  The 
aerial  roots  of  most  epiphytic  orchids  contain  chlorophyll  in  their  cortex 
throughout  their  length,  but  the  cortex  being  covered  by  a  '  velamen '  of  air- 
containing  cells  which  break  up  and  reflect  incident  light,  the  green  colour 
is  not  visible  through  this  opaque  coat.  When  moistened  the  cells  of  the 
velamen  take  up  water  and  the  green  colour  immediately  shows  through. 
Such  roots  do  not  however  possess  stomata.  The  roots  of  certain  species  of 
Anprcecum,  however,  contain  the  whole  of  the  assimilating  tissue  of  the 
plant." 


256  PHYSIOLOGICAL  DEVELOPMENT. 

matter,  is  primarily  due  to  the  unlike  actions  of  these  unlike 
parts  of  the  environment,  is,  then,  clearly  implied  by  observed 
facts — more  clearly,  indeed,  than  was  to  be  expected.  Con- 
sidering how  strong  must  be  the  inherited  tendency  of  a  plant 
to  assume  those  special  characters,  physiological  as  well  as 
morphological,  which  have  resulted  from  an  enormous  accu- 
mulation of  antecedent  actions,  it  may  be  even  thought 
surprising  that  this  tendency  can  be  counteracted  to  so  great 
an  extent  by  changed  conditions.  Such  a  degree  of  modifi- 
ability  becomes  comprehensible  only  when  we  remember 
how  little  a  plant's  functions  are  integrated,  and  how  much, 
therefore,  the  functions  going  on  in  each  part  may  be  altered 
without  having  to  overcome  the  momentum  of  the  functions 
throughout  the  whole  plant.  But  this  modifiability  being  as 
great  as  it  is,  we  can  have  no  difficulty  in  understanding 
how,  by  the  cumulative  aid  of  natural  selection,  this  primary 
differentiation  of  the  surface  in  plants  has  become  what  we 
see  it. 

§  273.  We  will  leave  now  these  contrasts  between  the  free 
surfaces  of  plants  and  their  attached  or  imbedded  surfaces, 
and  turn  our  attention  to  the  secondary  contrasts  existing 
between  different  parts  of  their  free  surfaces.  Were  a  full 
statement  of  the  evidence  practicable,  it  would  be  proper 
here  to  dwell  on  that  which  is  furnished  by  the  inferior 
classes.  It  might  be  pointed  out  in  detail  that  where,  as 
among  the  Algae,  the  free  surfaces  are  not  dissimilarly  con- 
ditioned, there  is  no  systematic  differentiation  of  them — that 
the  frond  of  an  Ulva,  the  ribbon-shaped  divisions  of  a 
Laminaria,  and  the  dichotomous  expansions  of  the  Fuci 
which  clothe  the  rocks  between  tide-marks,  are  alike  on  both 
sides;  because,  swayed  about  in  all  directions  as  they  are  by 
the  waves  and  tides,  their  sides  are  equally  affected.  Con- 
versely, from  the  Fungi  might  be  drawn  abundant  proof  that 
even  among  Thallophytes,  unlikenesses  arise  between  different 
parts  of  the  free  surfaces  when  their  circumstances  are  unlike. 


THE  OUTER  TISSUES  OF  PLANTS.  257 

In  such  laterally-growing  kinds  as  are  shown  in  Fig.  196&, 
the  honey-combed  under  surface  and  the  smooth  leathery 
upper  surface,  have  their  contrasts  related  to  contrasted  con- 
ditions; and  in  the  adjacently-figured  Agarics,  and  other 
stalked  genera,  the  pileus  exhibits  a  parallel  difference,  ex- 
plicable in  a  parallel  way.  But  passing  over  Cryptogams  it 
must  suffice  if  we  examine  more  at  length  these  traits  as  they 
are  displayed  by  Phasnogams.  Let  us  first  note  the  dis- 
similarities between  the  outer  tissues  of  stems  and  leaves. 

That  these  dissimilarities  arose  by  degrees,  as  fast  as  the 
units  of  which  the  phasnogamic  axis  is  composed  became 
integrated,  is  a  conclusion  in  harmony  with  the  truth  that  in 
every  shoot  of  every  plant,  they  are  at  first  slight  and  become 
gradually  marked.  Already,  in  briefly  tracing  the  contrasts 
between  the  outer  and  inner  tissues  of  plants,  some  facts 
have  been  named  showing,  by  implication,  how  the  cessation 
of  the  leaf-function  in  axes  is  due  to  that  change  of  condi- 
tions entailed  by  the  discharge  of  other  functions.  Here 
we  have  to  consider  more  closely  facts  of  this  class,  together 
with  others  immediately  to  the  point.  On  pulling 

off  from  a  stem  of  grass  the  successive  sheaths  of  its  leaves, 
the  more-inclosed  parts  of  which  are  of  a  fainter  green  than 
the  outer  parts,  it  will  be  found  that  the  tubular  axis  even- 
tually reached  is  of  a  still  fainter  green;  but  when  the  axis 
eventually  shoots  up  into  a  flowering  stem,  its  exposed  part 
acquires  as  bright  a  green  as  the  leaves.  In  other  Mono- 
cotyledons, the  leaf-sheaths  of  which  are  successively  burst 
and  exfoliated  by  the  swelling  axis,  it  may  be  observed  that 
where  the  dead  sheaths  do  not  much  obstruct  the  light  and 
air,  the  surface  of  the  axis  underneath  is  full  of  chlorophyll. 
Dendrobium  is  an  example.  But  when  the  dead  sheaths 
accumulate  into  an  opaque  envelope,  the  chlorophyll  is  ab- 
sent, and  also,  we  may  infer,  the  function  which  its  presence 
habitually  implies.  Carrying  with  us  this  evidence,  we  shall 
recognize  a  like  relation  in  Dicotyledons.  While  its  outer 
layer  remains  tolerably  transparent,  an  exogenous  stem  or 
63 


258  PHYSIOLOGICAL  DEVELOPMENT. 

branch  continues  to  show,  by  the  formation  of  chlorophyll, 
that  it  shares  in  the  duties  of  the  leaves;  but  in  proportion 
as  a  bark  which  the  light  cannot  penetrate  is  produced  by 
the  adherent  flakes  of  dead  skin,  or  by  the  actual  deposit  of 
a  protective  substance,  the  differentiation  of  duties  becomes 
more  decided.  Cactuses  and  Euphorbias  supply 

us  with  converse  facts  having  the  same  implication.  The 
succulent  axes  so  strangely  combined  in  these  plants,  main- 
tain for  a  long  time  the  translucency  of  their  outer  layers 
and  their  greenness ;  and  they  so  efficiently  perform  the  offices 
of  leaves  that  leaves  are  not  produced.  In  some  cases,  axes 
that  are  not  succulent  participate  largely  in  the  leaf-function, 
or  entirely  usurp  it — still,  however,  by  fulfilling  the  same 
essential  conditions.  Occasionally,  as  in  Statice  brassiccefolla, 
stems  become  fringed;  and  the  fringes  they  bear  assume, 
along  with  the  thinness  of  leaves,  their  darker  green  and 
general  aspect.  In  the  genus  Ruscus,  the  flattened  axis 
simulates  so  closely  the  leaf-structure,  that  were  it  not  for  the 
flower  borne  on  its  midrib,  or  edge,  its  axial  nature  would 
hardly  be  suspected.  And  let  us  not  omit  to  note  that  where 
axes  usurp  the  characters  of  leaves,  in  their  attitudes  as  well 
as  in  their  shapes  and  thicknesses,  there  are  contrasts  between 
their  under  and  upper  surfaces,  answering  to  the  contrasts 
between  the  relations  of  these  surfaces  to  the  light.  Of  this 
Ruscus  androgynus  furnishes  a  striking  example.  In  it  the 
difference  which  the  unaided  eye  perceives  is  much  less  con- 
spicuous than  that  disclosed  by  the  microscope;  for  I  find 
that  while  the  face  of  the  pseudo-leaf  has  no  stomata,  the  back 
is  abundantly  supplied  with  them.  One  more  illustration 
must  be  added.  Equally  for  the  morphological  and  physio- 
logical truths  which  it  enforces,  the  MiihlenbecJcia  platyclada 
is  one  of  the  most  instructive  of  plants.  In  it  the  simulation 
of  forms  and  usurpation  of  functions,  are  carried  out  in  a 
much  more  marvellous  way  than  among  the  Cactacece. 
Imagine  a  growth  resembling  in  outline  a  very  long  willow- 
leaf,  but  without  a  midrib,  and  having  its  two  surfaces  alike. 


THE  OUTER  TISSUES  OF  PLANTS.  259 

Imagine  that  across  this  thin,  green,  semi-transparent  struc- 
ture, there  are  from  ten  to  thirty  divisions,  which  prove  to  be 
the  successive  nodes  of  an  axis.  Imagine  that  along  the 
edges  of  this  leaf-shaped  aggregate  of  internodes,  there  arise 
axillary  buds,  some  of  which  unfold  into  flowers,  and  others 
of  which  shoot  up  vertically  into  growths  like  the  one  which 
bears  them.  Imagine  a  whole  plant  thus  seemingly  composed 
of  jointed  willow-leaves  growing  from  one  another's  edges, 
and  some  conception  will  be  formed  of  the  Muhleribeclcia. 
The  two  facts  which  have  meaning  for  us  here  are — first,  that 
the  performance  of  leaf-functions  by  these  axes  goes  along 
with  the  assumption  of  a  leaf-like  translucency ;  and,  second, 
that  these  flattened  axes,  retaining  their  upright  attitudes, 
and  therefore  keeping  their  two  faces  similarly  conditioned, 
have  these  two  faces  alike  in  colour  and  texture. 

That  physiological  differentiation  of  the  surface  which 
arises  in  Phaenogams  between  axial  organs  and  foliar  organs, 
is  thus  traceable  with  tolerable  clearness  to  those  differences 
between  their  conditions  which  integration  has  entailed — 
partly  in  the  way  above  described  and  partly  in  other  ways 
still  to  be  named.  By  its  relative  position,  as  being  shaded 
by  the  leaves,  the  axis  is  less-favourably  circumstanced  for 
performing  those  assimilative  actions  effected  by  the  aid  of 
light.  Further,  that  relatively-small  ratio  of  surface  to  mass 
in  the  axis,  which  is  necessitated  by  its  functions  as  a  support 
and  a  channel  for  circulation,  prevents  it  from  taking  in, 
with  the  same  facility  as  the  leaves,  those  surrounding  gases 
from  which  matter  is  to  be  assimilated.  Both  these  special 
causes,  however,  in  common  with  that  previously  assigned, 
fall  within  the  general  cause.  And  in  the  fact  that  where 
the  differential  conditions  do  not  exist,  the  physiological 
differentiation  does  not  arise,  or  is  obliterated,  we  have  clear 
proof  that  it  is  determined  by  unlikenesses  in  the  relations 
of  the  parts  to  the  environment. 

§  274.  From  this  most  general  contrast  between  aerial 


260  PHYSIOLOGICAL  DEVELOPMENT. 

surface-tissues — those  of  axes  and  those  of  folia — we  pass 
now  to  the  more  special  contrasts  of  like  kind  existing  in 
folia  themselves.  Leaves  present  us  with  superficial  differen- 
tiations of  structure  and  function;  and  we  have  to  consider 
the  relations  between  these  and  the  environing  forces. 

Over  the  whole  surface  of  every  phaenogamic  leaf,  as  over 
the  fronds  of  the  Pteridophyta,  there  extends  a  simple  or 
compound  epidermal  layer,  formed  of  cells  that  are  closely 
united  at  their  edges  and  devoid  (in  the  Flowering  Plants) 
of  that  granular  colouring  matter  (chlorophyll)  contained  in 
the  layers  of  cells  they  inclose:  the  result  being  that  the 
membrane  formed  of  them  is  comparatively  transparent. 
On  the  submerged  leaves  of  aquatic  Phaenogams,  this  outer 
layer  is  thin,  delicate,  and  permeable  by  water;  but  on 
leaves  exposed  to  the  air,  and  especially  on  their  upper  sur- 
faces, is  comparatively  strong,  dense,  often  smooth  and 
impermeable  by  water:  being  thus  fitted  to  prevent  the 
rapid  escape  of  the  contained  juices  by  evaporation.  Simi- 
larly, while  the  leaves  of  terrestrial  plants  which  live  in  tem- 
perate climates,  usually  have  comparatively  thin  coats  thus 
composed,  in  climates  that  are  both  hot  and  dry,  leaves  are 
commonly  clothed  with  a  very  thick  cuticle.  Nor  is  this  all. 
The  outside  of  an  aerial  leaf  differs  from  that  of  a  submerged 
leaf  by  containing  a  deposit  of  waxy  substance.  Whether 
this  be  exuded  by  the  exposed  surfaces  of  the  cells,  as  some 
contend,  or  whether  it  is  deposited  within  the  cells,  as  thought 
by  others,  matters  not  in  so  far  as  the  general  result  is  con- 
cerned. In  either  case  a  waterproof  coating  is  formed  at  the 
outermost  sides  of  these  outermost  cells;  and  in  many  cases 
produces  that  polish  by  which  the  upper  surface  of  the  leaf  is 
more  or  less  distinguished  from  the  under  surface.  This 

external  pellicle  presents  us  with  another  contrast  of  allied 
meaning.  On  the  upper  surfaces  of  leaves  subject  to  the 
direct  action  of  the  sun's  rays,  there  are  either  few  or  none 
of  those  minute  openings,  or  stomata,  through  which  gases 
can  enter  or  escape ;  but  on  the  under  surfaces  these  stomata 


THE  OUTER  TISSUES  OP  PLANTS.  261 

are  abundant:  a  distribution  which,  while  permitting  free 
absorption  of  the  needful  carbonic  acid,  puts  a  check  on  the 
exit  of  watery  vapour.  Two  general  exceptions  to  this  ar- 
rangement may  be  noted.  Leaves  that  float  on  the  water 
have  all  their  stomata  on  their  upper  sides,  and  leaves  that 
are  submerged  have  no  stomata — modifications  obviously  ap- 
propriate to  the  conditions.  What  is  to  be  said 
respecting  the  genesis  of  these  differentiations?  For  the 
last  there  seems  no  direct  cause:  its  cause  must  be  indirect. 
The  unlike  actions  to  which  the  upper  and  under  surfaces  of 
leaves  are  subject,  have  no  apparent  tendency  to  produce 
unlikeness  in  the  number  of  their  breathing  holes.  Here 
the  natural  selection  of  spontaneous  variations  furnishes  the 
only  feasible  explanation.  For  the  first,  however,  there  is  a 
possible  cause  in  the  immediate  actions  of  incident  forces, 
which  survival  of  the  fittest  continually  furthers. 

The  fluid  exhaling  through  the  walls  of  the  cells  next  the 
air,  will  be  likely  to  leave  behind  suspended  substances  on 
their  outer  surfaces.  On  remembering  the  pellicle  which  is  apt 
to  form  on  thick  solutions  or  emulsions  as  they  dry,  and  how 
this  pellicle  as  it  grows  retards  the  further  drying,  it  will  be 
perceived  that  the  deposit  of  waxy  matter  next  to  the  outer 
surfaces  of  the  cuticular  cells  in  leaves,  is  not  improbably 
initiated  by  the  evaporation  which  it  eventually  checks. 
Should  it  be  so,  there  results  a  very  simple  case  of  equilibra- 
tion. Where  the  loss  of  water  is  too  great,  this  waxy  pellicle 
left  behind  by  the  escaping  water  will  protect  most  those  in- 
dividuals of  the  species  in  which  it  is  thickest  or  densest ;  and 
by  inheritance  and  continual  survival  of  the  fittest,  there  will 
be  established  in  the  species  that  thickness  of  the  layer  which 
brings  the  evaporation  to  a  balance  with  the  supply  of  water. 

Another  superficial  differentiation,  still  more  familiar,  has 
to  be  noted.  Every  child  soon  learns  to  distinguish  by  its 
colour  the  upper  side  of  a  leaf  from  its  under  side,  if  the  leaf 
is  one  that  has  grown  in  such  way  as  to  establish  the  rela- 
tions of  upper  and  under.  The  upper  surfaces  of  leaves  are 


262  PHYSIOLOGICAL  DEVELOPMENT. 

habitually  of  a  deeper  green  than  the  under.  Microscopic 
examination  shows  that  this  deeper  green  results  from  the 
closer  clustering  of  those  parenchyma-cells  full  of  chlorophyll 
that  are  in  some  way  concerned  with  the  assimilative  actions ; 
while  beneath  them  are  more  numerous  intercellular  passages 
communicating  with  those  openings  or  stomata  through 
which  is  absorbed  the  needful  air.  Now  when  it  is  remem- 
bered that  the  formation  of  chlorophyll  is  clearly  traceable  to 
the  action  of  light — when  it  is  remembered  that  leaves  are  pale 
where  they  are  much  shaded  and  colourless  when  developed 
in  the  dark,  as  in  the  heart  of  a  Cabbage — when  it  is  remem- 
bered that  succulent  axes  and  petioles,  like  those  of  Sea-kale 
and  Celery,  remain  white  while  the  light  is  kept  from  them 
and  become  green  when  exposed;  it  cannot  be  questioned 
that  this  greater  production  of  chlorophyll  next  to  the  upper 
surface  of  a  leaf,  is  directly  consequent  on  the  greater 
amount  of  light  received.  Here,  as  in  so  many  other  cases, 
we  must  regard  the  differentiation  as  in  part  due  to  direct 
equilibration  and  in  part  to  indirect  equilibration.  Familiar 
facts  compel  us  to  conclude  that  from  the  beginning,  each 
individual  foliar  organ  has  undergone  a  certain  immediate 
adaptation  of  its  surfaces  to  the  incidence  of  light;  that 
when  there  arose  a  mode  of  growth  which  exposed  the  leaves 
of  successive  generations  in  similar  ways,  this  immediately- 
produced  adaptation,  ever  tending  to  be  transmitted,  was 
furthered  by  the  survival  of  individuals  inheriting  it  in  the 
greatest  degree ;  and  that  so  there  was  gradually  established 
that  difference  between  the  two  surfaces  which  each  leaf  dis- 
plays before  it  unfolds  to  the  light,  but  which  becomes  more 
marked  when  it  has  unfolded.* 

*  The  current  doctrine  that  chlorophyll  is  the  special  substance  concerned 
in  vegetal  assimilation,  either  as  an  agent  or  as  an  incidental  product,  must 
be  taken  with  considerable  qualification.  Besides  the  fact  that  among  the 
Algce  there  are  many  red  and  brown  kinds  which  thrive ;  and  besides  the 
fact  that  among  the  lower  Archegoniates  there  are  species  which  are  purple  or 
chocolate-coloured  ;  there  is  the  fact  that  Phsenogams  are  not  all  green.  We 
have  the  Copper-Beech,  we  have  the  black-purple  Colens  VerschafTelfii,  and 
we  have  the  red  variety  of  Cabb'oge,  which  seetns  to  flourish  as  well  as  the 


THE  OUTER  TISSUES  OF  PLANTS.  263 

From  the  ordinary  cases  let  us  now  pass  to  the  exceptional 
cases.  We  will  look  first  at  those  in  which  the  two  faces  of 
the  leaves  differ  but  little,  or  not  at  all — their  circumstances 
being  similar  or  equal.  Leaves  that  grow  in  approximately- 
upright  attitudes,  and  attitudes  which  do  not  maintain  the 
relative  positions  of  the  two  surfaces  with  constancy,  may  be 
expected  to  display  an  unusual  likeness  between  the  two 
surfaces;  and  among  tnem  we  see  it.  The  Grasses  may  be 
named  as  a  group  exemplifying  this  relation;  and  if,  instead 
of  comparing  them  as  a  group  with  other  groups,  we  compare 
those  dwarf  kinds  of  them  which  spread  out  their  leaves 
horizontally,  with  the  large  aspiring  kinds,  such  as  Arundo, 
we  trace  a  like  antithesis :  in  the  one  the  contrast  of  upper 
and  under  is  very  obvious,  while  in  the  other  it  is  scarcely  to 
be  detected.  Leaves  of  various  other  Monocotyledons  that 
grow  in  a  similar  way,  similarly  show  us  a  near  approach  to 
uniformity  of  the  two  surfaces ;  as  instance  the  genus  Olivia 
and  the  thinner-leaved  kinds  of  Yucca.  Where  the  con- 
trast of  upper  and  under  is  greatly  diminished  by  the  assump- 
tion of  a  rounded  or  cylindrical  form,  instead  of  a  flattened 
form,  the  same  thing  happens.  The  genus  Kleinia  furnishes 
illustrations.  It  may  be  remarked,  too,  that  even  within 
the  limits  of  this  genus  there  are  instructive  variations;  for 
while  in  Kleinia  ficoides  the  leaves,  shaped  like  pea-pods, 
are  broadest  in  a  vertical  direction,  and  have  their  lateral 
surfaces  alike  in  conditions  and  structure,  in  other  species 
the  leaves,  broader  horizontally  than  vertically,  exhibit 
unlikeness  between  the  upper  and  under  sides.  Equally 
to  the  point  is  the  evidence  furnished  by  vertically-growing 
leaves  that  are  cylindrical,  as  those  of  Sanseviera  cylindrica, 
or  as  those  of  the  Rush-tribe:  the  similarly-placed  surface 
has  all  around  a  similar  character.  Of  kindred  meaning, 

other  varieties.  Chlorophyll,  then,  must  be  regarded  simply  as  the  most 
general  of  the  colouring  matters  found  in  those  parts  of  plants  in  which 
assimilation  is  being  effected  by  the  ajrency  of  light.  Though  it  is  always 
present  alonn  with  the  red  and  brown  pigments,  yet  there  is  much  evidence  to 
show  that  these  are  the  actual  assimilative  pigments. 


264  PHYSIOLOGICAL  DEVELOPMENT. 

and  still  more  conclusive,  are  the  cases  in  which  the  under 
side  of  the  leaf,  being  more  exposed  to  light  than  the  upper 
side,  usurps  the  character  and  function  of  the  upper  side. 
If  a  common  Flag  be  pulled  to  pieces,  it  will  be  seen  that 
what  answers  to  the  face  in  other  leaves,  forms  merely  the 
inside  of  the  sheath  including  the  younger  leaves,  and  is 
obliterated  higher  up.  The  two  surfaces  of  the  blade  answer 
to  the  two  under  halves  of  a  leaf  that  has  been,  as  it  were, 
folded  together  lengthways,  with  the  two  halves  of  its  upper 
surface  in  contact.  And  here,  in  default  of  an  upper  surface, 
the  under  surface  acquires  its  character  and  discharges  its 
function.  A  like  substitution  occurs  in  Aristea  corymbosa; 
and  there  are  some  of  the  Orchids,  as  Lockhartia,  which  dis- 
play it  in  a  very  obvious  way. 

When  joined  with  the  foregoing  evidence,  the  evidence 
which  another  kind  of  substitution  supplies  is  of  great 
weight.  I  refer  to  that  which  occurs  in  the  Australian 
Acacias,  already  instanced  as  throwing  light  on  morpho- 
logical changes.  In  these  plants  the  leaves  properly  so  called 
are  undeveloped,  and  the  footstalks,  flattened  out  into  folia- 
ceous  shapes,  acquire  veins  and  midribs,  and  so  far  simulate 
leaves  as  ordinarily  to  be  taken  for  them:  a  fact  in  itself  of 
much  physiological  significance.  But  that  which  it  concerns 
us  especially  to  note,  is  the  absence  of  distinction  between 
the  two  faces  of  these  phyllodes,  as  they  are  named,  and 
the  cause  of  its  absence.  These  transformed  petioles  do  not 
flatten  themselves  out  horizontally,  so  as  to  acquire  under 
and  upper  sides,  as  most  true  leaves  do;  but  they  flatten 
themselves  out  vertically:  the  result  being  that  their  two 
sides  are  similarly  circumstanced  with  respect  to  light  and 
other  agencies;  and  there  is  consequently  nothing  to  cause 
their  differentiation.  And  then  we  find  an  analogous  case 
where  differential  conditions  arise,  and  where  some  differ- 
entiation results.  In  Oxalis  bupleurifolia,  Fig.  66,  there  is  a 
similar  flattening  out  of  the  petiole  into  a  pseudo-leaf;  but 
in  it  the  flattening  takes  place  in  the  same  plane  as  the  leaf, 


THE  OUTER  TISSUES  OF  PLANTS.  265 

so  as  to  produce  an  under  and  an  upper  surface;  and  here 
the  two  surfaces  of  the  pseudo-leaf  are  slightly  unlike — in 
contour  if  in  nothing  else. 

§  275.  We  now  come  to  such  physiological  differentiations 
among  the  outer  tissues  of  plants,  as  are  displayed  in  the 
contrasts  between  foliar  organs  on  the  same  axis,  or  on 
different  axes  —  contrasts  between  the  seed-leaves  and  the 
leaves  subsequently  formed,  between  submerged  and  aerial 
leaves  in  certain  aquatic  plants,  between  leaves  and  bracts, 
and  between  bracts  and  sepals.  To  deal  even  briefly  with 
these  implies  information  which  even  a  professed  botanist 
would  have  to  increase  by  special  inquiries,  before  attempting 
interpretations.  Here  it  must  suffice  to  say  something 
respecting  those  marked  unlikenesses  existing  between  the 
tissues  of  the  more  characteristic  parts  of  flowers,  and  the 
tissues  of  the  homologous  foliar  organs. 

It  was  pointed  out  in  §  196,  that  the  terminal  folia  of  a 
phsenogamic  axis  have  sundry  characters  in  common  with  such 
fronds  as  those  out  of  which  we  concluded  that  the  phomo- 
gamic  axis  has  arisen  by  integration — common  characters  of 
a  kind  to  be  expected.  In  their  simple  cellular  composition, 
comparative  want  of  chlorophyll,  and  deficiency  of  vascular 
structures,  the  undeveloped  ends  of  leaf-shoots  and  the  de- 
veloped ends  of  flower-shoots,  approach  to  the  fronds  of  the 
simpler  Archegoniates.  We  also  noted  between  them  another 
resemblance.  It  is  said  of  the  Jungermanniacea,  that 
"  though  under  certain  circumstances  of  a  pure  green,  they 
are  inclined  to  be  shaded  with  red,  purple,  chocolate,  or  other 
tints ; "  and  answering  to  this  we  have  the  facts  that  such 
colours  commonly  occur  in  the  terminal  folia  of  a  phseno- 
gamic  axis,  when  arrest  of  its  development  leads  to  the 
formation  of  a  flower,  and  that  very  frequently  they  are 
visible  at  the  ends  of  leaf-axes.  In  the  unfolding  parts  of 
shoots,  more  or  less  of  red,  or  copper-colour,  or  chocolate- 
colour,  may  generally  be  seen:  often,  indeed,  it  charac- 


266  PHYSIOLOGICAL  DEVELOPMENT. 

terizes  the  leaves  for  some  time  after  they  are  unfolded. 
Occasionally  the  traces  of  it  are  permanent;  and,  as  in 
the  scarlet  terminal  leaves  of  Poinsettia  pulcherrima,  we  see 
that  it  may  become,  and  continue,  extremely  conspicuous. 
The  question,  then,  now  to  be  asked  is — has  this  colouring 
by  which  the  immature  part  of  the  phaenogamic  axis  is  cha- 
racterized, anything  to  do  with  the  colouring  of  flowers? 
Has  this  difference  between  undeveloped  folia  and  folia  that 
are  further  developed,  been  increased  by  natural  selection 
where  an  advantage  accrued  from  it,  until  it  has  ended  in 
the  strong  contrast  we  now  see?  I  think  we  may  not  irra- 
tionally infer  that  this  has  happened. 

Facts,  very  numerous  and  varied,  united  to  warrant  us  in 
concluding  that  gamogenesis  commences  where  the  forces 
which  conduce  to  growth  are  nearly  equilibrated  by  the  forces 
which  resist  growth  (§  78) ;  and  the  induction  that  in  plants, 
fertilized  germs  are  produced  at  places  where  there  is  an 
approach  towards  this  balance,  we  found  to  be  in  harmony 
with  the  deduction  that  an  advantage  to  the  species  must  be 
gained  by  sending  off  migrating  progeny  from  points  where 
nutrition  is  failing.  Other  things  equal,  failure  of  nutrition 
may  be  expected  in  parts  which  have  the  most  remote  or  most 
indirect  access  to  the  materials  furnished  by  the  roots — 
materials  which  have  to  be  carried  great  distances  by  a  very 
imperfect  apparatus.  The  ends  of  lateral  axes  are  therefore 
the  probable  points  of  fructification,  in  aggregates  of  the 
third  order  that  have  taken  to  growing  vertically.  But  if 
these  points  at  which  nutrition  is  failing,  are  also  the  points 
at  which  the  colours  inherited  from  lower  types  are  likely  to 
recur  in  more  marked  degrees  than  elsewhere;  then  we  may 
infer  that  the  organs  of  fructification  will  not  unfrequently 
co-exist  with  such  colours  at  the  ends  of  such  axes.  How 
may  the  resulting  contrast  between  the  older  fronds  and  the 
fronds  next  the  germ-producing  organs  be  increased?  If 
uninterfered  with  it  would  be  likely  to  diminish.  These 
traits  inherited  from  remote  ancestry  might  be  expected 


THE  OUTER  TISSUES  OF  PLANTS.  267 

slowly  to  fade  away.     How,  then,  is  the  intensification  of 
them  to  be  explained  ? 

If  a  contrast  of  the  kind  described  favours  the  propagation 
of  a  race  in  which  it  exists,  it  will  be  maintained  and 
increased;  and  if  we  take  into  account  an  agency  of  which 
Mr.  Darwin  has  shown  the  great  importance — the  agency  of 
insects — we  shall  have  little  difficulty  in  understanding  how 
such  a  contrast  may  facilitate  propagation.  We  cannot,  of 
course,  here  assume  the  agency  of  insects  so  specialized  in 
their  habits  as  Bees  and  Butterflies;  for  their  specialized 
habits  imply  the  pre-existence  of  the  contrast  to  be  explained. 
But  there  is  an  insect-agency  of  a  more  general  kind  which 
may  be  fairly  counted  upon  as  coming  into  action.  Various 
small  Flies  and  Beetles  wander  over  the  surfaces  of  plants  in 
search  of  food.  It  is  a  legitimate  assumption  that  they  will 
frequent  most  those  parts  in  which  they  find  most  food,  or 
food  most  to  their  liking — especially  if  at  the  same  time 
they  gain  the  advantage  of  concealment.  Now  the  ends  of 
axes,  formed  of  young,  soft,  and  closely-packed  folia,  are  the 
parts  which  more  than  any  others  offer  these  several  advan- 
tages. They  afford  shelter  from  enemies;  they  frequently 
contain  exuded  juices;  and  when  they  do  not,  their  tissues 
are  so  tender  as  to  be  easily  pierced  in  search  of  the  sap. 
If,  then,  from  the  first,  as  at  present,  these  ends  of  axes 
have  been  favourite  haunts  of  small  insects;  and  if,  where 
the  closely-clustered  folia  contained  the  generative  organs, 
the  insects  frequenting  them  occasionally  carried  adherent 
fructifying  cells  from  one  plant  to  another,  and  so  aided 
fertilization;  it  would  follow  that  anything  which  made 
such  terminal  clusters  more  attractive  to  such  insects,  or 
more  conspicuous  to  them,  or  both,  would  further  the  multi- 
plication of  the  race,  and  would  so  be  continually  increased 
by  the  extra  multiplication  of  individuals  in  which  it  was 
greatest.  Here  we  find  the  clue.  This  contrast  of  colour 
between  the  folia  next  to  the  fructifying  parts  and  all  other 
folia,  must  constantly  have  facilitated  insect-agency;  sup- 


268  PHYSIOLOGICAL  DEVELOPMENT. 

posing  the  insects  to  have  had  the  power  of  distinguishing 
between  colours.  That  Bees  and  Butterflies  have  this  power 
is  manifest.  They  may  be  watched  flying  from  flower  to 
flower,  disregarding  all  other  parts  of  the  plants.  And  if 
the  less-specialized  insects  possessed  some  degree  of  such 
discrimination,  then  the  initial  contrasts  of  colour  above 
described  would  be  maintained  and  increased.  Let  such  a 
connexion  be  once  established,  and  it  must  tend  to  become 
more  decided.  Insects  most  able  to  discern  the  parts  of 
plants  which  afford  what  they  seek,  will  be  those  most  likely 
to  survive  and  leave  offspring.  Plants  presenting  most  of 
the  desired  food,  and  showing  most  clearly  where  it  lies,  will 
have  their  fertilization  and  multiplication  furthered  in  the 
greatest  degree.  And  so  the  mutual  adaptation  will  become 
ever  closer;  while  it  is  rendered  at  the  same  time  more 
varied  by  the  special  requirements  of  the  insects  and  of 
the  plants  in  each  locality,  under  each  change  of  con- 
ditions. Of  course,  the  genesis  of  the  sweet 
secretions  and  the  odours  of  flowers,  has  a  parallel  interpre- 
tation. The  simultaneous  production  of  honey,  or  some 
kindred  substance,  is  implied  above;  since,  unless  a  bait 
co-existed  with  the  colour,  the  colour  would  not  attract 
insects,  and  would  not  be  maintained  and  intensified  by 
natural  selection.  Gums,  and  resins,  and  balsams,  are  familiar 
products  of  plants;  apparently,  in  many  cases,  excreted  as 
useless  matters  from  various  parts  of  their  surfaces.  These 
substances,  admitting  of  wide  variations  in  quality,  as  they 
do,  afford  opportunities  for  the  action  of  natural  selection 
wherever  any  of  them,  attractive  to  insects,  happen  to  be 
produced  near  the  organs  of  fructification.  And  this  action 
of  natural  selection  once  set  up,  may  lead  to  the  establish- 
ment of  a  local  excretion,  to  the  production  of  an  excretion 
more  and  more  attractive,  and  to  the  disposal  of  the  organ 
containing  it  in  such  a  way  as  most  to  facilitate  the  carry- 
ing away  of  pollen.  Similarly  and  simultaneously  with 
odours.  Odours,  like  colours,  draw  insects  to  flowers.  After 


THE  OUTER  TISSUES  OF  PLANTS.  269 

observing  how  Bees  come  swarming  into  a  house  where 
honey  is  largely  exposed,  or  how  Wasps  find  their  way  into 
a  shop  containing  much  ripe  fruit,  it  cannot  be  questioned 
that  insects  are  to  a  considerable  extent  guided  by  scent. 
Being  thus  sensitive  to  the  aromatic  substances  which  flowers 
exhale,  they  may,  when  the  flowers  are  in  large  masses,  be 
attracted  by  them  from  distances  at  which  the  flowers  them- 
selves are  invisible.  And  manifestly,  the  flowers  which  so 
attract  them  from  the  greatest  distances,  increasing  thereby 
their  chances  of  efficient  fertilization,  will  be  most  likely  to 
perpetuate  themselves.  That  is  to  say,  survival  of  the  fittest 
must  tend  to  produce  perfumes  that  are  both  more  powerful 
and  more  attractive. 

These  physiological  differentiations,  then,  which  mark  off 
the  foliar  organs  constituting  flowers  from  other  foliar  organs, 
are  the  consequences  of  indirect  equilibration.  They  are  not 
due  to  the  immediate  actions  of  unlike  incident  forces  on 
the  parts  of  the  individual  plant;  but  they  are  due  to  the 
actions  of  such  unlike  incident  forces  on  the  aggregate  of 
individuals,  generation  after  generation.* 

§  276.  The  unity  of  interpretation  which  we  here  find  for 
phenomena  of  such  various  orders,  could  hardly  be  found 

*  This  seems  as  fit  a  place  as  any  for  noting  the  fact,  that  the  greater  part 
of  what  wo  call  beauty  in  the  organic  world,  is  in  some  way  dependent  on 
the  sexual  relation.  It  is  not  only  so  with  the  colours  and  odours  of  flowers. 
It  is  so,  too,  with  the  brilliant  plumage  of  birds  ;  and  it  is  probable  that  the 
colours  of  the  more  conspicuous  insects  are  in  part  similarly  determined.  The 
remarkable  circumstance  is,  that  these  characteristics,  which  have  originated 
by  furthering  the  production  of  the  best  offspring,  while  they  are  naturally 
those  which  render  the  organisms  possessing  them  attractive  to  one  another, 
directly  or  indirectly,  should  also  be  those  which  are  so  generally  attractive 
to  us — those  without  which  the  fields  and  woods  would  lose  half  their  charm. 
It  is  interesting,  too,  to  observe  how  the  conception  of  human  beauty  is  in  a 
considerable  degree  thus  originated.  And  the  trite  observation  that  the 
element  of  beauty  which  grows  out  of  the  sexual  relation  is  so  predominant 
in  aesthetic  products — in  music,  in  the  drama,  in  fiction,  in  poetry — gains  a 
new  meaning  when  we  see  how  deep  down  in  organic  nature  this  connexion 
extends. 


270  PHYSIOLOGICAL  DEVELOPMENT. 

were  the  phenomena  otherwise  caused.  That  the  stronger 
and  the  feebler  contrasts  among  the  different  parts  of  the 
outer  tissues  in  plants,  should  so  constantly  occur  along  with 
stronger  and  feebler  contrasts  among  the  incident  forces,  is 
in  itself  weighty  evidence  that  unlike  outer  actions  have 
caused  unlike  inner  actions,  and  correspondingly-unlike 
structures;  either  by  changing  the  functional  equilibrium  in 
the  individual,  or  by  changing  it  in  the  race,  or  by  both. 

Even  in  the  absence  of  more  direct  proof,  there  would  be 
great  significance  in  the  marked  differences  that  habitually 
exist  between  the  exposed  and  imbedded  parts  of  plants,  be- 
tween the  stems  and  the  leaves,  and  between  the  upper  and 
under  surfaces  of  the  leaves.  The  significance  of  these  differ- 
ences is  increased  when  we  discover  that  they  vary  in  degree 
as  the  differences  in  the  conditions  vary  in  degree.  Still 
greater  becomes  the  force  of  the  evidence  on  finding  that 
these  strongly-contrasted  parts  may,  when  placed  in  one 
another's  conditions,  and  kept  in  them  from  generation  to 
generation,  permanently  assume  one  another's  functions,  and, 
in  a  great  degree,  one  another's  structures.  Even  more  con- 
clusive yet  is  the  argument  rendered,  by  the  discovery  that, 
where  these  substitutions  of  function  and  structure  take  place, 
the  superinduced  modifications  differ  in  different  circum- 
stances ;  just  as  the  original  modifications  do.  The  fact  that 
a  flattened  stem  simulating  a  vertically-growing  leaf  has  its 
two  surfaces  alike,  while  when  it  simulates  a  horizontally- 
growing  leaf  its  upper  and  under  surfaces  differ,  is  a  fact 
which,  standing  alone,  might  prove  little,  but  proves  much 
when  joined  with  all  the  other  evidence.  And  its  profound 
meaning  becomes  the  more  obvious  on  discovering  that  the 
same  thing  happens  with  petioles  when  they  usurp  leaf- 
functions. 

Finally,  when  we  remember  how  rapidly  analogous  modi- 
fications of  function  and  structure  arise  in  the  superficial 
tissues  of  individual  plants,  the  general  inference  can  scarcely 
be  resisted.  When  we  meet  with  so  striking  a  case  as  that 


THE  OUTER  TISSUES  OF  PLANTS.  271 

of  the  Begonia-leaf,  a  fragment  of  which  stuck  in  the  ground 
produces  roots  from  its  under  surface  and  leaves  from  its 
upper  surface — when  we  see  that  though,  in  this  case,  the 
typical  structure  of  the  plant  presently  begins  to  control  the 
organizing  process,  yet  the  initial  differentiations  are  set  up 
by  the  differential  actions  of  the  environment;  the  presump- 
tion becomes  extremely  strong  that  the  heterogeneities  of 
surface  which  we  have  considered,  result,  as  alleged,  directly 
or  indirectly  from  heterogeneities  in  the  incident  forces. 


CHAPTEE  IV. 

DIFFERENTIATIONS   AMONG   THE   INNER   TISSUES   OF 
PLANTS.* 

§  277.  IN  passing  from  plants  formed  of  threads  or  thin 
lamina^  to  plants  having  some  massiveness,  we  find  that  after 
the  external  and  internal  parts  have  become  distinguished 
from  one  another,  there  arise  distinctions  among  the  internal 
parts  themselves,  as  well  as  among  the  external  parts 
themselves:  the  primarily-differentiated  parts  are  both  re- 
differentiated. 

From  types  of  very  low  organisation  illustrations  of  this 
may  be  drawn.  In  the  thinner  kinds  of  Laminaria  there 
exists  but  the  single  contrast  between  the  outer  layer  of  cells 
and  an  inner  layer;  but  in  larger  species  of  the  same  genus, 
as  L.  digitata,  there  are  three  unlike  layers  on  each  side  of  a 
central  layer  differing  from  them — augmentation  of  bulk  is 
accompanied  by  multiplication  of  concentric  internal  struc- 
tures, having  their  unlikenesses  obviously  related  to  unlike- 
nesses  in  their  conditions.  In  Furcellaria  and  various  Algce 
of  similarly  swollen  forms,  the  like  relation  may  be  traced. 

Just  indicating  the  generality  of  this  contrast,  but  not 

*  Students  of  vegetal  physiology,  familiar  with  the  controversies  respecting 
sundry  points  dealt  with  in  this  chapter,  will  probably  be  surprised  to  find 
taken  for  granted  in  it,  propositions  which  they  have  habitually  regarded  as 
open  to  doubt.  Hence  it  seems  needful  to  say  that  the  conclusions  here  set 
forth,  have  resulted  from  investigations  undertaken  for  the  purpose  of  form- 
ing opinions  on  several  unsettled  questions  which  I  had  to  treat,  but  which  I 
could  find  in  books  no  adequate  data  for  treating.  The  details  of  these 
investigations,  and  the  entire  argument  of  which  this  chapter  is  partly  an 
abstract,  will  be  found  in  Appendix  C. 


THE  INNER  TISSUES  OF  PLANTS.  273 

attempting  to  seek  in  these  lower  types  for  any  more  specific 
interpretation  of  it,  let  us  pass  to  the  higher  types.  The 
argument  will  be  amply  enforced  by  the  evidence  obtained 
from  them.  We  will  look  first  at  the  conditions  which  they 
have  to  fulfil;  and  then  at  the  ways  in  which  the  functions 
and  structures  adapting  them  to  these  conditions  arise. 

§  278.  A  terrestrial  plant  that  grows  vertically  needs  no 
marked  modification  of  its  internal  tissues,  so  long  as  the 
height  it  reaches  is  very  small.  As  we  before  saw,  the  spiral 
or  cylindrical  rolling  up  of  a  simple  cellular  frond,  or  the 
more  bulky  growth  of  a  simple  cellular  axis,  may  give  the 
requisite  strength ;  and  the  requisite  circulation  may  be  car- 
ried on  through  the  unchanged  cellular  tissue.  But  in  pro- 
portion as  the  height  to  be  attained  and  the  mass  to  be 
supported  increase,  the  supporting  part  must  acquire  greater 
bulk  or  greater  density,  or  both;  and  some  modification  that 
shall  facilitate  the  transfer  of  nutritive  liquids  must  take 
place.  Hence,  in  the  inner  tissues  of  plants  we  may  expect 
to  find  that  structural  changes  answering  to  these  require- 
ments become  marked,  as  the  growth  of  the  aerial  part 
becomes  great.  Facts  correspond  with  these  expectations. 

Among  the  humbler  Cormophytes,  which  creep  over  or 
raise  themselves  but  little  above,  the  surfaces  they  flourish 
upon,  there  is  scarcely  any  internal  differentiation:  the 
vascular  and  woody  structures,  if  not  in  all  cases  absolutely 
unrepresented,  are  rarely  and  very  feebly  indicated.  But 
among  the  higher  types  —  the  Ferns  and  Lycopodiums  — 
which  raise  their  fronds  to  considerable  heights,  there  are 
vascular  bundles  and  hard  tissues  like  wood;  and  by  the 
Tree-Ferns  massive  axes  are  developed.  That  the  relation 
which  thus  shows  itself  among  Cryptogams  is  habitual  among 
Phamogams,  scarcely  needs  saying. 

Phasnogams,  however,  are  not  universally  thus  charac- 
terized in  a  decided  way.  Besides  the  comparative  want  of 
woody  tissue  in  flowering  plants  of  humble  growth,  and 
64 


274:        PHYSIOLOGICAL  DEVELOPMENT. 

besides  the  paucity  of  vessels  in  ordinary  water-plants,  there 
are  cases  of  much  more  marked  divergence  from  this  typical 
internal  structure.  These  exceptional  cases  occur  under 
exceptional  conditions,  and  are  highly  instructive.  They  are 
of  two  kinds.  One  group  of  them  is  furnished  by 

certain  plants  which  are  parasitic  on  the  exposed  roots  of 
trees — parasitic  not  partially,  as  the  Mistletoe,  but  to  the 
extent  of  subsisting  wholly  on  the  sap  they  absorb.  Fungus- 
like  in  colour  and  texture,  and  having  scales  for  leaves,  these 
Balanophorce  and  Rafflesiacece  are  recognizable  as  Phaenogams 
by  scarcely  any  other  traits  than  their  fructifications.  Along 
with  their  aborted  leaves  and  absence  of  chlorophyll,  there 
is  a  great  degradation  of  those  internal  tissues  by  which 
Phasnogams  are  commonly  distinguished.  Though  Dr.  [now 
Sir  J.]  Hooker  has  shown  that  they  are  not,  as  some  botanists 
thought,  devoid  of  spiral  vessels;  yet,  as  shown  by  the  mis- 
take previously  made  in  classifying  them,  their  appliances  for 
circulation  are  rudimentary.  And  this  trait  goes  along  with  a 
greatly-simplified  distribution  of  nutriment.  In  the  absence 
of  leaves  there  can  be  but  little  down-current  of  sap,  such 
as  leaves  usually  supply  to  roots:  there  cannot  be  much 
beyond  an  upward  current  of  the  absorbed  juices.  The 

other  cases  occur  where  circulation  is  arrested  or  checked  in 
a  different  way ;  namely,  in  plants  that  are  wholly  submerged. 
These  are  the  PodosUmacece.  Clothing  as  they  do  the  sub- 
merged rocks,  their  roots  play  the  part  of  rhizomes,  being 
attached  to  the  substratum  by  hairs  and  other  processes,  and 
having  the  leaf-bearing  and  flower-bearing  shoots  on  their 
surfaces.  The  latter  spread  out  more  or  less  horizontally  and 
are  also  fixed  to  the  substratum  in  the  same  manner  as  the 
roots.  Observe  then  the  connexion  of  facts.  One  of  these 
Podostemacece  needs  no  internal  stiffening  substance,  for  it 
exists  in  a  medium  of  its  own  specific  gravity;  and  being  in 
a  position  to  absorb  water  over  its  entire  surface,  it  has  no 
need  for  a  circulation  of  crude  sap — nor,  indeed,  in  the 
absence  of  evaporation  from  any  part  of  its  surface,  could 


THE  INNER  TISSUES  OF  PLANTS.  375 

any  active  circulation  take  place.  Here,  accordingly,  the 
tracheal  and  mechanical  elements  are  undeveloped.  Though 
spiral  vessels  are  not  entirely  absent,  yet  they  are  so  rare  as 
to  do  no  more  than  verify  the  inference  of  phaenogamic  rela- 
tionship drawn  from  the  flowers. 

The  method  of  agreement,  the  method  of  difference,  and 
the  method  of  concomitant  variations,  thus  unite  in  proving 
a  direct  relation  between  the  demand  for  support  and  circu- 
lation, and  the  existence  of  these  vascular  woody  bundles 
which  the  higher  plants  habitually  possess.  The  question 
which  we  have  to  consider  is  —  Under  what  influences  are 
these  structures,  answering  to  these  requirements,  developed? 
How  are  these  internal  differentiations  caused  ?  The  inquiry 
may  be  conveniently  divided.  Though  the  supporting  tissues 
and  the  tissues  concerned  in  the  circulation  of  liquids  are 
closely  connected,  and  indeed  entangled,  with  one  another, 
we  may  fitly  deal  with  them  apart.  Let  us  take  first  the 
supporting  tissue. 

§  279.  Many  common-place  facts  indicate  that  the  me- 
chanical strains  to  which  upright  growing  plants  are  exposed, 
themselves  cause  increase  of  the  dense  deposits  by  which  such 
plants  are  enabled  to  resist  such  strains.  There  is  the  fact 
that  the  massiveness  of  a  tree-trunk  varies  according  to  the 
stress  habitually  put  upon  it.  If  the  contrast  between  the 
slender  stem  of  a  tree  growing  in  a  wood  and  the  bulky  stem 
of  a  kindred  tree  growing  in  the  fields,  be  ascribed  to  differ- 
ence of  nutrition  rather  than  difference  of  exposure  to  winds ; 
there  is  still  the  fact  that  a  tree  trained  against  a  wall  has  a 
less  bulky  stem  than  a  tree  of  the  same  kind  growing  un- 
supported; and  that  between  the  long  weak  branches  of  the 
one  and  the  stiff  ones  of  the  other  there  are  decided  contrasts. 
If  it  be  objected  that  a  tree  so  trained  and  branches  so  borne 
have  relatively  less  foliage,  and  that  therefore  these  unlike- 
nesses  also  are  due  to  unlikenesses  of  general  nutrition,  which 
may  in  part  be  true ;  there  are  still  such  cases  as  those  of 


276  PHYSIOLOGICAL  DEVELOPMENT. 

garden  plants,  which  when  held  up  by  tying  them  to  sticks 
have  weaker  stems  than  when  they  are  unpropped,  and  sink 
down  if  their  props  are  taken  away.  Again,  there  is  the 
evidence  supplied  by  roots.  Though  the  contrast  between 
the  feeble  roots  of  a  sheltered  tree  and  the  strong  roots  of 
an  exposed  tree,  may,  like  the  contrast  of  their  stems,  be 
mainly  due  to  difference  of  nutrition,  and  therefore  supplies 
but  doubtful  evidence,  we  get  tolerably  clear  evidence  where 
trees  growing  on  inclined  rocky  surfaces,  send  into  crevices 
that  afford  little  moisture  or  nutriment,  roots  which  never- 
theless become  thick  where  they  are  so  directed  as  to  bear 
great  strains.  Suspicion  thus  raised  is  strengthened 

into  conviction  by  special  evidences  occurring  in  the  places 
where  they  are  to  be  expected.  The  Cactuses,  with  their 
succulent  growths  that  pass  into  woody  growths  slowly  and 
irregularly,  give  us  the  opportunity  of  tracing  the  conditions 
under  which  the  wood  is  formed.  Good  examples  occur  in  the 
genus  Cereus,  and  especially  in  forms  like  C.  crenulatus. 
Here,  from  a  massive  vertically-growing  rod  of  fleshy  tissue, 
two  inches  or  more  in  diameter,  there  grow  at  intervals  lateral 
rods  similarly  bulky,  which,  quickly  curving  themselves,  take 
vertical  directions.  One  of  these  heavy  branches  puts  great 
strains  on  its  own  substance  and  that  of  the  stem  at  their 
point  of  junction ;  and  here  both  of  them  become  brown  and 
hard,  while  they  continue  green  and  succulent  all  around. 
Such  differentiations  may  be  traced  internally  before  they 
are  visible  on  the  surface.  If  a  joint  of  an  Opuntia  be  sliced 
through  longitudinally,  the  greater  resistance  to  the  knife 
all  around  the  narrow  neck,  indicates  there  a  larger  deposit 
of  lignin  than  elsewhere;  and  a  section  of  the  tissue  placed 
under  the  microscope,  exhibits  at  the  narrowest  part  a  con- 
centration of  the  woody  and  vascular  bundles.  Clear 
evidence  of  another  kind  has  been  noted  by  Mr.  Darwin,  in  the 
organs  of  attachment  of  climbing  plants.  Speaking  of  Sola- 
num  jasminoides  he  says : — "  When  the  flexible  petiole  of  a 
half-  or  a  quarter-grown  leaf  has  clasped  any  object,  in  three 


THE  INNER  TISSUES  OF  PLANTS.  277 

or  four  days  it  increases  much  in  thickness,  and  after  several 
weeks  becomes  wonderfully  hard  and  rigid;  so  that  I  could 
hardly  remove  one  from  its  support.  On  comparing  a  thin 
transverse  slice  of  this  petiole  with  one  from  the  next  or 
older  leaf  beneath,  which  had  not  clasped  anything,  its 
diameter  was  found  to  be  fully  doubled,  and  its  structure 
greatly  changed.  .  .  .  This  clasped  petiole  had  actually 
become  thicker  than  the  stem  close  beneath;  and  this  was 
chiefly  due  to  the  greater  thickness  of  the  ring  of  wood, 
which  presented,  both  in  transverse  and  longitudinal  sections, 
a  closely  similar  structure  in  the  petiole  and  axis.  The 
assumption  by  a  petiole  of  this  structure  is  a  singular 
morphological  fact;  but  it  is  a  still  more  singular  physio- 
logical fact  that  so  great  a  change  should  have  been  induced 
by  the  mere  act  of  clasping  a  support." 

If  there  is  a  direct  relation  between  mechanical  stress  and 
the  formation  of  wood,  it  ought  to  explain  for  us  the  internal 
distribution  of  the  wood.  Let  us  see  whether  it  does  this. 

When  seeking  in  mechanical  actions  and  reactions  the 
cause  of  that  indurated  structure  which  forms  the  verte- 
brate axis  (§§  254-7),  it  was  pointed  out  that  in  a  transverse- 
ly-strained mass,  the  greatest  pressures  and  tensions  are 
thrown  on  the  molecules  of  the  concave  and  convex  surfaces. 
Hence,  supposing  the  transversely-strained  mass  to  be  a  cylin- 
der, bent  backwards  and  forwards  not  in  one  plane  but  now 
in  this  plane  and  now  in  that,  its  peripheral  layers  will  be 
those  on  which  the  greatest  stress  falls.  An  ordinary  dicoty- 
ledonous axis  is  such  a  cylinder  so  strained.  The  main- 
tenance of  its  attitude  either  as  a  lateral  shoot  or  a  vertical 
shoot,  implies  subjection  to  the  bendings  caused  by  its  own 
weight  and  by  the  ever-varying  wind.  These  bendings 
imply  tensions  and  pressures  falling  most  severely  first  on 
one  side  of  its  outer  layers  and  then  on  another.  And  if  the 
dense  substance  able  to  resist  these  tensions  and  pressures  is 
deposited  most  where  they  are  greatest,  we  ought  to  find  it 
taking  the  shape  of  a  cylindrical  casing.  This  is  just  what 


278  PHYSIOLOGICAL   DEVELOPMENT. 

we  do  find.  On  cutting  across  a  shoot  in  course  of  formation, 
we  see  its  central  space  either  unoccupied  or  occupied  only 
by  soft  tissue.  That  the  layer  of  hard  tissue  surrounding 
this  is  not  the  outermost  layer,  is  true:  there  lies  beyond  it 
the  cambium  layer,  from  which  it  is  formed,  the  phloem, 
and  the  cortex.  But  outside  of  the  soft  phloem  there  is 
frequently  another  layer  of  dense  tissue  now  known  as  the 
pericyclic  fibres,  having  freq\iently  a  tenacity  greater  even 
than  that  of  the  wood — a  layer  which,  while  it  protects  the 
cambium  and  offers  additional  resistance  to  the  transverse 
strain,  admits  of  being  fissured  as  fast  as  the  cylinder  of 
wood  thickens.  That  is  to  say,  the  deposit  of  resisting  sub- 
stance is  as  completely  peripheral  as  the  exogenous  mode  of 
growth  permits.  So,  too,  in  general  arrangement  is  it  with 
the  ordinary  monocotyledonous  stem.  Different  as  is  here 
the  internal  structure,  there  yet  holds  the  same  general  dis- 
tribution of  tissues,  answering  to  the  same  mechanical  con- 
ditions. The  vascular  woody  bundles,  more  abundant  towards 
the  outside  of  the  stem  than  near  the  centre,  produce  a  harder 
casing  surrounding  a  softer  core.  In  the  supporting 

structures  of  leaves  we  find  significant  deviations  from  this 
arrangement.  While  axes  are  on  the  average  exposed  to 
equal  strains  on  all  sides,  most  leaves,  spreading  out  their 
surfaces  horizontally,  have  their  petioles  subject  to  strains 
that  are  not  alike  in  all  directions ;  and  in  them  the  hard 
tissue  is  differently  arranged.  Its  transverse  section  is 
not  ring-shaped  but  crescent-shaped:  the  two  horns  being 
directed  towards  the  upper  surface  of  the  petiole.  That  this 
arrangement  is  one  which  answers  to  the  mechanical  con- 
ditions, is  not  easy  to  demonstrate :  we  must  satisfy  ourselves 
by  noting  that  here,  where  the  distribution  of  forces  is  dif- 
ferent, the  distribution  of  resisting  tissue  is  different.  And 
then,  showing  conclusively  the  connexion  between  these  differ- 
ences, we  have  the  fact  that  in  petioles  growing  vertically 
and  supporting  peltate  leaves — petioles  which  are  therefore 


THE  IKNER  TISSUES  OF  PLANTS.  279 

subject  to  equal  transverse  strains  on  all  sides — the  vascular 
bundles  are  arranged  cylindrically,  as  in  axes. 

Such,  then,  are  some  of  the  reasons  for  concluding  that  the 
development  of  the  supporting  tissue  in  plants,  is  caused  by 
the  incident  forces  which  this  tissue  has  to  resist.  The 
individuals  in  which  this  direct  balancing  of  inner  and  outer 
actions  progresses  most  favourably,  are  those  which,  other 
things  equal,  are  most  likely  to  prosper;  and,  by  habitual 
survival  of  the  fittest,  there  is  established  a  systematic  and 
constant  distribution  of  a  deposit  adapted  to  the  circum- 
stances of  each  type. 

§  280.  The  function  of  circulation  may  now  be  dealt  with. 
We  have  to  consider  here  by  what  structures  this  is  dis- 
charged; and  what  connexion  exists  between  the  demand 
for  them  and  the  genesis  of  them. 

The  contrast  between  the  rates  at  which  a  dye  passes 
through  simple  cellular  tissiie  and  cellular  tissue  of  which  the 
units  have  been  elongated,  indicates  one  of  the  structural 
changes  required  to  facilitate  circulation.  If  placed  with  its 
cut  surface  in  a  coloured  liquid,  the  parenchyma  of  a  potato 
or  the  medullary  mass  of  a  cabbage-stalk,  will  absorb  the 
liquid  with  extreme  slowness ;  but  if  the  stalk  of  a  fungus  be 
similarly  placed,  the  liquid  runs  up  it,  and  especially  up  its 
loose  central  substance,  very  quickly.  On  comparing  the 
tissues  which  thus  behave  so  differently,  we  find  that  whereas 
in  the  one  case  the  component  cells,  packed  close  together, 
have  deviated  from  their  primitive  sphericity  only  as  much  as 
mutual  pressure  necessitates,  in  the  other  case  they  are  drawn 
out  into  long  tubules  with  narrow  spaces  among  them — the 
greatest  dimensions  of  the  tubules  and  the  spaces  being  in  the 
direction  which  the  dye  takes  so  rapidly.  That  which  we 
should  infer,  then,  from  the  laws  of  capillary  action,  is 
experimentally  shown :  liquid  moving  through  tissues  follows 
the  lines  in  which  the  elements  of  the  tissues  are  most 


280  PHYSIOLOGICAL  DEVELOPMENT. 

elongated.  It  does  this  for  two  reasons.  That  narrowing  of 
the  cells  and  intercellular  spaces  which  accompanies  their 
elongation,  facilitates  capillarity;  and  at  the  same  time  fewer 
of  the  septa  formed  by  the  joined  ends  of  the  cells  have  to  be 
passed  through  in  a  given  distance.  Hence  the 

general  fact  that  the  establishment  of  a  rudimentary  vascular 
system,  is  the  formation  of  bundles  of  cells  lengthened  in  the 
direction  which  the  liquid  is  to  take.  This  we  see  very 
obviously  among  the  lower  Cormophytes.  In  one  of  the 
lichen-like  Liverworts,  the  veins  which,  branching  through 
its  frond,  serve  as  communications  with  its  scattered  rootlets, 
are  formed  of  cells  longer  than  those  composing  the  general 
tissue  of  the  frond:  the  lengths  of  these  cells  corresponding 
in  their  directions  with  the  lengths  of  the  veins.  So,  too,  is 
it  with  the  midribs  of  such  fronds  as  assume  more  definite 
shapes;  and  so,  too,  is  it  with  the  creeping  stems  which 
unite  many  such  fronds.  That  is  to  say,  the  current  which 
sets  towards  the  growing  part  from  the  part  which  supplies 
certain  materials  for  growth,  sets  through  a  portion  of  the 
tissues  composed  of  units  that  are  longer  in  the  line  of  the 
current  than  at  right  angles  to  that  line.  The  like  is  true 

of  Phasnogams.  Omitting  all  other  characteristics  of  those 
parts  of  them  through  which  chiefly  the  currents  of  sap 
flow,  we  find  the  uniform  fact  to  be  that  they  consist  of  cells 
and  intercellular  spaces  distinguished  from  others  by  their 
lengths.  It  is  thus  with  veins,  and  midribs,  and  petioles; 
and  if  we  wish  proof  that  it  is  thus  with  stems,  we  have  but 
to  observe  the  course  taken  by  a  coloured  solution  into  which 
a  stem  is  inserted. 

What  is  the  original  cause  of  this  differentiation?  Is  it 
possible  that  this  modification  of  cell-structure  which  favours 
the  transfer  of  liquid  towards  each  place  of  demand,  is  itself 
caused  by  the  current  which  the  demand  sets  up?  Does  the 
stream  make  its  own  channel?  There  are  various  reasons 
for  thinking  that  it  does.  In  the  first  place,  the  simplest  and 
earliest  channels,  such  as  we  see  in  the  Liverworts,  do  not 


THE  INNER  TISSUES  OF  PLANTS.  281 

develop  in  any  systematic  way,  but  branch  out  irregularly, 
following  everywhere  the  irregular  lobes  of  the  fronds  as 
these  spread ;  and  on  examining  under  a  magnifier  the  places 
at  which  the  veins  are  lost  in  the  cellular  tissue,  it  will  be 
seen  that  the  cells  are  there  slightly  longer  than  those 
around:  suggesting  that  the  lengthening  of  them  which 
produces  an  extension  of  the  veins,  takes  place  as  fast  as 
the  growth  of  the  tissue  beyond  causes  a  current  to  pass 
through  them.  In  the  second  place,  a  disappearance  of  the 
granular  contents  of  these  cells  accompanies  their  union 
into  a  vein — a  result  which  the  transmission  of  a  current 
may  not  improbably  bring  about.  But  be  the  special  causes 
of  this  differentiation  what  they  may,  the  evidence  favours 
very  much  the  conclusion  that  the  general  cause  is  the 
setting  up  of  a  current  towards  a  place  where  the  sap  is 
being  consumed.  In  the  histological  development 

of  the  higher  plants  we  find  confirmation.  The  more 

finished  distributing  canals  in  Phasnogams  are  formed  of  cells 
previously  lengthened.  At  parts  of  which  the  typical  struc- 
ture is  fixed,  and  the  development  direct,  this  fact  is  not  easy 
to  trace;  the  cells  rapidly  take  their  elongated  structures  in 
anticipation  of  their  pre-determined  functions.  But  in  places 
where  new  vessels  are  required  in  adaptation  to  a  modifying 
growth,  we  may  clearly  trace  this  succession.  The  swelling 
root  of  a  turnip,  continually  having  its  vascular  system 
further  developed,  and  the  component  vessels  lengthened  as 
well  as  multiplied,  gives  us  an  opportunity  of  watching  the 
process.  In  it  we  see  that  the  reticulated  cells  which  unite 
to  form  ducts,  arise  in  the  midst  of  bundles  of  cells  that  have 
previously  become  elongated,  and  that  they  arise  by  trans- 
formation of  such  elongated  cells ;  and  we  also  see  that  these 
bundles  of  elongated  cells  have  an  arrangement  suggestive 
of  their  formation  by  passing  currents. 

Are  there  grounds  for  thinking  that  these  further  trans- 
formations by  which  strings  of  elongated  cells  pass  into 
vessels  lined  with  spiral,  annular,  reticulated,  or  other 


282  PHYSIOLOGICAL  DEVELOPMENT. 

frameworks,  are  also  in  any  way  determined  by  the  currents 
of  sap  carried?    There  are  some  such  grounds. 

As  just  indicated,  the  only  places  where  we  may  look 
for  evidence  with  any  rational  hope  of  finding  it,  are 
places  where  some  local  requirement  for  vessels  has  arisen 
in  consequence  of  some  local  development  which  the  type 
does  not  involve.  In  these  cases  we  find  such  evidence. 
Good  illustrations  occur  in  those  genera  of  the  Cactacece, 
which  simulate  leaves,  like  Epiphyllum  and  Pliyllocactus. 
A  branch  of  one  of  these  is  outlined  in  Fig.  256.  As  before 
explained  this  is  a  flattened  axis;  and  the  notches  along 
its  edges  are  the  seats  of  the  axillary  buds.  Most  of  these 
axillary  buds  are  arrested;  but  occasionally  one  of  them 
grows.  Now  if,  taking  an  E  pi pkyllum -shoot  which  bears  a 
lateral  shoot,  we  compare  the  parts  of  it  that  are  near  the 
aborted  axillary  buds  with  the  part  that  is  near  the  de- 
veloped axillary  bud,  we  find  a  conspicuous  difference.  In 
the  neighbourhood  of  an  aborted  axillary  bud  there  is  no 
external  sign  of  any  internal  differentiation;  and  on  hold- 
ing up  the  branch  against  the  light,  the  uniform  trans- 
lucency  shows  that  there  is  no  greater  amount  of  dense 
tissue  near  it  than  in  other  parts  of  the  succulent  mass. 
But  where  an  axillary  bud  has  developed,  a  prominent  rounded 
ridge  joins  the  midrib  of  the  lateral  branch  with  the  midrib 
of  the  parent  branch.  In  the  midst  of  this  rounded  ridge 
an  opaque  core  may  be  seen.  And  on  cutting  through  it,  this 
opaque  core  proves  to  be  full  of  vascular  bundles  imbedded 
in  woody  deposits.  Clearly,  these  clusters  of  vessels  imply 
transformations  of  the  tissues,  caused  by  the  passage  of  in- 
creased currents  of  sap.  The  vessels  were  not  there  when 
the  axillary  bud  was  formed;  they  would  not  have  de- 
veloped had  the  axillary  bud  proved  abortive;  but  they 
arise  as  fast  as  growth  of  the  axillary  bud  draws  the  sap 
along  the  lines  in  which  they  lie.  Verification  is  obtained 
by  examining  the  internal  structures.  If  longitudinal 
sections  be  made  through  a  growing  bud  of  Opuntia  or 


THE  INNER  TISSUES  OF  PLANTS.  283 

Cereus,  it  will  be  found  that  the  vessels  in  course  of  forma- 
tion converge  towards  the  point  of  growth,  as  they  would 
do  if  the  sap-currents  determined  their  formation;  that  they 
are  most  developed  near  their  place  of  convergence,  which 
they  would  be  if  so  produced;  and  that  their  terminations 
in  the  tissue  of  the  parent  shoot  are  partially-formed  lines 
of  irregular  elongated  cells,  like  those  out  of  which  the  ves- 
sels of  a  leaf  or  bud  are  developed. 

Concluding,  then,  that  sap-vessels  arise  along  the  lines  of 
least  resistance,  through  which  currents  are  drawn  or  forced, 
the  question  to  be  asked  is — What  physical  process  produces 
them  ?  Their  component  cells,  united  end  to  end  more  or  less 
irregularly  in  ways  determined  by  their  original  positions, 
form  a  channel  much  more  permeable,  both  longitudinally 
and  laterally,  than  the  tissue  around.  How  is -this  greater 
permeability  caused?  The  idea,  first  propounded 

I  believe  by  Wolff,  that  the  adjoined  ends  of  the  cells  are 
perforated  or  destroyed  by  the  passing  current,  is  one  for 
which  much  is  to  be  said.  Whether  these  septa  are  dissolved 
by  the  liquids  they  transmit,  or  whether  they  are  burst  by 
those  sudden  gushes  which,  as  we  shall  hereafter  see,  must 
frequently  take  place  along  these  canals,  need  not  be  dis- 
cussed :  it  is  sufficient  for  us  that  the  septa  do,  in  many  cases, 
disappear,  leaving  internal  ridges  showing  their  positions; 
and,  in  other  cases,  become  extremely  porous.  Though  it  is 
manifest  that  this  is  not  the  process  of  vascular  development 
in  tissues  that  unfold  after  pre-determined  types,  since,  in 
these,  the  dehiscences  or  perforations  of  septa  occur  before 
such  direct  actions  can  have  come  into  play;  yet  it  is  still 
possible  that  the  disappearances  of  septa  which  now  arise  by 
repetition  of  the  type  were  established  in  the  type  by  such 
direct  actions.  Be  this  as  it  may,  however,  a 

simultaneous  change  undergone  by  these  longitudinally- 
united  cells  must  be  otherwise  caused.  Frame-works  are 
formed  in  them — frame-works  which,  closely  fitting  their 
inner  surfaces,  may  consist  either  of  successive  rings,  or  con- 


284:  PHYSIOLOGICAL  DEVELOPMENT. 

tinuous  spiral  threads,  or  networks,  or  structures  between 
spirals  and  networks,  or  networks  with  openings  so  far 
diminished  that  the  cells  containing  them  are  distinguished 
as  fenestrated.  Their  differences  omitted,  however,  these 
structures  have  the  common  character  that,  while  support- 
ing the  coats  of  the  vessels,  they  also  give  special  facilities 
for  the  passage  of  liquids,  both  through  the  sides  of  the 
transformed  cells  and  through  their  united  ends,  where  these 
are  not  destroyed. 

To  attempt  any  physical  interpretation  of  this  change  is 
scarcely  safe:  the  conditions  are  so  complex.  There  are 
reasons  for  suspecting,  however,  that  it  arises  from  a  vacuo- 
lation  of  the  substance  deposited  on  the  cell-wall.  If  rapidly 
deposited,  as  it  is  likely  to  be  along  lines  where  sap  is  freely 
supplied,  this  may,  in  passing  from  the  state  of  a  soluble 
colloid  to  that  of  an  insoluble  colloid,  so  contract  as  to  leave 
uncovered  spaces  on  the  cell-membrane;  and  this  change, 
originally  consequent  on  a  physico-chemical  action,  may  be 
so  maintained  and  utilized  by  natural  selection,  as  to  result 
in  structures  of  definite  kinds,  regularly  formed  in  growing 
parts  in  anticipation  of  functions  to  be  afterwards  discharged. 
But,  without  alleging  any  special  cause  for  this  metamor- 
phosis, we  may  reasonably  conclude  that  it  is  in  some  way 
consequent  upon  the  carrying  of  sap.  If  we  examine  tissues 
such  as  that  in  the  interior  of  a  growing  turnip  that  has  not 
yet  become  stringy,  we  may,  in  the  first  place,  find  bundles 
of  elongated  cells  not  having  yet  developed  in  them  those 
fenestrated  or  reticulated  structures  by  which  the  ducts  are 
eventually  characterized.  Along  the  centres  of  adjacent 
bundles  we  may  find  incomplete  lines  of  such  cells — some 
that  are  partially  or  wholly  transformed,  with  some  between 
them  that  are  not  transformed.  In  other  bundles,  completed 
chains  of  such  transformed  cells  are  visible.  And  then,  in 
still  older  bundles,  there  are  several  complete  chains  running 
side  by  side.  All  which  facts  imply  a  metamorphosis  of  the 


THE  INNER  TISSUES  OP  PLANTS.  285 

elongated  cells,  indirectly  caused  by  the  continued  action  of 
the  currents  carried. 

§  281.  Here,  however,  presents  itself  a  further  problem. 
Taking  it  as  manifest  that  there  is  a  typical  distribution  of 
supporting  tissue  adapted  to  meet  the  mechanical  strains  a 
plant  is  exposed  to  by  its  typical  mode  of  growth,  and  also 
that  there  goes  on  special  adaptation  of  the  supporting  tissue 
to  the  special  strains  the  individual  plant  has  to  bear;  and 
taking  it  as  tolerably  evident  that  the  sap-channels  are 
originally  determined  by  the  passage  of  currents  along  lines 
of  least  resistance ;  there  still  remains  the  ultimate  question — 
Through  what  physical  actions  are  established  these  general 
and  special  adjustments  of  supporting  tissue  to  the  strains 
borne,  and  these  distributions  of  nutritive  liquid  required  to 
make  possible  such  adjustments?  Clearly,  if  the  external 
actions  produce  internal  reactions ;  and  if  this  play  of  actions 
and  reactions  results  in  a  balancing  of  the  strains  by  the 
resistances;  we  may  rationally  suspect  that  the  incident 
forces  are  directly  conducive  to  the  structural  changes  by 
which  they  are  met.  Let  us  consider  how  they  must  work. 

When  any  part  of  a  plant  is  bent  by  the  wind,  the  tissues 
on  its  convex  surface  are  subject  to  longitudinal  tension,  and 
these  extended  outer  layers  compress  the  layers  beneath 
them.  Such  of  the  vessels  or  canals  in  these  subjacent  layers 
as  contain  sap,  must  have  some  of  this  sap  expelled.  Part  of 
it  will  be  squeezed  through  the  more  or  less  porous  walls  of 
the  canals  into  the  surrounding  tissue,  thus  supplying  it  with 
assimilable  materials ;  while  part  of  it,  and  probably  the 
larger  part,  will  be  thrust  along  the  canals  longitudinally 
upwards  and  downwards.  When  the  branch  or  twig  or 
leaf-stalk  recoils,  these  vessels,  relieved  from  pressure,  expand 
to  their  original  diameters.  As  they  expand,  the  sap  rushes 
back  into  them  from  above  and  below.  In  whichever  of 
these  directions  least  has  been  expelled  by  the  compression, 


286  PHYSIOLOGICAL  DEVELOPMENT. 

from  that  direction  most  must  return  during  the  dilation; 
seeing  that  the  force  which  more  efficiently  resisted  the 
thrusting  back  of  the  sap  is  the  same  force  which  urges  it 
into  the  expanded  vessels  again,  when  they  are  relieved  from 
pressure.  At  the  next  bend  of  the  part  a  further  portion  of 
sar>  will  be  squeezed  out,  and  a  further  portion  thrust  fo"- 
wards  along  the  vessels.  This  rude  pumping  process  thus 
serves  for  propelling  the  sap  to  heights  which  it  could  not 
reach  by  capillary  action,  at  the  same  time  that  it  incident- 
ally serves  to  feed  the  parts  in  which  it  takes  place.  It 
strengthens  them,  too,  just  in  proportion  to  the  stress  to  be 
borne;  since  the  more  severe  and  the  more  repeated  the 
strains,  the  greater  must  be  the  exudation  of  sap  from  the 
vessels  or  ducts  into  the  surrounding  tissue,  and  the  greater 
the  thickening  of  this  tissue  by  secondary  deposits.  By 

this  same  action  the  movement  of  the  sap  is  determined 
either  upwards  or  downwards,  according  to  the  conditions. 
While  the  leaves  are  active  and  evaporation  is  going  on  from 
them,  these  oscillations  of  the  branches  and  petioles  urge 
forward  the  sap  into  them;  because  so  long  as  the  vessels  of 
the  leaves  are  being  emptied,  the  sap  in  the  compressed 
vessels  of  the  oscillating  parts  will  meet  with  less  resistance 
in  the  direction  of  the  leaves  than  in  the  opposite  direction. 
But  when  evaporation  ceases  at  night,  this  will  no  longer  be 
the  case.  The  sap  drawn  to  the  oscillating  parts,  to  supply 
the  place  of  the  exuded  sap,  must  come  from  the  directions 
of  least  resistance.  A  slight  breeze  will  bring  it  back  from 
the  leaves  into  the  gently-swaying  twigs,  a  stronger  breeze 
into  the  bending  branches,  a  gale  into  the  strained  stem  and 
roots — roots  in  which  longitudinal  tension  produces,  in  an- 
other way,  the  same  effects  that  transverse  tension  does  in 
the  branches. 

Two  possible  misinterpretations  must  be  guarded  against. 
It  is  not  to  be  supposed  that  this  force-pump  action  causes 
movement  of  the  sap  towards  one  point  rather  than  another : 
it  is  simply  an  aid  to  its  movement.  From  the  stock  of  sap 


THE  INNER  TISSUES  OF  PLANTS.  287 

distributed  through  the  plant,  more  or  less  is  everywhere 
being  abstracted — here  by  evaporation,  here  by  the  unfold- 
ing of  the  parts  into  their  typical  shapes,  here  by  both. 
The  result  is  a  tension  on  the  contained  liquid  columns,  which 
is  greatest  now  in  this  direction  and  now  in  that.  This 
tension  it  is  which  must  be  regarded  as  the  force  that  de- 
termines the  current  upwards  or  downwards ;  and  all  which 
the  mechanical  actions  do  is  to  facilitate  the  transfer  to  the 
places  of  greatest  demand.  Hence  it  happens  that  in  a  plant 
prevented  from  oscillating,  but  having  a  typical  tendency  to 
assume  a  certain  height  and  bulk,  the  demands  set  up  by  its 
unfolding  parts  will  still  cause  currents;  and  there  will  still 
be  alternate  ascents  and  descents,  according  as  the  varying 
conditions  change  the  direction  of  greatest  demand  —  the 
only  difference  being  that,  in  the  absence  of  oscillations,  the 
growth  will  be  less  vigorous.  Similarly,  it  must  not 

be  supposed  that  mechanical  actions  are  here  alleged  to  be 
the  sole  causes  of  wood-formation  in  the  individual  plant. 
The  tendency  of  the  individual  plant  to  form  wood  at  places 
where  wood  has  been  habitually  formed  by  ancestral  plants, 
is  manifestly  a  cause,  and,  indeed,  the  chief  cause.  In  this, 
as  in  all  other  cases,  inherited  structures  repeat  themselves 
irrespective  of  the  circumstances  of  the  individual:  absence 
of  the  appropriate  conditions  resulting  simply  in  imperfect 
repetition  of  the  structures.  Hence  the  fact  that  in  trained 
trees  and  hothouse  shrubs,  dense  substance  is  still  largely 
deposited;  though  not  so  largely  as  where  the  normal  me- 
chanical strains  have  acted.  Hence,  too,  the  fact,  that  in 
such  plants  as  the  Elephant's-foot  or  the  Welwitschia  mira- 
bilis,  which  for  untold  generations  can  have  undergone  no 
oscillations,  there  is  an  extensive  formation  of  wood  (though 
not  to  any  considerable  height  above  the  ground),  in  repeti- 
tion of  an  ancestral  type :  natural  selection  having  here  main- 
tained the  habit  as  securing  some  other  advantage  than  that 
of  support. 

Still,  it  must  be  borne  in  mind  that  though  intermittent 


288  PHYSIOLOGICAL  DEVELOPMENT. 

mechanical  strains  cannot  be  assigned  as  the  direct  causes  of 
these  internal  differentiations  in  plants  that  are  artificially 
sheltered  or  supported,  they  are  assignable  as  the  indirect 
causes;  since  the  inherited  structures,  repeated  apart  from 
such  strains,  are  themselves  interpretable  as  accumulated 
results  of  such  strains  acting  on  successive  generations  of 
ancestral  plants.  This  will  become  clear  on  combining  the 
several  threads  of  the  argument  and  bringing  it  to  a  close, 
which  we  may  now  do. 

§  282.  To  put  the  co-operative  actions  in  their  actual 
order,  would  require  us  to  consider  them  as  working  on  in- 
dividuals small  modifications  that  become  conspicuous  and 
definite  only  by  inheritance  and  gradual  increase;  but  it  will 
aid  our  comprehension  without  leading  us  into  error,  if  we 
suppose  the  whole  process  resumed  in  a  single  continuously- 
existing  plant. 

As  the  plant  erects  the  integrated  series  of  fronds  whose 
united  parts  form  its  rudimentary  axis,  the  increasing  area 
of  frond-surface  exposed  to  the  sun's  rays  entails  an  increas- 
ing draught  upon  the  liquids  contained  in  the  rudimentary 
axis.  The  currents  of  sap  so  produced,  once  established  along 
certain  lines  of  cells  that  offer  least  resistance,  render  them 
by  their  continuous  passage  more  and  more  permeable.  This 
establishment  of  channels  is  aided  by  the  wind.  Each  bend 
produced  by  it  while  yet  the  tissue  is  undifferentiated, 
squeezes  towards  the  place  of  growth  and  evaporation  the 
liquids  that  are  passing  by  osmose  from  cell  to  cell;  and 
when  the  lines  of  movement  become  denned,  each  bend  helps, 
by  forcing  the  liquid  along  these  lines,  to  remove  obstructions 
and  make  continuous  canals.  As  fast  as  this  transfer  of  sap 
is  facilitated,  so  fast  is  the  plant  enabled  further  to  raise  it- 
self, and  add  to  its  assimilating  surfaces;  and  so  fast  do  the 
transverse  strains,  becoming  greater,  give  more  efficient  aid. 
The  canals  thus  formed  can  be  neither  in  the  centre  of  the 
rudimentary  axis  nor  at  its  surface:  for  at  neither  of  these 


THE  INNER  TISSUES  OF  PLANTS.  289 

places  can  the  transverse  strains  produce  any  considerable 
compressions.  They  must  arise  along  a  tract  between  the 
outside  of  the  axis  and  its  core — a  tract  along  which  there 
occur  the  severest  squeezes  between  the  stretched  outer  layers 
and  the  internal  mass.  Just  that  distribution  which  we  find, 
is  the  distribution  which  these  mechanical  actions  tend  to 
establish. 

As  the  plant  gains  in  height,  and  as  the  mass  of  its  foliage 
accumulates,  the  strains  thrown  upon  its  axis,  and  especially 
the  lower  part  of  its  axis,  rapidly  increase.  Supposing  the 
forms  to  remain  similar,  the  strains  must  increase  in  the  ratio 
of  the  cubes  of  the  dimensions ;  or  even  in  a  somewhat  higher 
ratio.  One  consequence  must  be  that  the  compressions  to 
which  the  vessels  at  the  lower  part  of  the  incipient  stem  are 
subject,  become  greater  as  fast  as  the  height  to  which  the  sap 
has  to  be  raised  becomes  greater;  and  another  consequence 
must  be  that  the  local  exudation  of  sap  produced  by  the 
pressure  is  proportionately  augmented.  Hence  the  materials 
for  interstitial  nutrition  being  there  supplied  more  abun- 
dantly, we  may  expect  thickening  of  the  surrounding  tissues 
to  show  itself  there  first :  in  other  words,  wood  will  be 
formed  round  the  vessels  of  the  lower  part  of  the  incipient 
stem.  The  resulting  greater  ability  of  this  lower  part  of  the 
stem  to  bear  strains,  renders  possible  an  increase  of  height; 
and  while  after  an  increase  of  height  the  lowest  part  be- 
comes still  further  strained,  and  still  further  thickens,  the 
part  above  it,  exposed  to  like  actions,  undergoes  a  like 
thickening.  This  induration,  while  it  spreads  upwards,  also 
spreads  outwards.  As  fast  as  the  rude  cylinder  of  dense 
matter  formed  in  this  way,  begins  to  inclose  the  original 
vessels,  it  begins  to  play  the  part  of  a  resistant  mass,  which 
more  and  more  prevents  the  contained  vessels  from  being 
squeezed;  while  between  it  and  the  outer  layers  the  greatest 
compression  occurs  at  each  bend.  Thus  at  the  same  time 
that  the  original  vessels  become  useless,  the  peripheral  cells 
of  the  developing  wood  become  those  which  have  their  liquid 
65 


290  PHYSIOLOGICAL  DEVELOPMENT. 

contents  squeezed  out  longitudinally  and  laterally  with  in- 
creasing force;  and,  consequently,  amid  them  are  formed 
new  sap-channels,  from  which  there  is  the  most  active  local 
exudation,  producing  the  greatest  deposit  of  dense  matter. 

Thus  fusing  together,  as  it  were,  the  individualities  of 
successive  generations  of  plants,  and  recognizing  as  all- 
important  that  facilitation  of  the  process  which  natural 
selection  has  all  along  given,  we  are  enabled  to  interpret  the 
chief  internal  differentiations  of  plants  as  consequent  on  an 
equilibration  between  inner  and  outer  forces.  Here,  indeed, 
we  see  illustrated  in  a  way  more  than  usually  easy  to  follow, 
the  eventual  balancing  of  outer  actions  by  inner  reactions. 
The  relation  between  the  demand  for  liquid  and  the  formation 
of  channels  that  supply  liquid,  as  well  as  that  between  the 
incidence  of  strains  and  the  deposit  of  substance  which  resists 
strains,  are  among  the  clearest  special  examples  of  the  general 
truth  that  the  moving  equilibrium  of  an  organism,  if  not 
overthrown  by  an  incident  force,  must  eventually  be  adjusted 
to  it. 

The  processes  here  traced  out  are,  of  course,  not  to  be 
taken  as  the  only  differentiating  processes  to  which  the  inner 
tissues  of  plants  have  been  subject.  Besides  the  chief  changes 
we  have  considered,  various  less  conspicuous  changes  have 
taken  place.  These  must  be  passed  over  as  arising  in  ways 
too  involved  to  admit  of  specific  interpretations;  even  sup- 
posing them  to  have  been  produced  by  causes  of  the  kind 
assigned.  But  the  probability,  or  rather  indeed  the  certainty, 
is  that  some  of  them  have  not  been  so  produced.  Here,  as 
in  nearly  all  other  cases,  indirect  equilibration  has  worked  in 
aid  of  direct  equilibration ;  and  in  many  cases  indirect  equili- 
bration has  been  the  sole  agency.  Besides  ascribing  to 
natural  selection  the  rise  of  various  internal  modifications 
of  other  classes  than  those  above  treated,  we  must  ascribe 
some  even  of  these  to  natural  selection.  It  is  so  with  the 
dense  deposits  which  form  thorns  and  the  shells  of  nuts : 
these  cannot  have  resulted  from  any  inner  reactions  imme- 


THE  INNER  TISSUES  OF  PLANTS.  291 

diately  called  forth  by  outer  actions;  but  must  have  resulted 
mediately  through  the  effects  of  such  outer  actions  on  the 
species.  Let  it  be  understood,  therefore,  that  the  differ- 
entiations to  which  the  foregoing  interpretation  applies,  are 
only  those  most  conspicuous  ones  which  are  directly  related 
to  the  most  conspicuous  incident  forces.  They  must  be  taken 
as  instances  on  the  strength  of  wkich  we  may  conclude  that 
other  internal  differentiations  have  had  a  natural  genesis, 
though  in  ways  that  we  cannot  trace. 


CHAPTEK  V. 

PHYSIOLOGICAL   INTEGRATION   IN   PLANTS. 

§  283.  A  GOOD  deal  has  been  implied  on  this  topic  in  the 
preceding  chapters.  Here,  however,  we  must  for  a  brief 
space  turn  our  attention  immediately  to  it. 

Plants  do  not  display  integration  in  such  distinct  and 
multiplied  ways  as  do  animals.  But  its  advance  may  be 
traced  both  directly  and  indirectly — directly  in  the  increas- 
ing co-ordination  of  actions,  and  indirectly  in  the  effect  of 
this  upon  the  powers  and  habits. 

Let  us  group  the  facts  under  these  heads :  ascending  in 
both  cases  from  the  lower  to  the  higher  types. 

§  284.  The  inferior  Algce,  along  with  little  unlikeness  of 
parts,  show  us  little  mutual  dependence  of  parts.  Having 
surfaces  similarly  circumstanced  everywhere,  much  physio- 
logical division  of  labour  cannot  arise;  and  therefore  there 
cannot  be  much  physiological  unity.  Among  the  superior 
Algce,  however,  the  differentiation  between  the  attached  part 
and  the  free  part  is  accompanied  by  some  integration.  There 
is  evidently  a  certain  transfer  of  materials,  which  is  doubtless 
facilitated  by  the  elongated  forms  of  the  cells  in  the  stem, 
and  probably  leads  to  the  formation  of  dense  tissue  at  the 
places  of  greatest  strain,  in  a  way  akin  to  that  recently  ex- 
plained in  other  cases.  And  where  there  is  this  co-ordina- 
tion of  actions,  the  parts  are  so  far  mutually  dependent  that 


PHYSIOLOGICAL  INTEGRATION  IN  PLANTS.        293 

each  dies  if  detached  from  the  other.  That  though  the 
organization  is  so  low  neither  part  can  reproduce  the  other 
and  survive  by  so  doing,  is  probably  due  to  the  circumstance 
that  neither  part  contains  any  considerable  stock  of  untrans- 
formed  protoplasm,  out  of  which  new  tissues  may  be  pro- 
duced. 

Fungi  and  Lichens  present  no  very  significant  advances 
of  integration.  We  will  therefore  pass  at  once  to  the 
Archegoniates.  In  those  of  them  which,  either  as  single 
fronds  or  strings  of  fronds,  spread  over  surfaces,  and  which, 
rooting  themselves  as  they  spread,  do  not  need  that  each  part 
should  receive  aid  from  remote  parts,  there  is  no  developed 
vascular  system  serving  to  facilitate  transfer  of  nutriment: 
the  parts  being  little  differentiated  there  is  but  little  integra- 
tion. But  along  with  assumption  of  the  upright  attitude  and 
the  accompanying  specializations,  producing  vessels  for  dis- 
tributing sap  and  hard  tissue  for  giving  mechanical  support, 
there  arises  a  decided  physiological  division  of  labour ;  render- 
ing the  aerial  part  dependent  on  the  imbedded  part  and  the 
imbedded  part  dependent  on  the  aerial  part.  Here,  in- 
deed, as  elsewhere,  these  concomitant  changes  are  but  two 
aspects  of  the  same  change.  Always  the  gain  of  power  to  dis- 
charge a  special  function  involves  a  loss  of  power  to  perform 
other  functions;  and  always,  therefore,  increased  mutual  de- 
pendence constituting  physiological  integration,  must  keep 
pace  with  that  increased  fitting  of  particular  parts  to  particu- 
lar duties  which  constitutes  physiological  differentiation. 

Making  a  great  advance  among  the  Archegoniates,  this 
physiological  integration  reaches  its  climax  among  Phseno- 
gams.  In  them  we  see  interdependence  throughout 
masses  that  are  immense.  Along  with  specialized  appli- 
ances for  support  and  transfer,  we  find  an  exchange  of  aid  at 
great  distances.  We  see  roots  giving  the  vast  aerial  growth 
a  hold  tenacious  enough  to  withstand  violent  winds,  and 
supplying  water  enough  even  during  periods  of  drought;  we 
see  a  stem  and  branches  of  corresponding  strength  for  up- 


294  PHYSIOLOGICAL  DEVELOPMENT. 

holding  the  assimilating  organs  under  ordinary  and  extraor- 
dinary strains;  and  in  these  assimilating  organs  we  see 
elaborate  appliances  for  yielding  to  the  stem  and  roots  the 
materials  enabling  them  to  fulfil  their  offices.  As  a  con- 
sequence of  which  greater  integration  accompanying  the 
greater  differentiation,  there  is  ability  to  maintain  life  over 
an  immense  period  under  marked  vicissitudes. 

Even  more  conspicuously  exemplified  in  Phaenogams,  is 
that  physiological  integration  which  holds  together  the  func- 
tions not  of  the  individual  only  but  of  the  species  as  a  whole. 
The  organs  of  reproduction,  both  in  their  relations  to  other 
parts  of  the  individual  bearing  them  and  in  their  relations  to 
corresponding  parts  of  other  individuals,  show  us  a  kind  of 
integration  conducing  to  the  better  preservation  of  the  race; 
as  those  already  specified  conduce  to  the  better  preservation  of 
the  individual.  In  the  first  place,  this  greater  co-ordination 
of  functions  just  described,  itself  enables  Phaenogams  to  be- 
queath to  the  germs  they  cast  off,  stores  of  nutriment,  pro- 
tective envelopes,  and  more  or  less  of  organization :  so  giving 
them  greater  chances  of  rooting  themselves.  In  the  second 
place,  certain  differentiations  among  the  parts  of  fructifica- 
tion, the  meaning  of  which  Mr.  Darwin  has  so  admirably  ex- 
plained, give  to  the  individuals  of  the  species  a  kind  of  inte- 
gration that  makes  possible  a  mutual  aid  in  the  production  of 
vigorous  offspring.  And  it  is  interesting  to  observe  how,  in 
that  dimorphism  by  which  in  some  cases  this  mutual  aid  is 
made  more  efficient,  the  greater  degree  of  integration  is 
dependent  on  the  greater  degree  of  differentiation — not  simply 
differentiation  of  the  fructifying  organs  from  other  parts  of 
the  plant  bearing  them,  but  differentiation  of  these  fructify- 
ing organs  from  the  homologous  organs  of  neighbouring  indi- 
viduals of  the  same  race.  Another  form  of  this 
co-ordination  of  functions  which  conduces  to  the  maintenance 
of  the  species,  may  be  here  named — partly  for  its  intrinsic 
interest.  I  refer  to  the  strange  processes  of  multiplication 
occurring  in  the  genus  Bryophyllum.  It  is  well  known  that 


PHYSIOLOGICAL  INTEGRATION  IN  PLANTS.         295 

the  succulent  leaves  of  B.  calycinum,  borne  on  foot-stalks 
so  brittle  that  they  are  easily  snapped  by  the  wind,  send 
forth  from  their  edges  when  they  fall  to  the  ground,  buds 
which  root  themselves  and  grow  into  independent  plants.  The 
correlation  here  obviously  furthering  the  preservation  of  the 
race,  is  more  definitely  established  in  another  species  of  the 
gemis — B.  proliferum.  This  plant,  shooting  up  to  a  consider- 
able height,  and  having  a  stem  containing  but  little  woody 
fibre,  habitually  breaks  near  the  bottom  while  still  in  flower ; 
and  is  thus  generally  prevented  from  ripening  its  seeds.  The 
multiplication  is,  however,  secured  in  another  way.  Before 
the  stem  is  broken  young  plants  have  budded  out  from  the 
pedicels  of  the  flowers,  and  have  grown  to  considerable  lengths ; 
and  on  the  fall  of  the  parent  they  forthwith  commence  their 
separate  lives.  Here  natural  selection  has  established  a 
remarkable  kind  of  co-ordination  between  a  special  habit  of 
growth  and  decay,  and  a  special  habit  of  proliferation. 

§  285.  The  advance  of  physiological  integration  among 
plants  as  we  ascend  to  the  higher  types,  is  implied  by  their 
greater  constancy  of  structure,  as  well  as  by  the  stricter  limi- 
tations of  their  habitats  and  modes  of  life.  "  Complexity  of 
structure  is  generally  accompanied  with  a  greater  tendency 
to  permanence  in  form,"  says  Dr.  [now  Sir  J.]  Hooker;  or, 
conversely,  "  the  least  complex  are  also  the  most  variable." 
This  is  the  second  aspect  under  which  we  have  to  contem- 
plate the  facts. 

The  differences  between  the  simpler  Alga  and  Fungi  are 
so  feebly  marked  that  botanists  have  had  great  difficulty  in 
framing  definitions  of  these  classes.  This  structural  indefi- 
niteness  is  accompanied  by  functional  indefiniteness.  Algce, 
which  are  mostly  aquatic,  include  many  small  forms  that 
frequent  the  damp  places  preferred  by  Fungi.  Among  Fungi, 
there  are  kinds  which  lead  submerged  lives  like  the  Algce. 
Besides  this  indistinctness  of  the  classes,  there  is  great  varia- 
bility in  the  shapes  and  modes  of  life  of  their  species — a  vari- 


296  PHYSIOLOGICAL  DEVELOPMENT. 

ability  so  great  that  what  were  at  first  taken  to  be  different 
species,  or  different  genera,  or  even  different  orders,  have 
proved  to  be  merely  varieties  of  one  species.  So  inconstant 
in  structure  are  the  Algce  that  Schleiden  quotes  with  approval 
the  opinion  of  Kutzing,  that  "  there  are  no  species  but  merely 
forms  of  Algce : "  an  opinion  which  though  now  rejected 
sufficiently  implies  extreme  indefiniteness.  In  all  which 
facts  we  see  that  these  lowest  types  of  plants,  little  differ- 
entiated, are  also  but  little  integrated. 

Archegoniates  present  a  like  relation  between  the  small 
specialization  of  functions  which  constitutes  physiological 
differentiation,  and  the  small  combination  of  functions  which 
constitutes  physiological  integration.  "  Mosses,"  says  Mr. 
Berkeley,  "  are  no  less  variable  than  other  cryptogams,  and 
are  therefore  frequently  very  difficult  to  distinguish.  Not 
only  will  the  same  species  exhibit  great  diversity  in  the  size, 
mode  of  branching,  form  and  nervation  of  the  leaves,  but  the 
characters  of  even  the  peristome  itself  are  not  constant." 
And  concerning  the  classification  of  the  remaining  group, 
Filicales,  he  says : — "  Not  only  is  there  great  difficulty  in 
arranging  ferns  satisfactorily,  but  it  is  even  more  difficult  to 
determine  the  limits  of  species." 

After  this  vagueness  of  separation  as  well  as  inconstancy 
of  structure  and  habit  among  the  lower  plants,  the  stability 
of  structure  and  habit  and  divisibility  of  groups  among  the 
higher  plants,  appear  relatively  marked.  Though  Phaenogams 
are  much  more  variable  than  most  botanists  have  until 
lately  allowed,  yet  the  definitions  of  species  and  genera 
may  be  made  with  far  greater  precision,  and  the  forms  are 
far  less  capable  of  change,  than  among  Cryptogams.  And 
this  comparative  fixity  of  type,  implying,  as  it  does,  a  closer 
combination  of  the  component  functions,  we  see  to.be  the 
accompaniment  of  the  greater  differentiation  of  those  func- 
tions and  of  the  structures  performing  them.  That  these 
characters  are  correlatives  is  further  shown  by  the  fact  that 
the  higher  plants  are  more  restricted  in  their  habitats  than 


PHYSIOLOGICAL  INTEGRATION  IN  PLANTS.         297 

the  lower  plants,  both  in  space  and  time.  "  The  much 
narrower  delimitation  in  area  of  animals  than  plants,"  says 
Sir  J.  Hooker,  "  and  greater  restriction  of  Faunas  than 
Floras,  should  lead  us  to  anticipate  that  plant-types  are,  geo- 
logically speaking,  more  ancient  and  permanent  than  the 
higher  animal  types  are,  and  so  I  believe  them  to  be,  and  I 
would  extend  the  doctrine  even  to  plants  of  highly  complex 
structure."  "  Those  classes  and  orders  which  are  the  least 
complex  in  organization  are  the  most  widely  distributed." 

§  286.  Thus  that  which  the  general  doctrine  of  evolution 
leads  us  to  anticipate,  we  find  implied  by  the  facts.  The 
physiological  division  of  labour  among  parts,  can  go  on  only 
in  proportion  to  the  mutual  dependence  of  parts;  and  the 
mutual  dependence  of  parts  can  progress  only  as  fast  as  there 
arise  structures  by  which  the  parts  are  efficiently  combined, 
and  the  mutual  utilization  of  their  actions  made  easy. 

To  say  definitely  by  what  process  is  brought  about  this 
co-ordination  of  functions  which  accompanies  their  specializa- 
tion, is  hardly  practicable.  Direct  and  indirect  equilibration 
doubtless  co-operate  in  establishing  it.  We  may  see,  for 
example,  that  every  increase  of  fitness  for  function  produced 
in  the  aerial  part  of  a  plant  by  light,  as  well  as  every  increase 
of  fitness  for  function  produced  in  its  imbedded  part  by  the 
direct  action  of  the  moist  earth,  must  conduce  to  an  increased 
current  of  the  liquid  evaporated  from  the  one  and  supplied 
by  the  other — must  serve,  therefore,  to  aid  the  formation  of 
sap-channels  in  the  ways  already  described;  that  is — must 
serve  to  develop  the  structures  through  which  mutual  aid 
of  the  parts  is  given:  the  additional  differentiation  tends 
immediately  to  bring  about  the  additional  integration.  Con- 
trariwise, it  is  obvious  that  an  inter-dependence  such  as  we 
see  between  the  secretion  of  honey  and  the  fertilization  of 
germs,  or  between  the  deposit  of  albumen  in  the  cotyledons 
of  an  embryo-plant  and  its  subsequent  striking  root,  is  a 
kind  of  integration  in  the  actions  of  the  individual  or  of  the 


298  PHYSIOLOGICAL  DEVELOPMENT. 

species,  which  no  differentiation  has  a  direct  tendency  to 
initiate.  Hence  we  must  regard  the  total  results  as  due  to  a 
plexus  of  influences  acting  simultaneously  on  the  individual 
and  on  the  species :  some  chiefly  affecting  the  one  and  some 
chiefly  affecting  the  other. 

[NOTE. — In  Nature  for  June  11,  1896,  Dr.  Maxwell  Mas- 
ters, in  an  essay  on  "  Plant  Breeding,"  names  an  instructive 
fact  concerning  the  production  of  varieties  by  selection  of 
slightly  divergent  forms.  He  says : — 

"  To  the  untrained  eye,  the  primordial  differences  noted  are 
often  very  slight;  even  the  botanist,  unless  his  attention  be 
specially  directed  to  the  matter,  fails  to  see  minute  differences 
which  are  perceptible  enough  to  the  raiser  or  his  workmen. 
Nor  must  it  be  thought  that  these  variations,  difficult  as  they 
are  to  recognise  in  the  beginning,  are  unimportant.  On  the 
contrary,  they  are  interesting,  physiologically,  as  the  potential 
origin  of  new  species,  and  very  often  they  are  commercially 
valuable  also.  These  apparently  trifling  morphological  dif- 
ferences are  often  associated  with  physiological  variations 
which  render  some  varieties,  say  of  wheat,  much  better 
enabled  to  resist  mildew  and  disease  generally  than  others. 
Some,  again,  prove  to  be  better  adapted  for  certain  soils  or 
for  some  climates  than  others ;  some  are  less  liable  to  injury 
from  predatory  birds  than  others,  and  so  on." 

Thus  we  are  shown  that,  to  a  much  greater  degree  than 
might  be  supposed,  minute  changes  of  forms  and  functions 
in  one  part  of  a  plant  are  correlated  with  changes  of  forms 
and  functions  throughout  it.  The  inter-dependence — that  is 
to  say,  the  physiological  integration — is  very  close  at  the 
same  time  that  it  is  very  complex. 

Here  while  naming  these  facts  in  illustration  of  physio- 
logical integration  in  plants  I  name  them  because  they 
illustrate  an  important  truth  bearing  upon  the  general  ques- 
tion of  heredity  which  I  have  dealt  with  in  Appendix  G,  and 
to  which  I  now  especially  draw  attention.] 


CHAPTER  VI. 

DIFFERENTIATIONS   BETWEEN   THE   OUTER   AND   INNER 
TISSUES   OF   ANIMALS. 

§287.  WHAT  was  said  respecting  the  primary  physiological 
differentiation  in  plants,  applies  with  little  beyond  change  of 
terms  to  animals.  Among  Protozoa,  as  among  Protophyta, 
the  first  definite  contrast  of  parts  is  that  between  outside 
and  inside.  The  speck  of  jelly  or  sarcode  which  appears 
to  constitute  the  simplest  animal,  proves,  on  closer  examina- 
tion, to  be  a  mass  of  substance  containing  a  nucleus — a 
periplast  in  the  midst  of  which  there  is  a  minute  endoplast, 
consisting  of  a  spherical  membrane  and  its  contents. 

This  parallel,  only  just  traceable  among  these  Ehizopods, 
which  are  perpetually  changing  the  distribution  of  their  outer 
substance,  becomes  at  once  marked  in  those  higher  Protozoa 
which  have  fixed  shapes,  and  maintain  constant  relations 
between  their  surfaces  and  their  environments.  Indeed  the 
Rhizopods  themselves,  on  passing  into  a  state  of  quiescence 
in  which  the  relations  of  outer  and  inner  parts  are  fixed, 
become  encysted:  there  is  formed  a  hardened  outer  coat 
different  from  the  matter  which  it  contains.  And  what  is 
here  a  temporary  character  answering  to  a  temporary  definite- 
ness  of  conditions,  is  in  the  Infusoria  a  constant  character, 
answering  to  definite  conditions  that  are  constant.  Each  of 
these  minute  creatures,  though  not  coated  by  a  distinct 
membrane,  has  an  outer  layer  of  excreted  substance  forming 
a  delicate  cuticle. 

§  288.  The  early  establishment  of  this  primary  contrast  of 


300  PHYSIOLOGICAL  DEVELOPMENT. 

tissues  answering  to  this  primary  contrast  of  conditions,  is 
no  less  conspicuous  in  aggregates  of  the  second  order.  The 
feebly-integrated  units  of  a  Sponge,  with  individualities  so 
little  merged  in  that  of  the  whole  they  form  that  most  of 
them  still  retain  their  separate  activities,  nevertheless  show 
us,  in  the  unlikeness  that  arises  between  the  outermost  layer 
and  the  contained  mass,  the  effect  of  converse  with  unlike 
conditions.  This  outermost  layer  is  composed  of  units  some- 
what flattened  and  united  into  a  continuous  membrane — a 
kind  of  rudimentary  skin. 

Secondary  aggregates  in  which  the  lives  of  the  units  are 
more  subordinate  to  the  life  of  the  whole,  carry  this  dis- 
tinction further.  The  leading  physiological  trait  of  every 
ccelenterate  animal  is  the  divisibility  of  its  substance  into 
endoderm  and  ectoderm — the  part  next  the  food  and  the 
part  next  the  environment.  Fig.  147  (§  201),  representing  a 
portion  of  the  body-wall  of  a  Hydra  seen  in  section,  gives 
some  idea  of  this  fundamental  differentiation.  The  creature 
consists  of  a  simple  sac,  the  cavity  of  which  is  in  communi- 
cation with  the  surrounding  water;  and  hence  the  unlike- 
ness between  the  outer  and  inner  layers  has  not  become 
great.  The  essential  contrast  is  that  between  the  differen- 
tiated parts  of  what  was  originally  the  same  part — a  uniform 
membrane  composed  of  juxtaposed  cells. 

For  here,  indeed,  we  are  shown  unmistakably  how  the 
primary  contrast  of  structures  follows  upon  the  primary  con- 
trast of  conditions.  The  ordinary  form  from  which  low 
types  of  the  Metazoa  set  out,  is  a  hollow  sphere  formed  of 
cells  packed  side  by  side — a  blastula,  as  it  is  called :  all  these 
cells  being  similarly  exposed  to  the  environment.  The 
blastula  presently  changes  into  what  is  called  a  gastrula — a 
form  resulting  from  the  introversion  of  one  of  the  sides  of 
the  blastula.  If  there  be  taken  a  small  ball  of  vulcanized 
india-rubber,  say  an  inch  or  more  in  diameter,  and  having  a 
hole  in  it  through  which  the  air  may  escape,  and  if  one  side 
of  it  be  thrust  inwards  so  as  to  produce  a  cup,  and  if  the 


THE  OUTER  AND  INNER  TISSUES  OP  ANIMALS.    301 

wide  opening  of  the  cup  be  supposed  to  contract,  thus 
becoming  a  narrow  opening,  there  will  result  something  like 
the  gastrula  form.  Manifestly  that  part  of  the  original  layer 
which  has  become  internal  is  differently  conditioned  from 
the  rest  which  remains  external:  the  one  continuing  to  hold 
converse  with  the  forces  of  the  environment,  while  the  other 
begins  to  hold  converse  with  the  nutritive  matters  taken 
into  the  sac-formed  chamber — the  archenteron  or  primitive 
stomach.  Interesting  evidence  of  the  primitive  externality 
of  the  digestive  cavity  is  yielded  by  the  fact  that  whereas 
the  blastula  consisted  of  ciliated  cells,  and  whereas  the  cilia- 
tion  persists  throughout  life  on  the  outer  layer,  or  parts  of  it, 
in  sundry  low  types — even  in  some  Chastopods — it  persists 
also  on  the  alimentary  tract  of  sundry  low  types :  not  only 
in  the  Hydra  but  commonly  in  Nemertines,  in  some  Platy- 
helminthes,  and  even  in  some  leeches. 

Besides  being  enabled  thus  to  understand  how  an  aggre- 
gate of  Amoeba-form  units,  originally  consisting  of  a  single 
layer,  may  pass  into  an  aggregate  consisting  of  a  double 
layer;  we  may  also  understand  under  what  influences  the 
transition  takes  place.  If  the  habit  which  some  of  the 
primary  aggregates  have,  of  wrapping  themselves  round 
masses  of  nutriment,  is  followed  by  a  secondary  aggregate, 
there  will  naturally  arise  just  that  re-differentiation  which 
the  Hydra  shows  us. 

§  289.  This  account  of  the  primary  differentiation  carries 
us  only  half-way  towards  a  true  conception  of  the  distinction 
between  outer  and  inner  tissues.  Though,  using  words  in 
their  current  senses,  this  introverted  part  of  the  primitive 
layer  has  become  internal  in  contrast  with  the  remainder, 
which  continues  external,  yet  this  introverted  part  has  not 
become  internal  in  the  strict  physiological  sense.  For  it 
remains  subject  to  the  actions  of  those  environing  matters 
which  are  taken  in  as  food:  such  environing  matters,  when 
they  happen  to  be  moving  prey,  acting  upon  it  much  as  they 


302  PHYSIOLOGICAL  DEVELOPMENT. 

might  act  upon  the  exterior.  So  that  this  introverted  part 
has  a  quasi-externality.  It  has  not  the  same  absolute  in- 
ternally as  have  those  parts  which  never  come  in  contact 
with  products  of  the  outer  world.  Here  we  must  briefly 
recognize  the  distinction  between  these  parts  and  the  parts 
thus  far  considered. 

Eeverting  to  our  symbol,  the  india-rubber  ball,  it  will  be 
seen  that  the  introversion  may  be  so  complete  that  the  cavity 
is  obliterated,  with  the  result  that  the  internal  surfaces  of 
the  outer  and  inner  layers  come  in  contact.  This  is  the 
state  reached  in  the  simplest  ccelenterate  animal,  the  Hydra: 
there  being  in  it  nothing  more  than  a  thin  structureless 
lamella  between  the  ectoderm  and  endoderm,  as  shown  in 
Fig.  147.  This  lamella  represents  all  that  there  is  of  strictly 
internal  tissues.  But  the  introversion,  instead  of  bringing 
the  inner  surfaces  of  the  ball  into  contact,  may  be  so  far  in- 
complete as  to  leave  a  space,  and  in  various  creatures  and 
embryos  of  others,  symbolized  by  this  arrangement,  this  space 
becomes  occupied  by  a  tissue  formed  from  one  or  other  or 
both  of  the  two  primary  tissues — the  mesoblast  or  meso- 
derm.  This  intermediate  layer,  sometimes,  as  in  the  Medusa, 
growing  into  a  mass  of  jelly  serving  as  a  fulcrum  for  the 
creature's  contractions,  or,  as  in  the  Sponge,  giving  a  passive 
basis  to  the  active  tissues,  becomes  in  higher  animals  the 
layer  out  of  which  the  structures  that  support  the  body  and 
move  it  about,  as  well  as  those  that  distribute  prepared 
nutriment,  are  developed.  From  it  arise  the  bones,  the 
muscles,  and  the  vascular  system — the  masses  of  differen- 
tiated tissue  which  are  truly  internal  and  occupy  what  is 
called  the  body-cavity  or  peri-visceral  space. 

In  the  higher  types  of  animals  this  space  comes  to  be 
partially  occupied  by  a  structure  that  may  be  described  as  a 
cavity  within  a  cavity — the  coelom.  Most  zoologists  regard 
this  as  arising  by  a  re-introversion  of  the  archenteron  or 
primary  alimentary  sac.  It  is  easily  to  be  perceived  that 
after  the  introversion  which  produces  this  digestive  cavity,  the 


THE  OUTER  AND  INNER  TISSUES  OF  ANIMALS.    303 

wall  of  the  cavity  may  be  again  introverted  in  such  way  as  to 
intrude  into  the  peri- visceral  space.  The  ccelom  thus  formed 
is  subsequently  shut  off.  Becoming  included  among  the  more 
truly  internal  structures,  and  in  part  giving  origin  to  certain 
lining  membranes,  it  has  for  its  chief  function  the  formation 
of  organs  for  the  excretion  and  emission  of  nitrogenous  waste 
and  of  the  generative  products :  some  portions  of  it  retaining, 
as  a  consequence,  indirect  connexions  with  the  environment 
and  characters  usually  accompanying  such  connexions. 

Here  we  are  not  concerned  with  further  details :  the  aim 
being  simply  to  indicate  the  way  in  which  out  of  the  original 
layer,  wholly  external,  there  arise,  by  primary  and  secondary 
introversions,  and  the  formation  of  intermediate  membranes 
and  spaces,  the  chief  contrasts  between  outer  and  inner  tis- 
sues, and  how  there  simultaneously  go  on  the  differentia- 
tions accompanying  different  conditions. 

§  289a.  Another  all-important  differentiation  between 
outer  tissues  and  inner  tissues  has  now  to  be  set  forth — that 
by  which  the  nervous  system  becomes  established  and  dis- 
tinguished. Strangely  enough,  like  the  one  above  described, 
it  is  sequent  upon  an  introversion:  the  nervous  system  is 
primarily  a  skin-structure  and  develops  by  the  infolding  of 
this  skin-structure. 

In  creatures  possessing  the  earliest  rudiments  of  nerves 
these  exist  in  certain  superficial  cells.  Each  has  a  small 
tubular  orifice  from  which  projects  a  minute  hair,  and  each 
has  on  its  under  side  processes  running  into  the  tissue 
below,  and  serving,  as  it  seems,  to  conduct  impressions  from 
the  projecting  hair  when  it  is  disturbed  by  contacts  with 
foreign  bodies.  A  plexus  of  fibres  bringing  the  inner  pro- 
cesses of  such  cells  into  communication  arises,  and  forms 
something  like  a  nervous  layer  capable  of  propagating  im- 
pulses in  all  directions.  At  a  subsequent  stage  some  of  the 
superficial  cells,  ceasing  to  be  themselves  the  recipients  of 
external  stimuli,  sink  inwards  and  become  ganglion-cells  con- 


304  PHYSIOLOGICAL  DEVELOPMENT. 

nected  with  the  nervous  plexus — agents,  as  we  must  suppose, 
for  the  reception,  multiplication,  and  diffusion  of  the  impulses 
received  from  the  outer  cells. 

As  thus  far  developed,  the  nervous  structure  is  one  fitted 
only  for  a  vague  stimulation  of  dispersed  contractile  fibres, 
causing  movements  of  an  undirected  kind.  A  concentration 
of  these  superficial  nervous  structures  is  a  probable  prelim- 
inary to  the  next  change — an  all-important  change.  For  a 
part  of  the  surface  begins  to  sink  inwards,  forming,  in  the 
Vertebrata,  a  groove ;  and  from  the  lining  cells  of  this  groove, 
which  presently  closes  over,  the  central  parts  of  the  nervous 
system  arise:  definite  nerves  having  meantime,  as  we  may 
suppose,  been  developed  out  of  the  indefinite  nervous  plexus. 

Neglecting  what  there  is  in  this  of  a  speculative  nature,  it 
is  sufficient  for  the  present  purpose  to  recognize  the  un- 
doubted fact  that  the  nervous  system  is  developed  from  the 
ectoderm,  and  that,  originally  external,  it  is  made  internal  by 
a  process  of  sinking  in  or  by  a  process  of  definite  introversion. 

§  290.  Whether  direct  equilibration  or  indirect  equilibra- 
tion has  had  the  greater  share  in  producing  these  fundamental 
contrasts  between  the  inner  and  outer  tissues  of  animals, 
must  be  left  undecided.  The  two  causes  have  all  along  co- 
operated— modification  of  the  individual  accumulated  by 
inheritance  predominating  in  some  cases,  and  in  other  cases 
modification  of  the  race  by  survival  of  the  incidentally  fittest. 
On  the  other  hand,  the  action  of  the  medium  on  the  organism 
cannot  fail  to  change  its  surface  more  than  its  centre,  and  so 
differentiate  the  two;  while,  on  the  other  hand,  the  surfaces 
of  organisms  inhabiting  the  same  medium  display  extreme 
unlikenesses  which  cannot  be  due  to  the  immediate  actions 
of  their  medium.  Let  us  dwell  a  moment  on  the  antithesis. 

We  have  abundant  evidence  that  animal  protoplasm  is 
rapidly  modified  by  light,  heat,  air,  water,  and  the  salts 
contained  in  water — coagulated,  turned  from  soluble  into  in- 
soluble, partially  changed  into  isomeric  compounds,  or  other- 


THE  OUTER  AND  INNER  TISSUES  OF  ANIMALS.    30& 

wise  chemically  altered.  Immediate  metamorphoses  of  this 
kind  are  often  obviously  produced  in  ova  by  changes  of  their 
media.  At  the  outset,  therefore,  before  yet  there  existed 
any  such  differentiation  as  that  which  now  usually  arises  by 
inheritance,  these  environing  agencies  must  have  tended  to 
originate  a  protective  envelope.  For  a  modification  produced 
by  them  on  the  superficial  part  of  the  protoplasm,  must 
either  have  been  a  decomposition  or  else  the  formation  of  a 
compound  which  remained  stable  under  their  subsequent 
action.  There  would  be  generated  an  outer  layer  of  substance 
that  was  so  molecularly  immobile  as  to  be  incapable  of  further 
metamorphoses,  while  it  would  shield  the  contained  proto- 
plasm from  that  too-great  action  of  external  forces  which,  by 
rapidly  changing  the  unstable  equilibrium  of  its  molecules 
into  a  relatively  stable  equilibrium,  would  arrest  development. 
Evidently  organic  evolution,  whether  individual  or  general, 
must  always  and  everywhere  have  been  subordinate  to  these 
physical  necessities.  Though  natural  selection,  beginning 
with  minute  portions  of  protoplasm,  must  all  along  have 
tended  to  establish  a  molecular  composition  apt  to  undergo 
this  differentiation  of  surface  from  centre  to  the  most  favour- 
able extent,  yet  it  must  all  along  have  done  so  while  con- 
trolled by  this  process  of  direct  equilibration. 

Contrariwise,  the  many  and  great  unlikenesses  among  the 
dermal  structures  of  creatures  inhabiting  the  same  element, 
cannot  be  ascribed  to  any  such  cause.  The  contrasts  between 
naked  and  shelled  Gastropods,  between  marine  Worms  and 
Crustaceans,  between  soft-skinned  Fishes  and  Fishes  in 
armour  like  the  Pterichthys,  must  have  been  produced  entirely 
by  natural  selection.  Environing  forces  are,  as  before,  the 
ultimate  causes;  but  the  forces  are  now  not  so  much  those 
exercised  by  the  medium  as  those  exercised  by  the  other  in- 
habitants of  the  medium ;  and  they  do  not  act  by  modifying 
the  surface  of  the  individual,  but  by  killing  off  individuals 
whose  surfaces  are  least  fitted  to  the  requirements :  thus 
slowly  affecting  the  species.  Still  the  dermal  skeleton  bristling 
66 


306  PHYSIOLOGICAL  DEVELOPMENT. 

with  spines,  which  protects  the  Diodon  or  the  Cyclichthys 
from  enemies  it  could  not  escape,  comes  within  the  general 
formula  of  an  outer  tissue  differentiated  from  inner  tissues 
by  the  outer  actions  to  which  the  creature  is  exposed:  the 
differentiation  having  gone  on  until  there  is  equilibrium 
between  the  destructive  forces  to  be  met  and  the  protective 
forces  which  meet  them. 

If  we  venture  to  apportion  the  respective  shares  which 
mediate  and  immediate  actions  have  had  in  differentiating 
outer  from  inner  tissues,  we  shall  probably  not  be  far  wrong 
in  ascribing  that  part  of  the  result  which  is  alike  in  all 
animals,  mainly  to  the  direct  actions  of  their  media,  while 
we  ascribe  the  multitudinous  unlikenesses  of  the  results  in 
various  animals,  partly  to  the  indirect  actions  of  the  media, 
and  partly  to  the  indirect  actions  of  other  animals  by  which 
the  media  are  inhabited.  That  is  to  say,  while  assigning  the 
specialities  of  the  differentiations  to  the  specialities  of  con- 
verse with  the  agencies  in  the  environment,  most  of  them 
organic,  we  may  assign  to  the  constant  and  universal  con- 
verse with  its  inorganic  agencies,  the  universal  characteristic 
of  tegumentary  structures — their  growth  outwards  from  a 
layer  lying  below  the  surface  which  continually  produces  new 
substance  to  replace  the  substance  worn  away  or  cast  off. 

Here  let  me  add  a  piece  of  evidence  which  strengthens 
the  general  argument,  at  the  same  time  that  it  justifies  this 
apportionment.  When  ulceration  has  gone  deep  enough  to 
destroy  the  tegumentary  structures,  these  are  never  repro- 
duced. The  puckered  surface  formed  where  an  ulcer  heals, 
or  where  a  serious  burn  has  destroyed  the  skin,  consists  of 
modified  connective  tissue,  which,  as  the  healing  goes  on, 
spreads  inwards  from  the  edges  of  the  ulcer:  some  of  it, 
perhaps,  growing  from  the  portions  of  connective  tissue  that 
dip  down  between  the  muscular  bundles.  This  connective 
tissue  is  normally  covered  by  the  epidermis  and  thus  sheltered 
from  environing  actions.  What  has  happened  to  it  ?  It  has 
now  become  the  outermost  layer.  And  how  does  it  comport 


THE  OUTER  AND  INNER  TISSUES  OF  ANIMALS.    307 

itself  under  its  new  conditions?  It  produces  a  superficial 
substance  which  plays  the  part  of  the  epidermis  and  grows 
outwardly.  For  since  the  surface,  subject  to  friction  and 
exfoliation,  has  to  be  continually  renewed,  there  must  be  a 
continual  reproduction  of  an  outermost  layer  from  a  layer 
beneath.  That  is  to  say,  the  contact  of  this  deep-seated  tissue 
with  outer  agencies,  produces  in  it  some  approach  towards 
that  character  which  we  find  universally  characterizes  outer- 
tissue.  But  while  we  see  under  this  exposure  to  the  con- 
ditions common  to  all  integument,  a  tendency  to  assume 
the  structure  common  to  all  integument,  we  see  no  tendency 
to  assume  any  of  the  specialities  of  tegumentary  structure: 
no  rudiments  of  glands  or  hair  sacs  make  their  appearance. 

Analogous  conclusions  may  be  drawn  respecting  the  pro- 
cesses of  differentiation  by  which  from  the  outer  layer 
nervous  tissue  and  finally  a  nervous  system  are  evolved. 
Here,  also,  both  direct  and  indirect  equilibration  appear  to 
have  operated.  Two  reasons  may  be  assigned  for  the  belief 
that  the  transformation  of  certain  superficial  cells  into  sensi- 
tive cells  was  initiated  by  exposure  to  external  stimuli.  The 
first  is  that,  extremely  unstable  as  protoplasm  is,  disturb- 
ances received  by  the  outer  side  of  a  specially-exposed  cell 
could  scarcely  fail  to  cause  changes  passing  through  it 
towards  the  interior  mass  of  the  body,  and  that  perpetual 
repetition  of  such  changes  would  tend  to  generate  channels 
of  easy  transmission  through  the  protoplasm.  The  second 
reason  is  that,  if  we  do  not  assume  this  process  of  initiation 
but  assume  that  survival  of  the  fittest  was  the  sole  agency, 
then  no  reason  can  be  assigned  why  the  nervous  system 
should  not  have  been  at  the  outset  formed  internally  instead 
of  being  initiated  externally  and  then  transferred  to  the  in- 
terior: the  roundabout  process  would  be  inexplicable.  At 
the  same  time  the  production  of  a  central  nervous  system  by 
introversion  of  superficial  sensitive  cells  cannot  be  ascribed 
to  the  differentiating  effects  of  external  stimuli,  but  must  be 
ascribed  to  natural  selection.  No  perpetual  repetition  of 


308  PHYSIOLOGICAL  DEVELOPMENT. 

outer  disturbances  would  cause  the  sinking  inwards,  and 
covering  up,  of  the  specially-sensitive  area  and  the  plexus 
below  it.  But  it  is  manifest  that  since  these  nervous  struc- 
tures, at  once  all-important  and  easily  injured,  would  be 
safer  if  removed  from  the  surface,  survival  of  the  fittest,  con- 
tinually preserving  those  in  which  they  were  more  deeply 
seated,  would  tend  to  produce  an  arrangement  in  which  all 
parts  but  the  actual  receivers  of  external  stimuli  became 
internal. 

Hence,  contemplating  generally  these  two  fundamental 
differentiations  of  inner  from  outer  tissues,  we  may  conclude 
that  though  their  first  stages  resulted  from  direct  equilibra- 
tion, their  subsequent  and  higher  stages  resulted  from  in- 
direct equilibration. 


CHAPTER  VII. 

DIFFERENTIATIONS   AMONG   THE   OUTER   TISSUES   OF 
ANIMALS. 

§  291.  THE  outer  tissues  of  animals,  originally  homo- 
geneous over  their  whole  surfaces,  pass  into  a  heterogeneity 
which  fits  their  respective  parts  to  their  respective  conditions. 
So  numerous  and  varied  are  the  implied  differentiations,  that 
it  is  impracticable  here  to  deal  with  them  all  even  in  outline. 
To  trace  them  up  through  classes  of  animals  of  increasing 
degrees  of  aggregation,  would  carry  us  into  undue  detail. 

Did  space  permit,  it  would  be  possible  to  point  out  among 
the  Protozoa,  various  cases  analogous  to  that  of  the  Arcella; 
which  may  be  described  as  like  a  microscopic  Limpet,  having 
a  sarcode  body  of  which  the  upper  surface  has  become  horny, 
while  the  lower  surface  with  its  protruding  pseudopodia, 
retains  the  primitive  jelly-like  character.  That  differentia- 
tions of  this  kind  have  been  gradually  established  among 
these  minute  creatures  through  the  unlike  relations  of  their 
parts  to  the  environment,  is  an  inference  supported  by  a 
form  which,  while  the  rest  of  the  body  has  a  scarcely  dis- 
tinguishable coating,  "  agrees  with  Arcella  and  Difflugia  in 
having  the  pseudopodia  protrusible  from  one  extremity  only 
of  the  body." 

Many  parallel  specializations  of  surface  among  aggregates 
of  the  second  order  might  be  instanced  from  the  Coelenterata. 
In  the  Hydra,  the  ectoderm  presents  over  its  whole  area  no 
conspicuous  unlikenesses ;  but  there  usually  exist  in  the 
hydroid  polypes  of  superior  types,  decided  contrasts  between 

309 


310  PHYSIOLOGICAL  DEVELOPMENT. 

the  higher  and  lower  parts.  While  the  higher  parts  retain 
their  original  characters,  the  lower  parts  excrete  hard  outer 
layers  yielding  support  and  protection.  Various  stages  of 
the  differentiation  might  be  followed.  "  In  Hydractinia" 
says  Prof.  Green,  this  horny  layer  "  becomes  elevated  at  in- 
tervals to  form  numerous  rough  processes  or  spines,  while 
over  the  general  surface  of  the  ectoderm  its  presence  is 
almost  imperceptible."  In  other  types,  as  in  Cordylophora, 
it  spreads  part  way  up  the  animal's  sides,  ending  indefinitely. 
In  Bimeria  it  "  extends  itself  so  as  to  enclose  the  entire  body 
of  each  polypite,  leaving  bare  only  the  mouth  and  tips  of  the 
tentacles."  While  in  Campanularia  it  has  become  a  partially- 
detached  outer  cell,  into  which  the  creature  can  retract  its 
exposed  parts. 

But  it  is  as  needless  as  it  would  be  wearisome  to  trace 
through  the  several  sub-kingdoms  the  rise  of  these  multiform 
contrasts,  with  the  view  of  seeking  interpretations  of  them. 
It  will  suffice  if  we  take  a  few  groups  of  the  illustrations 
furnished  by  the  higher  animals. 


J.  We  may  begin  with  those  modifications  of  surface 
which  subserve  respiration.  Though  we  ordinarily  think  of 
respiration  as  the  quite  special  function  of  a  quite  special 
organ,  yet  originally  it  is  not  so.  Little-developed  animals 
part  with  their  carbonic  acid  and  absorb  oxygen,  through  the 
general  surface  of  the  body.  Even  in  the  lower  types  of  the 
higher  classes,  the  general  surface  of  the  body  aids  largely  in 
aerating  the  blood ;  and  the  parts  which  discharge  the  greater 
part  of  this  function  are  substantially  nothing  more  than 
slightly  altered  and  extended  portions  of  the  skin. 

Such  differentiations,  marked  in  various  degrees,  are  to  be 
seen  among  Mollusca.  In  the  Pteropoda  the  only  modification 
which  appears  to  facilitate  respiration,  is  the  minute  vascu- 
larity  of  one  part  of  the  skin.  Higher  types  possess  special 
skin-developments.  The  Doris  has  appendages  developed 
into  elaborately-branched  forms — small  trees  of  blood-vessels 


THE  OUTER  TISSUES  OP  ANIMALS.  3H 

covered  by  slightly-changed  dermal  tissues.  And  these 
arborescent  branchiae  are  gathered  together  into  a  single 
cluster.  Thus  there  is  evidence  that  large  external  respira- 
tory organs  have  arisen  by  degrees  from  simple  skin:  as, 
indeed,  they  do  arise  during  the  development  of  each  indi- 
vidual having  them.  Just  as  gradually  as  in  the  embryo 
a  simple  bud  on  the  integument,  with  its  contained  vascular 
loop,  passes  by  secondary  buddings  into  a  tree-like  growth 
penetrated  everywhere  by  dividing  and  sub-dividing  blood- 
vessels; so  gradually  has  there  probably  proceeded  the 
differentiation  which  has  turned  part  of  the  outer  surface 
into  an  organ  for  excreting  carbonic  acid  and  absorbing 
oxygen. 

Certain  inferior  vertebrate  animals  present  us  with  a  like 
metamorphosis  of  tissues.  These  are  the  Amphibia.  The 
branchiae  here  developed  from  the  skin,  are  covered  with  cel- 
lular epidermis,  not  much  thinner  than  that  covering  the  rest 
of  the  body.  Like  it  they  have  their  surfaces  speckled  with 
pigment-cells;  and  are  not  even  conspicuous  by  their  extra 
vascularity — where  they  are  temporary  at  least.  They  facili- 
tate the  exchange  of  gases  in  scarcely  any  other  way  than  by 
affording  a  larger  area  of  contact  with  the  water,  and  inter- 
posing a  rather  thinner  layer  of  tissue  between  the  water 
and  the  blood-vessels.  Those  very  simple  branchia  of  the 
larval  Amphibia  that  have  them  but  for  a  short  time, 
graduate  into  the  more  complex  ones  of  those  that  have  them 
for  a  long  time  or  permanently ;  showing,  as  before,  the  small 
stages  by  which  this  heterogeneity  of  surface  accompanying 
heterogeneity  of  function  may  arise. 

In  what  way  are  such  differentiations  established  ?  Main- 
ly, no  doubt,  by  natural  selection ;  but  also  to  some  degree,  I 
think,  by  the  inheritance  of  direct  adaptations.  That  a  por- 
tion of  the  integument  at  which  aeration  is  favoured  by  local 
conditions,  should  thereby  be  led  to  grow  into  a  larger 
surface  of  aeration,  appears  improbable.  Survival  of  those 
individuals  which  happen  to  have  this  portion  of  the  integu- 


312  PHYSIOLOGICAL  DEVELOPMENT. 

ment  somewhat  more  developed,  seems  here  the  only  likely 


§  293.  Among  the  conspicuous  modifications  by  which  the 
originally-uniform  outer  layer  is  rendered  multiform,  are  the 
protective  structures.  Let  us  look  first  at  the  few  cases  in 
which  the  formation  of  these  is  ascribable  mainly  to  direct 
equilibration. 

Already  reference  has  been  more  than  once  made  to  those 
thickenings  that  occur  where  the  skin  is  exposed  to  unusual 
pressure  and  friction.  Are  these  adaptations  inheritable? 
and  may  they,  by  accumulation  through  many  generations, 
produce  permanent  dermal  structures  fitted  to  permanent  or 
frequently- recurring  stress?  Take,  for  instance,  the  callosi- 
ties on  the  knuckles  of  the  Gorilla,  which  are  adapted  to 
its  habit  of  partially  supporting  itself  on  its  closed  hands 
when  moving  along  the  ground.  Shall  we  suppose  that  these 
defensive  thickenings  are  produced  afresh  in  each  individual 
by  the  direct  actions;  or  that  they  are  inherited  modifica- 
tions caused  by  such  direct  actions;  or  that  they  are  wholly 
due  to  the  natural  selection  of  spontaneous  variations  ?  The 
last  supposition  does  not  seem  a  probable  one.  Such  thicken- 
ings, if  spontaneous,  would  be  no  more  likely  to  occur  on  the 
knuckles  than  on  any  other  of  the  hundred  equal  areas  form- 
ing the  skin-surface  at  large;  and  the  chances  against  their 
simultaneous  occurrence  on  all  eight  knuckles  would  be  in- 
calculable. Moreover,  the  implication  would  be  that  those 
slight  extra  thicknesses  of  skin  on  the  knuckles,  with  which 
we  must  suppose  the  selection  to  have  commenced,  were  so 
advantageous  as  to  cause  survivals  of  the  individuals  having 
them,  in  presence  of  other  superiorities  possessed  by  other 
individuals.  Then  that  survivals  so  caused,  if  they  ever 
occurred  at  all,  should  have  occurred  with  the  frequency 
requisite  to  establish  and  increase  the  variation,  is  hardly 
supposable.  And  if  we  reject,  as  also  unlikely,  the  repro- 
duction of  these  callosities  de  novo  in  each  individual  (for 


THE  OUTER  TISSUES  OF  ANIMALS.  313 

this  would  imply  that  after  a  thousand  generations  each 
young  gorilla  began  with  knuckles  having  skin  no  thicker 
than  elsewhere),  there  remains  only  the  inference  that  they 
have  arisen  by  the  transmission  and  accumulation  of  func- 
tional adaptations.  Another  case  which  seems 
interpretable  only  in  an  analogous  way,  is  that  of  the  spurs 
that  are  developed  on  the  wings  of  certain  birds — on  those 
of  the  Chaja  screamer  for  example.  These  are  weapons  of 
offence  and  defence.  It  is  a  familiar  fact  that  some  birds 
strike  with  their  wings,  often  giving  severe  blows ;  and  in  the 
birds  named,  the  blows  are  made  more  formidable  by  the 
horny,  dagger-shaped  growths  standing  out  from  those  points 
on  the  wings  which  deliver  them.  Are  these  spurs  directly 
or  indirectly  adaptive?  To  conclude  that  natural  selection 
of  spontaneous  variations  has  caused  them,  is  to  conclude 
that,  without  any  local  stimulus,  thickenings  of  the  skin 
occurred  symmetrically  on  the  two  wings  at  the  places 
required;  that  such  thickenings,  so  localized,  happened  to 
arise  in  birds  given  to  using  their  wings  in  fight;  and  that 
on  their  first  appearance  the  thickenings  were  decided 
enough  to  give  appreciable  advantages  to  the  individuals  dis- 
tinguished by  them — advantages  in  bearing  the  reactions  of 
the  blows  if  not  in  inflicting  the  blows.  But  to  conclude  this 
is,  I  think,  to  conclude  against  probability.  Contrariwise, 
if  we  assume  that  the  thickening  of  the  epidermis  produced 
by  habitual  rough  usage  is  inheritable,  the  development  of 
these  structures  presents  no  difficulty.  The  points  of  impact 
would  become  indurated  in  wings  used  for  striking  with 
unusual  frequency.  The  callosities  of  surface  thus  generated, 
rendering  the  parts  less  sensitive,  would  enable  the  bird  in 
which  they  arose  to  give,  without  injury  to  itself,  more 
violent  blows  and  a  greater  number  of  them :  so,  in  some 
cases,  helping  it  to  conquer  and  multiply.  Among  its  descend- 
ants, inheriting  the  modification  and  the  accompanying  habit, 
the  thickening  would  be  further  increased  in  the  same  way: 
survival  of  the  fittest  tending  ever  to  accelerate  the  process. 


314  PHYSIOLOGICAL  DEVELOPMENT. 

Presently  the  horny  nodes  so  formed,  hitherto  defensive  only 
in  their  effects,  would,  by  their  prominence,  become  offensive 
— would  make  the  blows  given  more  hurtful.  And  now 
natural  selection,  aiding  more  actively,  would  mould  the 
nodes  into  spurs :  the  individuals  in  which  the  nodes  were 
most  pointed  would  be  apt  to  survive  and  propagate ;  and  the 
pointedness  generation  after  generation  thus  increased,  would 
end  in  the  well-adapted  shape  we  see. 

But  if  in  these  cases  the  differentiations  which  fit  par- 
ticular parts  of  the  outer  tissues  to  bear  rough  usage  are 
caused  mainly  by  the  direct  balancing  of  external  actions  by 
internal  reactions,  then  we  may  suspect  that  the  like  is  true 
of  other  modifications  that  occur  where  special  strains  and 
abrasions  have  to  be  met.  Possibly  it  is  true  of  sundry  parts 
that  are  formed  of  hardened  epidermis,  such  as  the  nails, 
claws,  hoofs,  and  hollow  horns  of  Mammals ;  "  all  of  which," 
says  Prof.  Huxley,  "  are  constructed  on  essentially  the  same 
plan,  being  diverticula  of  the  whole  integument,  the  outer 
layer  of  whose  ecderon  has  undergone  horny  metamorphosis." 
Leaving  open,  however,  the  question  what  tegumentary 
structures  are  due  to  direct  equilibration,  furthered  and  con- 
trolled by  indirect  equilibration,  it  is  tolerably  clear  that 
direct  equilibration  has  been  one  of  the  factors. 

§  294.  Dermal  structures  of  another  class  are  developed 
mainly,  if  not  wholly,  by  the  actions  of  external  causes 
on  species  rather  than  on  individuals.  These  are  the 
various  kinds  of  clothing — hairs,  feathers,  quills,  scales, 
scutes.  Though  it  is  no  longer  thought  as  at  one  time  that 
all  these  various  tegumentary  structures  are  homologous  with 
one  another,  yet  it  is  unquestionable  that  sundry  of  the  more 
conspicuous  ones  are.  Those  which  are  extremely  unlike 
may  be  seen  linked  together  by  a  long  series  of  graduated 
forms.  A  retrograde  metamorphosis  from  feathers  to  ap- 
pendages that  are  almost  scale-like,  is  well  seen  in  the  coat 
of  the  Penguin.  There  is  manifest  a  transition  from  the 


THE  OUTER  TISSUES  OP  ANIMALS.  315 

bird-like  covering  to  the  fish-like  covering — a  transition  so 
gradual  that  no  place  can  be  found  where  an  appreciable 
break  occurs;  and  if  the  scale-like  appendages  are  not 
truly  scales  yet  they  exemplify  an  extreme  metamorphosis. 
Less  striking,  perhaps,  but  scarcely  less  significant,  are 
the  modifications  through  which  we  pass  from  feathers  to 
hairs,  on  the  surfaces  of  the  Ostrich  and  the  Cassowary. 
The  skin  of  the  Porcupine  shows  us  hairs  and  quills  united 
by  a  series  of  intermediate  structures,  differing  from  one 
another  inappreciably.  Even  more  remarkable  are  certain 
other  alliances  of  dermal  structures.  "  It  may  be  taken  as 
certain,  I  think,"  says  Prof.  Huxley,  "  that  the  scales,  plates, 
and  spines  of  all  fishes  are  homologous  organs ;  nor  as  less  so 
that  the  tegumentary  spines  of  the  Plagiostomes  are  homo- 
logous with  their  teeth,  and  thence  with  the  teeth  of  all 
vertebrata." 

Further  details  concerning  these  tegumentary  structures 
are  not  needful  for  present  purposes,  and  are  indeed  but  in- 
directly relevant  to  the  subject  of  physiological  development. 
Here  they  are  of  interest  to  us  only  by  involving  the  general 
question — "What  physical  influences  have  brought  them  into 
existence?  Still  with  a  view  to  definite  presentation  of  the 
problem,  it  will  be  well  to  contemplate  the  mode  of  de- 
velopment common  to  the  most  familiar  of  them. 

Suppose  a  small  pit  to  be  formed  on  the  previously  flat 
skin;  and  suppose  that  the  growth  and  casting  off  of  horny 
cells  which  goes  on  over  the  skin  in  general,  continues  to 
go  on  at  the  usual  rate  over  the  depressed  surface  of 
this  pit.  Clearly  the  quantity  of  horny  matter  produced 
within  this  hollow,  will  be  greater  than  that  produced  on  a 
level  portion  of  the  skin  subtending  an  equal  area  of  the 
animal's  outside.  Suppose  such  a  pit  to  be  deepened 
until  it  becomes  a  small  sac.  If  the  exfoliation  goes  on  as 
before,  the  result  will  be  that  the  horny  matter,  expelled,  as 
it  must  be,  through  the  mouth  of  the  sac,  which  now  bears 
a  small  proportion  to  the  internal  surface  of  the  sac,  will  be 


316  PHYSIOLOGICAL  DEVELOPMENT. 

large  in  quantity  compared  with  that  exfoliated  from  a 
portion  of  the  skin  equal  in  area  to  the  mouth  of  the  sac: 
there  will  be  a  conspicuous  thrusting  forth  of  horny  matter. 
Suppose  once  more  that  the  sac,  instead  of  remaining  simple, 
has  its  bottom  pushed  up  into  its  interior,  like  the  bottom  of 
a  wine-bottle — the  introversion  being  carried  so  far  that  the 
introverted  part  reaches  nearly  to  the  external  opening,  and 
leaves  scarcely  any  space  between  the  introverted  part  and 
the  walls  of  the  sac.  It  is  easy  to  see  that  the  exfoliation 
continuing  from  the  surface  of  the  introverted  part,  as  well  as 
from  the  inside  of  the  sac  generally,  the  horny  matter  cast 
off  will  form  a  double  layer;  and  will  come  out  of  the  sac 
in  the  shape  of  a  tube  having  within  its  lower  end  the  intro- 
verted part,  as  the  core  on  which  it  is  moulded,  and  from  the 
apex  of  which  is  cast  off  the  substance  filling,  less  densely, 
its  interior.  The  structure  resulting  will  be  what  we  know 
as  a  hair.  Manifestly  by  progressive  enlargement  of  the  sac, 
and  further  complication  of  that  introverted  part  on  which 
the  excreted  substance  is  moulded,  the  protruding  growth  may 
be  rendered  larger  and  more  involved,  as  we  see  it  in  quills 
and  feathers.  So  that  insensible  steps,  thus  indicated  in 
principle,  carry  us  from  the  exfoliation  of  epidermis  by  a  flat 
surface,  to  the  exfoliation  of  it  by  a  hollow  simple  sac,  an 
introverted  sac,  and  a  sac  further  complicated ;  each  of  which 
produces  its  modified  kind  of  tegumentary  appendage. 

But  now,  after  contemplating  this  typical  illustration, 
we  return  to  the  general  question.  What  are  the  agencies 
which  have  been  operative  in  developing  these  skin- 
structures?  Indirect  equilibration  must  have  worked  almost 
alone  in  producing  them.  No  direct  incidence  of  forces  can 
have  developed  the  enamelled  armour  of  the  Lepidosteus  or 
the  tesselated  plates  of  the  Glypiodon  and  its  modern  allies. 
Survival  of  the  fittest  must  here  and  in  multitudinous  other 
cases  be  regarded  as  the  sole  cause. 

§  295.  Among  many  other  differentiations  of  the  outer 


THE  OUTER  TISSUES  OP  ANIMALS.  317 

tissues,  the  most  worthy  to  be  noticed  in  the  space  that  re- 
mains, are  those  by  which  organs  of  sense  are  formed.  We 
will  begin  with  the  simplest  and  most  closely  allied  to  the 
foregoing. 

Every  hair  that  is  not  too  long  or  flexible  to  convey  to  its 
rooted  end  a  strain  put  upon  its  free  end,  is  a  rudimentary 
tactual  organ;  as  may  be  readily  proved  by  touching  one  of 
those  growing  on  the  back  of  the  hand.  If,  then,  a  creature 
has  certain  hairs  so  placed  that  they  are  habitually  touched 
by  the  objects  with  which  it  deals,  or  amid  which  it  moves, 
an  advantage  is  likely  to  accrue  if  these  hairs  are  modified 
in  a  way  that  enables  them  the  better  to  transmit  the  im- 
pressions derived.  Such  modified  hairs  we  have  in  the 
vibrissce,  or,  as  they  are  commonly  called,  the  "whiskers" 
possessed  by  Cats  and  feline  animals  generally,  as  well  as  by 
Seals  and  many  Rodents.  These  hairs  are  long  enough  to 
reach  objects  at  considerable  distances;  they  are  so  stiff  that 
forces  applied  to  their  free  ends,  cause  movements  of  their 
imbedded  ends ;  and  the  sacs  containing  their  imbedded  ends 
being  well  covered  with  nerve-fibres,  these  developed  hairs 
serve  as  instruments  of  exploration.  By  constant  use  of  them 
the  animal  learns  to  judge  of  the  relative  positions  of  objects 
past  which,  or  towards  which,  it  is  moving.  When  stealthily 
approaching  prey  or  stealthily  escaping  enemies,  such  aids  to 
perception  are  obviously  important:  indeed  their  importance 
has  been  proved  by  the  diminished  power  of  self-guidance  in 
the  dark,  that  results  from  cutting  them  off.  These,  then,  are 
dermal  appendages  originally  serving  the  purpose  of  cloth- 
ing, but  afterwards  differentiated  into  sense-organs. 

That  eyes  are  essentially  dermal  structures  seems  scarcely 
conceivable.  Yet  an  examination  of  their  rudimentary  types, 
and  of  their  genesis  in  creatures  that  have  them  well  deve- 
loped, shows  us  that  they  really  arise  by  successive  modifica- 
tions of  the  double  layer  composing  the  integument.  They 
make  their  first  appearance  among  the  simpler  animals  as 
specks  of  pigment,  covered  by  portions  of  epidermis  slightly 


318  PHYSIOLOGICAL  DEVELOPMENT. 

convex  and  a  little  more  transparent  than  that  around  it. 
Here  their  fundamental  community  of  structure  with  the 
skin  is  easy  to  trace;  and  the  formation  of  them  by  differen- 
tiation of  it  presents  no  difficulty.  Not  so  far 
in  advance  of  these  as  much  to  obscure  the  relationship,  are 
the  eyes  which  the  Crustaceans  possess.  In  every  fish- 
monger's shop  we  may  see  that  the  eyes  of  a  Lobster  are 
carried  on  pedicles ;  and  when  the  Lobster  casts  its  shell,  the 
outer  coat  of  each  eye,  being  continuous  with  the  epidermis 
of  its  pedicle,  is  thrown  off  along  with  the  rest  of  the  exo- 
skeleton.  Beneath  the  transparent  epidermic  layer,  there 
exists  a  group  of  eyes  of  the  kind  which  we  see  in  an 
insect ;  and  these,  according  to  a  high  authority,  are  inclosed 
in  the  dermal  system.  Describing  the  arrangement  of  the 
parts,  M.  Milne  Edwards  writes : — "  But  the  most  remarkable 
circumstance  is,  that  the  large  cavity  within  which  the  whole 
of  these  parallel  columns,  every  one  of  which  is  itself  a  per- 
fect eye,  are  contained,  is  closed  posteriorly  by  a  membrane, 
which  appears  to  be  neither  more  nor  less  than  the  middle 
tegumentary  membrane,  pierced  for  the  passage  of  the  optic 
nerve;  so  that  the  ocular  chamber  at  large  results  from 
the  separation  at  a  point  of  the  two  external  layers  of  the 
general  envelope."  Thus  too  is  it,  in  the  main, 
even  with  the  highly-developed  eyes  of  the  Vertebrata. 
"  The  three  pairs  of  sensory  organs  appertaining  to  the 
higher  senses/'  says  Prof.  Huxley — "  the  nasal  sacs,  the  eyes, 
and  the  ears — arise  as  simple  ccecal  involutions  of  the  ex- 
ternal integument  of  the  head  of  the  embryo.  That  such 
is  the  case,  so  far  as  the  olfactory  sacs  are  concerned,  is 
obvious,  and  it  is  not  difficult  to  observe  that  the  lens  and 
the  anterior  chamber  of  the  eye  are  produced  in  a  perfectly 
similar  manner.  It  is  not  so  easy  to  see  that  the  labyrinth 
of  the  ear  arises  in  this  way,  as  the  sac  resulting  from  the 
involution  of  the  integument  is  small,  and  remains  open  but 
a  very  short  time.  But  I  have  so  frequently  verified 
Huschke's  and  Remak's  statement  that  it  does  so  arise,  that 


THE   OUTER  TISSUES  OF  ANIMALS.  319 

I  entertain  no  doubt  whatever  of  the  fact.  The  outer  ends 
of  the  olfactory  sacs  remain  open,  but  those  of  the  ocular 
auditory  sacs-  rapidly  close  up,  and  shut  off  their  contents 
from  all  direct  communication  with  the  exterior."  That  is 
to  say,  the  eye  considered  as  an  optical  apparatus  is  pro- 
duced by  metamorphoses  of  the  skin:  the  only  parts  of  it 
not  thus  produced,  being  the  membranes  lying  between  the 
sclerotic  and  the  vitreous  humour,  including  those  retinal 
structures  formed  in  them.  All  is  tegumentary  save  that 
which  has  to  appreciate  the  impressions  which  the  modified 
integument  concentrates  upon  it. 

Thus,  as  Prof.  Huxley  has  somewhere  pointed  out,  there 
is  a  substantial  parallelism  between  all  the  sensory  organs  in 
their  modes  of  development;  as  there  is,  too,  between  their 
modes  of  action.  A  vibrissa  may  be  taken  as  their  common 
type.  Increased  impressibility  by  an  external  stimulus, 
requires  an  increased  peripheral  expansion  of  the  nervous 
system  on  which  the  stimulus  may  fall;  and  this  is  secured 
by  an  introversion  of  the  integument,  forming  a  sac  on  the 
walls  of  which  a  nerve  may  ramify.  That  the  more  extended 
sensory  area  thus  constituted  may  be  acted  upon,  there 
requires  some  apparatus  conveying  to  it  from  without  the 
appropriate  stimulus;  and  in  the  case  of  the  vibrissa,  this 
apparatus  is  the  epidermic  growth  which,  under  the  form  of 
a  hair,  protrudes  from  the  sac.  And  that  the  greatest 
sensitiveness  may  be  obtained,  the  external  action  must  be 
exaggerated  or  multiplied  by  the  apparatus  which  conveys  it 
to  the  recipient  nerve;  as,  in  the  case  of  the  vibrissa,  it  is  by 
the  development  of  a  hair  into  an  elastic  lever,  that  trans- 
forms the  slight  force  acting  through  considerable  space  on 
its  exposed  end,  into  a  greater  force  acting  through  a  smaller 
space  at  its  rooted  end.  Similarly  with  the  organs  of  the 
higher  senses.  In  a  rudimentary  eye,  the  slightly  modified 
sense  cell  has  but  a  rudimentary  nerve  to  take  cognizance  of 
the  impression;  and  to  concentrate  the  impression  upon  it, 
there  is  nothing  beyond  a  thickening  of  the  epidermis  into  a 


320  PHYSIOLOGICAL  DEVELOPMENT. 

lens-shape.  But  the  developed  eye  shows  us  a  termination 
of  the  nerve  greatly  expanded  and  divided  to  receive  the 
external  stimulus.  It  shows  us  an  introverted  portion  of  the 
integument  containing  the  apparatus  by  which  the  external 
stimulus  is  conveyed  to  the  recipient  nerve.  The  structure 
developed  in  this  sac  not  only  conveys  the  stimulus,  but  also, 
like  its  homologue,  concentrates  it;  and  in  the  one  case  as 
in  the  other,  the  structure  which  does  this  is  an  epidermic 
growth  from  the  bottom  of  the  sac.  Even  with  the  ear  it  is 
the  same.  Again  we  have  an  introverted  portion  of  the  inte- 
gument, on  the  walls  of  which  the  nerve  is  distributed  in 
the  primitive  ear.  The  otolithes  contained  in  the  sac  thus 
formed,  are  bodies  which  are  set  in  motion  by  the  vibrations 
of  the  surrounding  water,  and  convey  these  vibrations  in 
an  exaggerated  form  to  the  nerves.  And  though  it  is  not 
alleged  that  these  otolithes  are  developed  from  the  epidermic 
lining  of  the  chamber,  yet  as,  if  not  so  developed,  they  are 
concretions  from  the  contents  of  an  epidermic  sac,  they  must 
still  be  regarded  as  epidermic  products. 

Whether  these  differentiations  are  due  wholly  to  indirect 
equilibration,  or  whether  direct  equilibration  has  had  a  share 
in  working  them,  are  questions  that  must  be  left  open. 
Possibly  a  short  hair  so  placed  on  a  mammal's  face  as  to  be 
very  often  touched,  may,  by  conveying  excitations  to  the 
nerves  and  vessels  at  its  root,  cause  extra  growth  of  the 
bulb  and  its  appendages,  and  so  the  development  of  a  vibrissa 
may  be  furthered.  Possibly,  too,  the  light  itself,  to  which 
the  tissues  of  some  inferior  animals  are  everywhere  sensitive, 
may  aid  in  setting  up  certain  of  the  modifications  by  which 
the  nervous  parts  of  visual  organs  are  formed :  producing,  as 
it  must,  the  most  powerful  effects  at  those  points  on  the  sur- 
face which  the  movements  of  the  animal  expose  to  the  greatest 
and  most  frequent  contrasts  of  light  and  shade;  and  propa- 
gating from  those  points  currents  of  molecular  change 
through  the  organism.  But  it  seems  clear  that  the  complexities 


THE  OUTER  TISSUES  OP  ANIMALS.  321 

of  the  sensory  organs  arc  not  thus  explicable.      They  must 
have  arisen  by  the  natural  selection  of  favourable  variations. 

§  296.  A  group  of  facts,  serving  to  elucidate  those  put 
together  in  the  several  foregoing  sections,  has  to  be  added. 
I  have  reserved  this  group  to  the  last,  partly  because  it  is 
transitional — links  the  differentiations  of  the  literally  outer 
tissues  with  those  of  the  truly  inner  tissues.  Though  physi- 
cally internal,  the  mucous  coat  of  the  alimentary  canal  has 
a  ^Mast-externality  from  a  physiological  point  of  view.  As 
was  pointed  out  in  the  last  chapter,  the  skin  and  the  assimi- 
lating surface  have  this  in  common,  that  they  come  in  direct 
contact  with  matters  not  belonging  to  the  organism;  and 
we  saw  that  along  with  this  community  of  relation  to  alien 
substances,  there  is  a  certain  community  of  structure  and 
development.  The  like  holds  with  the  linings  of  all  internal 
cavities  and  canals  that  have  external  openings. 

The  transition  from  the  literally  outer  tissues  to  those 
tissues  which  are  intermediate  between  them  and  the  truly 
inner  tissues,  is  visible  at  all  the  orifices  of  the  body;  where 
skin  and  mucous  membrane  are  continuous,  and  the  one 
passes  insensibly  into  the  other.  This  visible  continuity  is 
associated  not  simply  with  a  great  degree  of  morphological 
continuity,  but  also  with  a  great  degree  of  physiological  con- 
tinuity. That  is  to  say,  these  literally  outer  and  quasi-outer 
layers  are  capable  of  rapidly  assuming  one  another's  struc- 
tures and  functions  when  subject  to  one  another's  conditions. 
Mucous  surfaces,  normally  kept  covered,  become  skin-like  if 
exposed  to  the  air;  but  resume  more  or  less  fully  their 
normal  characters  when  restored  to  their  normal  positions. 
These  are  truths  familiar  to  pathologists.  They  continually 
meet  with  proofs  that  permanent  eversion  of  the  mucous 
membrane,  even  where  it  is  by  prolapse  of  a  part  deeply 
seated  within  the  body,  is  followed  by  an  adaptation  eventu- 
ally almost  complete:  originally  moist,  tender  to  the  touch, 
and  irritated  by  the  air,  the  surface  gradually  becomes  covered 
67 


322  PHYSIOLOGICAL  DEVELOPMENT. 

with  a  thick,  dry  cuticle;  and  is  then  scarcely  more  sensitive 
than  ordinary  integument. 

Whether  this  equilibration  between  new  outer  forces  and 
reactive  inner  forces,  which  is  thus  directly  produced  in  in- 
dividuals, is  similarly  produced  in  races,  must  remain  as  a 
question  not  to  be  answered  in  a  positive  way.  On  the  one 
hand,  we  have  the  fact  that  among  the  higher  animals  there 
are  cases  of  quasi-outer  tissues  which  are  in  one  species 
habitually  ensheathed,  while  in  another  species  they  are  not 
ensheathed;  and  that  these  two  tissues,  though  unquestion- 
ably homologous,  differ  as  much  as  skin  and  mucous  mem- 
brane differ.  On  the  other  hand,  there  are  certain  analogous 
changes  of  surface,  as  on  the  abdomen  of  the  Hermit-Crab, 
which  give  warrant  to  the  supposition  that  survival  of  the 
fittest  is  the  chief  agent  in  establishing  such  differentiations ; 
since  the  abdomen  of  a  Hermit-Crab,  bathed  by  water  within 
the  shell  it  occupies,  is  not  exposed  to  physical  conditions 
that  directly  tend  to  differentiate  its  surface  from  the  surface 
of  the  thorax.  But  though  in  cases  like  this  last,  we  must 
assign  the  result  to  the  natural  selection  of  variations  arising 
incidentally;  we  may,  I  think,  legitimately  assign  the  result 
to  the  immediate  action  of  changed  conditions  where,  as  in 
cases  like  the  first,  we  see  these  producing  in  the  individual, 
effects  of  the  kinds  observed  in  the  race. 

However  this  may  be,  the  force  of  the  general  argument 
remains  the  same.  In  these  exchanges  of  structure  and 
function  between  the  outer  and  quasi-outer  tissues,  we  get 
Tindeniable  proof  that  they  are  easily  differentiate.  And 
seeing  this,  we  are  enabled  the  more  clearly  to  see  how  there 
have,  in  course  of  time,  arisen  those  extreme  and  multi- 
tudinous differentiations  of  the  outer  tissues  which  have  been 
glanced  at. 


CHAPTER  VIII. 

DIFFERENTIATIONS   AMONG    THE    INNER   TISSUES   OP 
ANIMALS. 

§  297.  THE  change  from  the  outside  of  the  lips  to  their 
inside,  introduces  us  to  a  new  series  of  interesting  and  in- 
structive facts,  joining  on  to  those  with  which  the  last  chapter 
closed.  They  concern  the  differentiations  of  those  coats  of 
the  alimentary  canal  which,  as  we  have  seen,  are  physiologi- 
cally outer,  though  physically  inner. 

These  coats  are  greatly  modified  at  different  parts;  and 
their  modifications  vary  greatly  in  different  animals.  In 
the  lower  types,  where  they  compose  a  simple  tube  running 
from  end  to  end  of  the  body,  they  are  almost  uniform  in  their 
histological  characters;  but  on  ascending  from  these  types, 
we  find  them  presenting  an  increasing  variety  of  minute 
structures  between  their  two  ends.  The  argument  will  be 
adequately  enforced  if  we  limit  ourselves  to  the  leading 
modifications  they  display  in  some  of  the  higher  animals. 

The  successive  parts  of  the  alimentary  canal  are  so  placed 
with  respect  to  its  contents,  that  the  physical  and  chemical 
changes  undergone  by  its  contents  while  passing  from  one 
end  to  the  other,  inevitably  tend  to  transform  its  originally 
homogeneous  surface  into  a  heterogeneous  surface.  Clearly, 
the  effect  produced  on  the  food  at  any  part  of  the  canal  by 
trituration,  by  adding  a  secretion,  or  by  absorbing  its  nutri- 
tive matters,  implies  the  delivery  of  the  food  into  the  next 
part  of  the  canal  in  a  state  more  or  less  unlike  its  previous 


324  PHYSIOLOGICAL  DEVELOPMENT. 

states — implies  that  the  surface  with  which  it  now  comes  in 
contact  is  differently  affected  by  it  from  the  preceding  sur- 
faces— implies,  that  is,  a  differentiating  action.  To  use  con- 
crete language ; — food  that  is  broken  down  in  the  mouth  acts 
on  the  oesophagus  and  stomach  in  a  way  unlike  that  which 
it  would  have  done  had  it  been  swallowed  whole;  the  masti- 
cated food,  to  which  certain  solvents  or  ferments  are  added, 
becomes  to  the  intestine  a  different  substance  from  that  which 
it  must  have  otherwise  been;  and  the  altered  food,  resolved 
by  these  additions  into  its  proximate  principles,  cannot  have 
those  proximate  principles  absorbed  in  the  next  part  of  the 
intestine,  without  the  remoter  parts  being  affected  as  they 
would  not  have  been  in  the  absence  of  absorption.  It  is  true 
that  in  developed  alimentary  canals,  such  as  the  reasoning 
here  tacitly  assumes,  these  marked  successive  differentiations 
of  the  food  are  themselves  the  results  of  pre-established 
differentiations  in  the  successive  parts  of  the  canal.  But  it  is 
also  true  that  actions  and  reactions  like  those  here  so  definite- 
ly marked,  must  go  on  indefinitely  in  an  undeveloped  alimen- 
tary canal.  If  the  food  is  changed  at  all  in  the  course  of  its 
transit,  which  it  must  be  if  the  creature  is  to  live  by  it,  then  it 
cannot  but  act  dissimilarly  on  the  successive  tracts  of  the 
alimentary  canal,  and  cannot  but  be  dissimilarly  reacted  on 
by  them.  Inevitably,  therefore,  the  uniformity  of  the  surface 
must  lapse  into  greater  or  less  multiformity :  the  differentia- 
tion of  each  part  tending  ever  to  initiate  differentiations  of 
other  parts. 

Not,  indeed,  that  the  implied  process  of  direct  equilibra- 
tion can  be  regarded  as  the  sole  process.  Indirect  equilibra- 
tion aids ;  and,  doubtless,  there  are  some  of  the  modifications 
which  only  indirect  equilibration  can  accomplish.  But  we 
have  here  one  unquestionable  cause — a  cause  that  is  known 
to  work  in  individuals,  changes  of  the  kind  alleged.  Where, 
for  instance,  cancerous  disease  of  the  oesophagus  so  narrows 
the  passage  into  the  stomach  as  to  prevent  easy  descent 
of  the  food,  the  oesophagus  above  the  obstmction  becomes 


THE  INNER  TISSUES  OF  ANIMALS.  325 

enlarged  into  a  kind  of  pouch;  and  the  inner  surface  of  this 
pouch  begins  to  secrete  juices  that  produce  in  the  food  a  kind 
of  rude  digestion.  Again,  stricture  of  the  intestine,  when  it 
arises  gradually,  is  followed  by  hypertrophy  of  the  muscular 
coat  of  the  intestine  above  the  constricted  part :  the  ordinary 
peristaltic  movements  being  insufficient  to  force  the  food 
forwards,  and  the  lodged  food  serving  as  a  constant  stimulus 
to  contraction,  the  muscular  fibres,  habitually  more  exercised, 
become  more  bulky.  The  deduction  from  general  principles 
being  thus  inductively  enforced,  we  cannot,  I  think,  resist 
the  conclusion  that  the  direct  actions  and  reactions  between 
the  food  and  the  alimentary  canal  have  been  largely  instru- 
mental in  establishing  the  contrasts  among  its  parts.  And 
we  shall  hold  this  view  with  the  more  confidence  on  observ- 
ing how  satisfactorily,  in  pursuance  of  it,  we  are  enabled  to 
explain  one  of  the  most  striking  of  these  differentiations, 
which  we  will  take  as  a  type  of  the  class. 

The  gizzard  of  a  bird  is  an  expanded  portion  of  the  alimen- 
tary canal,  specially  fitted  to  give  the  food  that  trituration 
which  the  toothless  mouth  of  a  bird  cannot  give.  Besides 
having  a  greatly-developed  muscular  coat,  this  grinding- 
chamber  is  lined  with  a  thick,  hard  cuticle,  capable  of  bear- 
ing the  friction  of  the  pebbles  swallowed  to  serve  as  grind- 
stones. This  differentiation  of  the  mucous  coat  into  a  ridged 
and  tubercled  layer  of  horny  matter — a  differentiation  which, 
in  the  analogous  organs  of  certain  Mollusca,  is  carried  to  the 
extent  of  producing  from  this  membrane  cartilaginous  plates, 
and  even  teeth — varies  in  birds  of  different  kinds,  according 
to  their  food.  It  is  moderate  in  birds  that  feed  on  ftesh  and 
fish,  and  extreme  in  granivorous  birds  and  others  that  live 
on  hard  substances.  How  does  this  immense  modification  of 
the  alimentary  canal  originate?  In  the  stomach 

of  a  mammal,  the  macerating  and  solvent  actions  are  united 
with  that  triturating  action  which  finishes  what  the  teeth 
have  mainly  done ;  but  in  the  bird,  unable  to  masticate,  these 
internal  functions  are  specialized,  and  while  the  crop  is  the 


326  PHYSIOLOGICAL   DEVELOPMENT. 

macerating  chamber,  the  gizzard  becomes  a  chamber  adapted 
to  triturate  more  effectually.  This  adaptation  requires  simply 
an  exaggeration  of  certain  structures  and  actions  which 
characterize  stomachs  in  general,  and,  in  a  less  degree, 
alimentary  canals  throughout  their  whole  lengths.  The 
massive  muscles  of  the  gizzard  are  simply  extreme  develop- 
ments of  the  muscular  tunic,  which  is  already  considerably 
developed  over  the  stomach,  and  incloses  also  the  oesophagus 
and  the  intestine.  The  indurated  lining  of  the  gizzard, 
thickened  into  horny  buttons  at  the  places  of  severest  pres- 
sure, is  nothing  more  than  a  greatly  strengthened  and 
modified  epithelium.  And  the  grinding  action  of  the  gizzard 
is  but  a  specialized  form  of  that  rhythmical  contraction  by 
which  an  ordinary  stomach  kneads  the  contained  food,  and 
which  in  the  oesophagus  effects  the  act  of  swallowing,  while 
in  the  intestine  it  becomes  the  peristaltic  motion.  Allied  as 
the  gizzard  thus  clearly  is  in  structure  and  action  to  the 
stomach  and  alimentary  canal  in  general;  and  capable  of 
being  gradually  differentiated  from  a  stomach  where  a  grow- 
ing habit  of  swallowing  food  unmasticated  entails  more 
trituration  to  be  performed  before  the  food  passes  the  pylorus ; 
the  question  is — Does  this  change  of  structure  arise  by  direct 
adaptation?  There  is  warrant  for  the  belief  that  it  does. 
Besides  such  collateral  evidence  as  that  mucous  membrane 
becomes  horny  on  the  toothless  gums  of  old  people,  when 
subject  to  continual  rough  usage,  and  that  the  muscular  coat 
of  the  intestine  thickens  where  unusual  activity  is  demanded 
of  it,  we  have  the  direct  evidence  of  experiment.  Hunter 
habituated  a  sea-gull  to  feed  on  grain,  and  found  that  the 
lining  of  its  gizzard  became  hardened,  while  the  gizzard- 
muscles  doubled  in  thickness.  A  like  change  in  the  diet  of 
a  kite  was  followed  by  like  results.  Clearly,  if  differentiations 
so  produced  in  the  individuals  of  a  race  under  changed  habits, 
are  in  any  degree  inheritable,  a  structure  like  a  gizzard  will 
originate  through  the  direct  actions  and  reactions  between 
the  food  and  the  alimentary  canal. 


THE  INNER  TISSUES  OP  ANIMALS.  327 

Another  case — a  very  interesting  one,  somewhat  allied  to 
this — is  presented  by  the  ruminating  animals.  Here  several 
dilatations  of  the  alimentary  canal  precede  the  true  stomach ; 
and  in  them  large  quantities  of  unmasticated  food  are  stored, 
to  be  afterwards  returned  to  the  mouth  and  masticated  at 
leisure.  What  conditions  have  made  this  specialization 
advantageous?  and  by  what  process  has  it  been  established? 
To  both  these  questions  the  facts  indicate  answers  which  are 
not  unsatisfactory.  Creatures  that  obtain  their 

food  very  irregularly — now  having  more  than  they  can  con- 
sume, and  now  being  for  long  periods  without  any — must, 
in  the  first  place,  be  apt,  when  very  hungry,  to  eat  to  the 
extreme  limits  of  their  capacities;  and  must,  in  the  second 
place,  profit  by  peculiarities  which  enable  them  to  compensate 
themselves  for  long  fasts,  past  and  future.  A  perch  which, 
when  its  stomach  is  full  of  young  frogs,  goes  on  filling  its 
oesophagus  also;  or  a  trout  which,  rising  to  the  fisherman's 
fly,  proves  when  taken  off  the  hook  to  be  full  of  worms  and 
insect-larvae  up  to  the  very  mouth,  gains  by  its  ability  to  take 
in  such  unusual  supplies  of  food  when  it  meets  with  them — 
obviously  thrives  better  than  it  would  do  could  it  never  eat 
more  than  a  stomachful.  That  this  ability  to  feed  greatly  in 
excess  of  immediate  requirement,  is  one  that  varies  in  indi- 
viduals of  the  same  race,  we  see  in  the  marked  contrast 
between  our  own  powers  in  this  respect,  and  the  powers  of 
uncivilized  men;  whose  fasting  and  gorging  are  to  us  so 
astonishing.  Carrying  with  us  these  considerations,  we  shall 
not  be  surprised  at  finding  dilatations  of  the  oesophagus  in 
vultures  and  eagles,  which  get  their  prey  at  long  intervals 
in  large  masses ;  and  we  may  naturally  look  for  them,  too,  in 
birds  like  pigeons,  which,  coming  in  flocks  upon  occasional 
supplies  of  grain,  individually  profit  by  devouring  the 
greatest  quantity  in  a  given  time.  Now  where  the  trituration 
of  the  food  is,  as  in  these  cases,  carried  on  in  a  lower  part  of 
the  alimentary  canal,  nothing  further  is  required  than  the 
storing-chamber;  but  for  a  mammal,  having  its  grinding 


328  PHYSIOLOGICAL  DEVELOPMENT. 

apparatus  in  its  mouth,  to  gain  by  the  habit  of  hurriedly 
swallowing  unmasticated  food,  it  must  also  have  the  habit  of 
regurgitating  the  food  for  subsequent  mastication.  This 
correlation  of  habits  with  their  answering  structures,  may, 
as  we  shall  see,  arise  in  a  very  simple  way.  The 

starting  point  of  the  explanation  is  a  familiar  fact — the  fact 
that  indigestion,  often  resulting  from  excess  of  food,  is  apt 
to  cause  that  reversed  peristaltic  action  known  as  vomiting. 
From  this  we  pass  to  the  fact,  also  within  the  experience  of 
most  persons,  that  during  slight  indigestion  the  stomach 
sometimes  quietly  regurgitates  a  small  part  of  its  contents  as 
far  as  the  back  of  the  mouth — giving  an  unpleasant  acquaint- 
ance with  the  taste  of  the  gastric  juices.  Exceptional  facts  of 
the  same  class  help  the  argument  a  step  further.  "  There  are 
certain  individuals  who  are  capable  of  returning,  at  will,  a 
greater  or  smaller  portion  of  the  contents  of  the  digesting 
stomach  into  the  cavity  of  the  mouth.  ...  In  some  of  these 
cases,  the  expulsion  of  the  food  has  required  a  violent  effort. 
In  the  majority  it  has  been  easily  evoked  or  suppressed. 
While  in  others,  it  has  been  almost  uncontrollable;  or  its 
non-occurrence  at  the  habitual  time  has  been  followed  by  a 
painful  feeling  of  fulness,  or  by  the  act  of  vomiting." 
Here  we  have  a  certain  physiological  action,  occasionally 
happening  in  most  persons  and  in  some  developed  into 
a  habit  more  or  less  pronounced:  indigestion  being  the 
habitual  antecedent.  Suppose,  then,  that  gregarious 

animals,  living  on  innutritive  food  such  as  grass,  are  subject 
to  a  like  physiological  action,  and  are  capable  of  like  varia- 
tions in  the  degree  of  it.  What  will  naturally  happen? 
They  wander  in  herds,  now  over  places  where  food  is  scarce 
and  now  coming  to  places  where  it  is  abundant.  Some  mas- 
ticate their  food  completely  before  swallowing  it,  while  some 
masticate  it  incompletely.  If  an  oasis,  presently  bared  by 
their  grazing,  has  not  supplied  to  the  whole  herd  a  full  meal, 
then  the  individuals  which  masticate  completely  will  have 
had  less  than  those  which  masticate  incompletely — will  not 


THE  INNER  TISSUES  OF  ANIMALS.  329 

have  had  enough.  Those  which  masticate  incompletely  and 
distend  their  stomachs  with  food  difficult  to  digest,  will  be 
liable  to  these  regurgitations ;  but  if  they  re-masticate  what 
is  thus  returned  to  the  mouth  (and  we  know  that  animals 
often  eat  again  what  they  have  vomited),  then  the  extra 
quantity  of  food  taken,  eventually  made  digestible,  will  yield 
them  mor£  nourishment  than  is  obtained  by  those  which 
masticate  completely  at  first.  The  habit  initiated  in  this 
natural  way,  and  aiding  survival  when  food  is  scarce,  will 
be  apt  to  cause  modifications  of  the  alimentary  canal. 
We  know  that  dilatations  of  canals  readily  arise  under 
habitual  distensions.  We  know  that  canals  habitually 
distended  become  gradually  more  tolerant  of  the  contained 
masses  that  at  first  irritated  them.  And  we  know  that 
there  commonly  take  place  adaptive  modifications  of  their 
surfaces.  Hence  if  a  habit  of  this  kind  and  the  structural 
changes  resulting  from  it,  are  in  any  degree  inheritable,  it  is 
clear  that,  increasing  in  successive  generations,  both  imme- 
diately by  the  cumulative  effect  of  repetitions  and  mediately 
by  survival  of  the  individuals  in  which  they  are  most  decided, 
they  may  go  on  until  they  end  in  the  peculiarities  which 
Ruminants  display. 

§  298.  There  are  structures  belonging  to  the  same  group 
which  cannot,  however,  be  accounted  for  in  this  way.  They 
are  the  organs  that  secrete  special  products  facilitating  diges- 
tion— the  liver,  pancreas,  and  various  smaller  glands.  All 
these  appendages  of  the  alimentary  canal,  large  and  inde- 
pendent as  some  of  them  seem,  really  arise  by  differentia- 
tions from  its  coats.  The  primordial  liver  consists  of  nothing 
more  than  bile-cells  scattered  along  a  tract  of  the  intestinal 
surface.  Accumulation  of  these  bile-cells  is  accompanied  by 
increased  growth  of  the  surface  which  bears  them — a  growth 
which  at  first  takes  the  form  of  a  cul-de-sac,  having  an  outside 
that  projects  from  the  intestine  into  the  peri-visceral  cavity. 
As  the  mass  of  bile-cells  becomes  greater,  there  arise  se- 


330  PHYSIOLOGICAL  DEVELOPMENT. 

condary  lateral  cavities  opening  into  the  primary  one,  and 
through  it  into  the  intestine;  until,  eventually,  these  cavities 
with  their  coatings  of  bile-cells,  become  ramifying  ducts  dis- 
tributed through  the  solid  mass  we  know  as  a  liver.  How  is 
this  differentiation  caused  ? 

Before  attempting  any  answer  to  this  question,  it  is 
requisite  to  inquire  the  nature  of  bile.  Is  that  which  the 
liver  throws  into  the  intestines  a  waste  product  of  the  organic 
actions  ?  or  is  it  a  secretion  aiding  digestion  ?  or  is  it  a  mix- 
ture of  these?  Modern  investigations  imply  that  it  is  most 
likely  the  last.  The  liver  is  found  to  have  a  compound  func- 
tion. Bernard  has  proved  to  the  satisfaction  of  physiologists, 
that  there  goes  on  in  it  a  formation  of  glycogen — a  substance 
which  is  transformed  into  sugar  before  it  leaves  the  liver  and 
is  afterwards  carried  away  by  the  blood  to  eventually  dis- 
appear in  the  active  organs,  chiefly  the  muscles.  It  is  also 
shown,  experimentally,  that  there  are  generated  in  the  liver 
certain  biliary  acids;  and  by  the  aid  either  of  these  or  of 
some  other  compounds,  it  is  clear  that  bile  renders  certain 
materials  more  absorbable.  Its  effect  on  fat  is  demonstrable 
out  of  the  body;  and  the  greatly  diminished  absorption  of 
fat  from  the  food  when  the  discharge  of  bile  into  the 
intestine  is  prevented,  is  probably  one  of  the  causes  of  that 
pining  away  which  results.  But  while  recognizing  the  fact 
that  the  bile  consists  in  part  of  a  solvent,  or  solvents,  aid- 
ing digestion,  there  is  abundant  evidence  that  one  element 
of  it  is  an  effete  product;  and  probably  this  is  the  primary 
element.  The  yellow-green  substance  called  biliverdine  in 
herbivora  and  bilirubin  in  man  and  carnivora,  which  gives 
its  colour  to  bile,  is  a  product  the  greater  part  of  which  is 
normally  cast  out  from  the  system  continually,  as  is  shown 
by  the  contrast  between  the  normal  and  abnormal  colours 
of  faecal  matters,  and  as  is  still  more  strikingly  shown  by 
the  effects  on  the  system  when  there  is  a  stoppage  of  the 
excretion,  and  an  attack  of  jaundice.  Hence  we  are  war- 
ranted in  classing  biliverdine  as  a  waste  product,  and  we 


THE  INNER  TISSUES  OF  ANIMALS.  331 

may  fairly  infer  that  the  excretion  of  it  is  the  original 
function  of  the  liver. 

One  further  preliminary  is  requisite.  We  must  for  a 
moment  return  to  those  physico-chemical  data  set  down  in 
the  first  chapter  of  this  work  (§§  7—8).  We  there  saw  that 
the  complex  and  large-atomed  colloids  which  mainly  compose 
living  organic  matter,  have  extremely  little  molecular  mo- 
bility; and,  consequently,  extremely  little  power  of  diffusing 
themselves.  Whereas  we  saw  not  only  that  those  absorbed 
matters,  gaseous  and  liquid,  which  further  the  decomposition 
of  living  organic  matter,  have  very  high  diffusibilities,  but 
also  that  the  products  of  the  decomposition  are  much  more 
diffusible  than  the  components  of  living  organic  matter. 
And  we  saw  that,  as  a  consequence  of  this,  the  tissues  give 
ready  entrance  to  the  substances  which  decompose  them,  and 
ready  exit  to  the  substances  into  which  they  are  decomposed. 
Hence  it  follows  that,  under  its  initial  form,  uncomplicated 
by  nervous  and  other  agencies,  the  escape  of  effete  matters 
from  the  organism,  is  a  physical  action  parallel  to  that  which 
goes  on  among  mixed  colloids  and  crystalloids  that  are  dead 
or  even  inorganic.  Excretion  is  a  specialized  form  of  this 
spontaneous  action ;  and  we  have  to  inquire  how  the  special- 
ization arises. 

Two  causes  conspire  to  establish  it.  The  first  is  that  these 
products  of  decomposition  are  diffusible  in  widely  different 
degrees.  While  the  carbonic  acid  and  water  permeate  the 
tissues  with  ease  in  all  directions,  and  escape  more  or  less 
from  the  exposed  surfaces,  urea,  and  other  waste  substances 
incapable  of  being  vaporized,  cannot  escape  thus  readily. 
The  second  is  that  the  different  parts  of  the  body,  being 
subject  to  different  physical  conditions,  are  from  the  outset 
sure  severally  to  favour  the  exit  of  these  various  products  of 
decomposition  in  various  degrees.  How  these  causes  must 
have  co-operated  in  localizing  the  excretions,  we  shall  see  on 
remembering  how  they  now  co-operate  in  localizing  the  sepa- 
ration of  morbid  materials.  The  characteristic  substances  of 


332  PHYSIOLOGICAL  DEVELOPMENT. 

gout  and  rheumatism  have  their  habitual  places  of  deposit. 
Tuberculous  matter,  though  it  may  be  present  in  various 
organs,  gravitates  towards  some  much  more  than  towards 
others.  Certain  products  of  disease  are  habitually  got  rid  of 
by  the  skin,  instead  of  collecting  internally.  Mostly,  these 
have  special  parts  of  the  skin  which  they  affect  rather  than 
the  rest;  and  there  are  those  which,  by  breaking  out  sym- 
metrically on  the  two  sides  of  the  body,  show  how  definitely 
the  places  of  their  excretion  are  determined  by  certain 
favouring  conditions,  which  corresponding  parts  may  be  pre- 
sumed to  furnish  in  equal  degrees.  Further,  it  is  to  be 
observed  of  these  morbid  substances  circulating  in  the  blood, 
that  having  once  commenced  segregating  at  particular  places, 
they  tend  to  continue  segregating  at  those  places.  Assum- 
ing, then,  as  we  may  fairly  do,  that  this  localization  of 
excretion,  which  we  see 'continually  commencing  afresh  with 
morbid  matters,  has  always  gone  on  with  the  matters  pro- 
duced by  the  waste  of  the  tissues,  let  us  take  a  further  step, 
and  ask  how  localizations  become  fixed.  Other  things  equal, 
that  which  from  its  physical  conditions  is  a  place  of  least 
resistance  to  the  exit  of  an  effete  product,  will  tend  to  become 
established  as  the  place  of  excretion;  since  the  rapid  exit 
of  an  effete  product  will  profit  the  organism.  Other  things 
equal,  a  place  at  which  the  excreted  matter  produces  least 
detrimental  effect  will  become  the  established  place.  If  at 
any  point  the  excreted  matter  produces  a  beneficial  effect, 
then,  other  things  equal,  survival  of  the  fittest  will  determine 
it  to  this  point.  And  if  facility  of  escape  anywhere  goes 
along  with  utilization  of  the  escaping  substance,  then,  other 
things  equal,  the  excretion  will  be  there  localized  still  more 
decisively  by  survival  of  the  fittest. 

Such  being  the  conditions  of  the  problem,  let  us  ask  what 
will  happen  with  the  lining  membrane  of  the  alimentary 
canal.  This,  physiologically  considered,  is  an  external  sur- 
face; and  matters  thrown  off  from  it  make  their  way  out  of 
the  body.  It  is  also  a  surface  along  which  is  moving  the  food 


THE  INNER  TISSUES  OF  ANIMALS.  333 

to  be  digested.  Now,  among  the  various  waste  products 
continually  escaping  from  the  living  tissues,  some  of  the 
more  complex  ones,  not  very  stable  in  composition,  are  likely, 
if  added  to  the  food,  to  set  up  changes  in  it.  Such  changes 
may  either  aid  or  hinder  the  preparation  of  the  food  for 
absorption.  If  an  effete  matter,  making  its  exit  through  the 
wall  of  the  intestine,  hinders  the  digestive  process,  the  en- 
feeblement  and  disappearance  of  individuals  in  which  this 
happens,  will  prevent  the  intestine  from  becoming  the  esta- 
blished place  for  its  exit.  While  if  it  aids  the  digestive 
process,  the  intestine  will,  for  converse  reasons,  become  more 
and  more  the  place  to  which  its  exit  is  limited.  Equally 
manifest  is  it  that  if  there  is  one  part  of  this  alimentary 
canal  at  which,  more  than  at  any  other  part,  the  favourable 
effect  results,  this  will  become  the  place  of  excretion. 

Thus,  then,  reverting  to  the  case  in  question,  we  may 
understand  how  a  product  to  be  cast  out,  such  as  biliverdine, 
if  it  either  directly  or  indirectly  serves  a  useful  purpose, 
when  poured  into  a  particular  part  of  the  intestine,  may  lead 
to  the  formation  of  a  patch  of  excreting  cells  on  its  wall; 
and  once  this  place  of  excretion  having  been  established,  the 
development  of  a  liver  is  simply  a  question  of  time  and 
natural  selection. 

§  299.  A  differentiation  of  another  order  occurring  in  the 
alimentary  canal,  is  that  by  which  a  part  of  it  is  developed 
into  a  lateral  chamber  or  chambers,  through  which  carbonic 
acid  exhales  and  oxygen  is  absorbed.  Comparative  anatomy 
and  embryology  unite  in  showing  that  a  lung  is  formed,  just 
as  a  liver  or  other  appendage  of  the  alimentary  canal  is 
formed,  by  the  growth  of  a  hollow  bud  into  the  peri-visceral 
cavity,  or  space  between  the  alimentary  canal  and  the  wall  of 
the  body.  The  interior  of  this  bud  is  simply  a  cul-de-sac  of 
the  alimentary  canal,  with  the  mucous  lining  of  which  its 
own  mucous  lining  is  continuous.  And  the  development  of 
this  cul-de-sac  into  an  air-chamber,  simple  or  compound,  is 


334:  PHYSIOLOGICAL  DEVELOPMENT. 

merely  a  great  extension  of  area  in  the  internal  surface  of 
the  cul-de-sac,  along  with  that  specialization  which  fits  it 
for  excreting  and  absorbing  substances  different  from  those 
which  other  parts  of  the  mucous  surface  excrete  and 
absorb.  These  lateral  air-chambers,  universal 

among  the  higher  Vertebrata  and  very  general  among  the 
lower,  and  everywhere  attached  to  the  alimentary  canal 
between  the  mouth  and  the  stomach,  have  not  in  all  cases  the 
respiratory  function.  In  most  fishes  that  have  them  they 
are  what  we  know  as  swim-bladders.  In  some  fishes  the 
cavities  of  these  swim-bladders  are  completely  shut  off  from 
the  alimentary  canal :  nevertheless  showing,  by  the  communi- 
cations which  they  have  with  it  during  the  embryonic  stages, 
that  they  are  originally  diverticula  from  it.  In  other  fishes 
there  is  a  permanent  ductus  pneumaticus,  uniting  the  cavity 
of  the  swim-bladder  with  that  of  the  gullet:  the  function, 
however,  being  still  not  respiratory  in  an  appreciable  degree, 
if  at  all.  But  in  certain  still  extant  representatives  of  the 
sauroid  fishes,  as  the  Lepidosteus,  the  air-bladder  is  "  divided 
into  two  sacs  that  possess  a  cellular  structure,"  and  "  the 
trachea  which  proceeds  from  it  opens  high-up  in  the  throat, 
and  is  surrounded  with  a  glottis."  In  the  Amphibia  the 
corresponding  organs  are  chambers  over  the  surfaces  of  which 
there  are  saccular  depressions,  indicating  a  transition  towards 
the  air-cells  characterizing  lungs;  and  accompanying  this 
advance  we  see,  as  in  the  common  Triton,  the  habit  of  coming 
up  to  the  surface  and  taking  down  a  fresh  supply  of  air  in 
place  of  that  discharged. 

How  are  the  internal  air-chambers,  respiratory  or  non- 
respiratory,  developed?  Upwards  from  the  amphibian  stage, 
in  which  they  are  partially  refilled  at  long  intervals,  there  is 
no  difficulty  in  understanding  how,  by  infinitesimal  steps, 
they  pass  into  complex  and  ever-moving  lungs.  But 
how  is  the  differentiation  that  produces  them  initiated? 
How  comes  a  portion  of  the  internal  surface  to  be  specialized 
for  converse  with  a  medium  to  which  it  is  not  naturally 


THE  INNER  TISSUES  OF  ANIMALS.  335 

exposed?  The  problem  appears  a  difficult  one;  but  there  is 
a  not  unsatisfactory  solution  of  it. 

When  many  gold-fish  are  kept  in  a  small  aquarium,  as 
with  thoughtless  cruelty  they  frequently  are,  they  swim 
close  to  the  surface,  so  as  to  breathe  that  water  which  is  from 
instant  to  instant  absorbing  fresh  oxygen.  In  doing  this 
they  often  put  their  mouths  partly  above  the  surface,  so  that 
in  closing  them  they  take  in  bubbles  of  air;  and  sometimes 
they  may  be  seen  to  continue  doing  this — the  relief  due  to 
the  slight  extra  aeration  of  blood  so  secured,  being  the 
stimulus  to  continue.  Air  thus  taken  in  may  be  detained. 
If  a  fish  that  has  taken  in  a  bubble  turns  its  head  down- 
wards, the  bubble  will  ascend  to  the  back  of  its  mouth,  and 
there  lodge;  and  coming  within  reach  of  the  contractions  of 
the  oesophagus,  it  may  be  swallowed.  If,  then,  among  fish 
thus  naturally  led  upon  occasion  to  take  in  air-bubbles,  there 
are  any  having  slight  differences  in  the  alimentary  canal  that 
facilitate  lodgment  of  the  air,  or  slight  nervous  differences 
such  as  in  human  beings  cause  an  accidental  action  to  be- 
come "  a  trick,"  it  must  happen  that  if  an  advantage  accrues 
from  the  habitual  detention  of  air-bubbles,  those  individuals 
most  apt  to  detain  them  will,  other  things  equal,  be  more 
likely  than  the  rest  to  survive;  and  by  the  survival  of 
descendants  inheriting  their  peculiarities  in  the  greatest  de- 
grees, and  increasing  them,  an  established  structure  and  an 
established  habit  may  arise.  And  that  they  do  in  some 
way  arise  we  have  proof.  The  common  Loach  swallows  air, 
which  it  afterwards  discharges  loaded  with  carbonic  acid. 

From  air  thus  swallowed  the  advantages  that  may  be 
derived  are  of  two  kinds.  In  the  first  place,  the  fish  is  made 
specifically  lighter,  and  the  muscular  effort  needed  to  keep  it 
from  sinking  is  diminished — or,  indeed,  if  the  bubble  is  of 
the  right  size,  is  altogether  saved.  The  contrast  between  the 
movements  of  a  Goby,  which,  after  swimming  up  towards  the 
surface,  falls  rapidly  to  the  bottom  on  ceasing  its  exertions, 
and  the  movements  of  a  Trout,  which  remains  suspended  just 


336  PHYSIOLOGICAL  DEVELOPMENT. 

balancing  itself  by  slight  undulations  of  its  fins,  shows  how 
great  an  economy  results  from  an  internal  float,  to  fishes 
which  seek  their  food  in  mid-water  or  at  the  surface.  Hence 
the  habit  of  swallowing  air  having  been  initiated  in  the  way 
described,  we  see  why  natural  selection  will,  in  certain  fishes, 
aid  modifications  of  the  alimentary  canal  favouring  its 
lodgment — modifications  constituting  air-sacs.  In 

the  second  place,  while  from  air  thus  lodged  in  air-sacs  thus 
developed,  the  advantage  will  be  that  of  flotation  only  if  the 
air  is  infrequently  changed  or  never  changed,  the  advantage 
will  be  that  of  supplementary  respiration  if  the  air-sacs  are 
from  time  to  time  partially  emptied  and  refilled.  The  re- 
quirements of  the  animal  will  determine  which  of  the  two 
functions  predominates.  Let  us  glance  at  the  different  sets 
of  conditions  under  which  these  divergent  modifications  may 
be  expected  to  arise. 

The  respiratory  development  is  not  likely  to  take  place  in 
fishes  that  inhabit  seas  or  rivers  in  which  the  supply  of 
aerated  water  never  fails:  there  is  no  obvious  reason  why 
the  established  branchial  respiration  should  be  replaced  by  a 
pulmonic  respiration.  Indeed,  if  a  fish's  branchial  respiration 
is  adequate  to  its  needs,  a  loss  would  result  from  the  effort  of 
coming  to  the  surface  for  air;  especially  during  those  first 
stages  of  pulmonic  development  when  the  extra  aeration 
achieved  was  but  small.  Hence  in  fishes  so  circumstanced, 
the  air-chambers  arising  in  the  way  described  would  naturally 
become  specialized  mainly  or  wholly  into  floats.  Their  con- 
tained air  being  infrequently  changed,  no  advantage  would 
arise  from  the  development  of  vascular  plexuses  over  their 
surfaces ;  nothing  would  be  gained  by  keeping  open  the  com- 
munication between  them  and  the  alimentary  canal;  and 
there  might  thus  eventually  result  closed  chambers  the 
gaseous  contents  of  which,  instead  of  being  obtained  from 
without,  were  secreted  from  their  walls,  as  gases  often  are 
from  mucous  membranes.  Contrariwise,  aquatic 

vertebrates  in  which  th<3  swallowing  of  air-bubbles,  becoming 


THE  INNER  TISSUES  OP  ANIMALS.  337 

habitual,  had  led  to  the  formation  of  sacs  that  lodged  the 
bubbles;  and  which  continued  to  inhabit  waters  not  always 
supplying  them  with  sufficient  oxygen,  might  be  expected  to 
have  the  sacs  further  developed,  and  the  practice  of  chang- 
ing the  contained  air  made  regular,  if  either  of  two  advan- 
tages resulted — either  the  advantage  of  being  able  to  live  in 
old  habitats  that  had  become  untenable  without  this  modifi- 
cation, or  the  advantage  of  being  able  to  occupy  new  habitats. 
Now  it  is  just  where  these  advantages  are  gained  that  we 
see  the  pulmonic  respiration  coming  in  aid  of  the  branchial 
respiration,  and  in  various  degrees  replacing  it.  Shallow 
•waters  are  liable  to  three  changes  which  conspire  to  make 
this  supplementary  respiration  beneficial.  The  summer's  sun 
heats  them,  and  raising  the  temperatures  of  the  animals  they 
contain,  accelerates  the  circulation  in  these  animals,  exalts 
their  functional  activities,  increases  the  production  of  car- 
bonic acid,  and  thus  makes  aeration  of  the  blood  more  need- 
ful than  usual.  Meanwhile  the  heated  water,  instead  of 
yielding  to  the  highly  carbonized  blood  brought  to  the 
branchiae  the  usual  quantity  of  oxygen,  yields  less  than 
usual ;  for  as  the  heat  of  the  water  increases,  the  quantity  of 
air  it  contains  diminishes.  And  this  greater  demand  for 
oxygen  joined  with  smaller  supply,  pushed  to  an  extreme 
where  the  water  is  nearly  all  evaporated,  is  at  last  still  more 
intensely  felt  in  consequence  of  the  excess  of  carbonic  acid 
discharged  by  the  numerous  creatures  congregated  in  the 
muddy  puddles  that  remain.  Here,  then,  it  is,  that  the  habit 
of  taking  in  air-bubbles  is  likely  to  become  established,  and 
the  organs  for  utilizing  them  developed;  and  here  it  is,  ac- 
cordingly, that  we  find  all  stages  of  the  transition  to  aerial 
respiration.  The  Loach  before-mentioned,  which  swallows 
air,  frequents  small  waters  liable  to  be  considerably  warmed. 
The  Amphipnous  Cuchia,  an  anomalous  eel-shaped  fish,  which 
has  vascular  air-sacs  opening  out  at  the  back  of  the  mouth, 
"  is  generally  found  lurking  in  holes  and  crevices,  on  the 
muddy  banks  of  marshes  or  slow-moving  rivers " ;  and 


338  PHYSIOLOGICAL  DEVELOPMENT. 

though  its  air-sacs  are  not  morphological  equivalents  of  those 
above  described,  yet  they  equally  well  illustrate  the  relation 
between  such  organs  and  the  environing  condition.  Still 
more  significant  is  the  fact  that  the  Lepidosiren,  or  "  mud- 
fish "  as  it  is  called  from  its  habits,  though  it  is  a  true  fish 
nevertheless  has  lungs.  But  it  is  among  the  Amphibia  that 
we  see  most  conspicuously  this  relation  between  the  develop- 
ment of  air-breathing  organs,  and  the  peculiarities  of  the 
habitats.  Pools,  more  or  less  dissipated  annually,  and  so 
rendered  uninhabitable  by  most  fishes,  are  very  generally 
peopled  by  these  transitional  types.  Just  as  we  see,  too, 
that  in  various  climates  and  in  various  kinds  of  shallow 
waters,  the  supplementary  aerial  respiration  is  needful  in  dif- 
ferent degrees;  so  do  we  find  among  the  Amphibia  many 
stages  in  the  substitution  of  the  one  respiration  for  the  other. 
The  facts,  then,  are  such  as  give  to  the  hypothesis  a  vrai- 
semblance  greater  than  could  have  been  expected. 

The  relative  effects  of  direct  and  indirect  equilibration  in 
establishing  this  further  heterogeneity,  must,  as  in  many 
other  cases,  remain  undecided.  The  habit  of  taking  in  bub- 
bles is  scarcely  interpretable  as  a  result  of  spontaneous  varia- 
tion: we  must  regard  it  as  arising  accidentally  during  the 
effort  to  obtain  the  most  aerated  water;  as  being  persevered 
in  because  of  the  relief  obtained ;  and  as  growing  by  repetition 
into  a  tendency  bequeathed  to  offspring,  and  by  them,  or 
some  of  them,  increased  and  transmitted.  The  formation  of 
the  first  slight  modifications  of  the  alimentary  canal  favour- 
ing the  lodgment  of  bubbles,  is  not  to  be  thus  explained.  Some 
favourable  variation  in  the  shape  of  the  passage  must  here 
have  been  the  initial  step.  '  But  the  gradual  increase  of  this 
structural  modification  by  the  survival  of  individuals  in 
which  it  is  carried  furthest,  will,  I  think,  be  all  along  aided 
by  immediate  adaptation.  The  part  of  the  alimentary  canal 
previously  kept  from  the  air,  but  now  habitually  in  contact 
with  the  air,  must  be  in  some  degree  modified  by  the  action 
of  the  air ;  and  the  directly-produced  modification,  increasing 


THE  INNER  TISSUES  OF  ANIMALS.  339 

in  the  individual  and  in  successive  individuals,  cannot  cease 
until  there  is  a  complete  balance  between  the  actions  of  the 
changed  agency  and  the  changed  tissue. 

§  300.  We  come  now  to  differentiations  among  the  truly 
inner  tissues — the  tissues  which  have  direct  converse  neither 
with  the  environment  nor  with  the  foreign  substances  taken 
into  the  organism  from  the  environment.  These,  speaking 
broadly,  are  the  tissues  which  lie  between  the  double  layer 
forming  the  integument  with  its  appendages,  and  the  double 
layer  forming  the  alimentary  canal  with  its  diverticula.  We 
will  take  first  the  differentiation  which  produces  the  vascular 
system. 

Certain  forces  producing  and  aiding  distribution  of  liquids 
in  animals,  come  into  play  before  any  vascular  system  exists ; 
and  continue  to  further  circulation  after  the  development  of 
a  vascular  system.  The  first  of  these  is  osmotic  exchange, 
acting  locally  and  having  an  indirect  general  action;  the 
second  is  local  variation  of  pressure,  which  movement  of  the 
body  throws  on  the  tissues  and  their  contained  liquids.  A 
few  words  are  needed  in  elucidation  of  each.  If  in 

any  creature,  however  simple,  different  changes  are  going  on 
in  parts  that  are  differently  conditioned — if,  as  in  a  Hydra, 
one  surface  is  exposed  to  the  surrounding  medium  while  the 
other  surface  is  exposed  to  dissolved  food;  then  between  the 
unlike  liquids  which  the  dissimilarly-placed  parts  contain, 
osmotic  currents  must  arise;  and  a  movement  of  liquid 
through  the  intermediate  tissue  must  go  on  as  long  as  an 
unlikeness  between  the  liquids  is  kept  up.  This  primary 
cause  of  re-distribution  remains  one  of  the  causes  of  re-dis- 
tribution in  every  more-developed  organism:  the  passage  of 
matters  into  and  out  of  the  capillaries  is  everywhere  thus 
set  up.  And  obviously  in  producing  these  local  currents, 
osmose  must  also  indirectly  produce  general  currents,  or  aid 
them  if  otherwise  produced.  In  the  absence  of  a  pumping 
organ,  this  force  is  probably  an  important  aid  to  that 


340  PHYSIOLOGICAL  DEVELOPMENT. 

movement  of  the  nutritive  liquids  which  the  functions  set 
up.  How  the  second  cause — the  changes  of  internal 

pressure  which  an  animal's  movements  produce — furthers 
circulation,  will  be  sufficiently  manifest.  That  parts  which 
are  bent  or  strained  necessarily  have  their  contained  vessels 
squeezed,  has  been  shown  (§  281)  ;  and  whether  the  bend  or 
strain  is  caused,  as  in  a  plant,  by  an  external  force,  or,  as 
usually  in  an  animal,  by  an  internal  force,  there  must  be  a 
thrusting  of  liquids  towards  places  of  least  resistance — com- 
monly places  of  greatest  consumption.  This  which  in  animals 
without  hearts  is  a  main  agent  of  circulation,  continues  to 
further  it  very  considerably  even  among  the  highest  animals. 
In  these  the  effect  becomes  as  it  were  systematized.  The  valves 
in  the  veins  necessitate  perpetual  propulsions  towards  the 
heart. 

Even  in  such  simple  types  as  the  Hydrozoa,  cavities  in  the 
tissues  faintly  indicate  a  structure  which  facilitates  the  trans- 
fer of  nutritive  matters.  ,  These  cavities  become  reservoirs 
filled  with  the  plasma  that  slowly  oozes  through  the  substance 
of  the  body ;  and  every  movement  of  the  animal,  accompanied 
as  it  must  be  by  changed  pressures  and  tensions  on  these 
reservoirs,  tends  here  to  fill  them  and  there  to  squeeze  out 
their  contents  in  that  or  the  other  direction — possibly  aiding 
to  produce,  by  union  of  several  cavities,  those  lacunae  or 
irregular  canals  which  the  body  in  some  cases  presents. 

Irregular  canals  of  this  kind,  not  lined  with  any  mem- 
branes but  being  simply  cavities  running  through  the  flesh, 
mainly  constitute  the  vascular  system  in  Polyzoa  and  Brach- 
iopoda  and  some  Mollusca.  Though  the  central  parts  of  a 
vascular  system  are  rudely  developed,  yet  its  peripheral  parts 
consist  of  sinuses  permeating  the  tissues.  The  higher  orders 
of  Mollusca  have  a  more  developed  system  of  vessels  or 
arteries,  which  run  into  the  substance  of  the  body  and  end 
in  lacunae  or  simple  fissures.  This  ending  in  lacunae  takes 
place  at  various  distances  from  the  vascular  centre.  In  some 
genera  the  arterial  structure  is  carried  to  the  periphery  of 
the  blood-system,  while  in  others  it  stops  short  midway. 


THE  INNER  TISSUES  OF  ANIMALS.  341 

Throughout  most  orders  of  the  Mollusca  the  back  current  of 
blood  continues  to  be  carried  by  channels  of  the  original 
kind:  there  are  no  true  veins,  but  the  blood  having  been 
delivered  into  the  tissues,  finds  its  way  back  to  the  peri- 
visceral  cavity  through  inosculating  sinuses.  Among  the 
Cephalopods,  however,  the  afferent  blood-canals,  as  well  as 
the  efferent  ones,  acquire  distinct  walls.  On  putting 

together  these  facts,  we  may  conceive  pretty  clearly  the 
stages  of  vascular  development.  From  the  original  reservoir 
of  nutritive  liquid  between  the  alimentary  canal  and  the 
wall  of  the  body,  a  portion  partially  shut  off  becomes  a  con- 
tractile vessel;  and  by  its  actions  there  is  produced  a  more 
rapid  transfer  of  the  nutritive  liquid  than  was  originally 
produced  by  the  motions  of  the  animal.  Clearly,  the  exten- 
sion of  this  contractile  tube  and  the  development  from  it  of 
branches  running  hither  and  thither  into  the  tissues,  must, 
by  denning  the  channels  of  blood  throughout  a  part  of  its 
course,  render  its  distribution  more  regular  and  active.  As 
fast  as  this  centrifugal  growth  advances,  so  fast  are  the  effer- 
ent currents  of  blood,  prevented  from  escaping  laterally, 
obliged  to  move  from  the  centre  towards  the  circumference; 
and  so  fast  also  does  the  less-developed  set  of  channels  become, 
of  necessity,  occupied  by  afferent  currents.  When,  by  a  paral- 
lel increase  of  defmiteness,  the  lacunae  and  irregular  sinuses 
through  which  the  afferent  currents  pass,  become  trans- 
formed into  veins,  the  accompanying  disappearance  of  all 
stagnant  or  slow-moving  collections  of  blood,  implies  a  fur- 
ther improvement  in  the  circulation. 

By  what  agency  is  effected  this  differentiation  of  a  definite 
vascular  system?  No  sufficient  reply  is  obvious.  The 
genesis  of  the  primordial  heart  is  not  comprehensible  as  a 
result  of  direct  equilibration,  and  we  cannot  readily  see  our 
way  to  it  as  a  result  of  indirect  equilibration ;  for  it  is  diffi- 
cult to  imagine  what  favourable  variation  natural  selection 
could  have  seized  hold  of  to  produce  such  a  structure.  A 
contractile  tube  that  aided  the  distribution  of  nutritive 


342  PHYSIOLOGICAL  DEVELOPMENT. 

liquid,  having  been  once  established,  survival  of  the  fittest 
would  suffice  for  its  gradual  extension  and  its  successive 
modifications.  But  what  were  the  early  stages  of  the  con- 
tractile tube,  while  it  was  yet  not  sufficiently  formed  to  help 
circulation,  and  while  it  must  nevertheless  have  had  some 
advantage  without  which  no  selective  process  could  go  on? 
The  question  seems  insoluble.  To  another  part  of 

the  question,  however,  an  answer  may  be  ventured.  If  we 
ask  the  origin  of  these  ramifying  channels  which,  first 
appearing  as  simple  lacunae,  eventually  become  vessels 
having  definite  walls,  a  reply  admitting  of  considerable 
justification,  is,  that  the  currents  of  nutritive  liquid  forced 
and  drawn  hither  and  thither  through  the  tissues,  themselves 
initiate  these  channels.  We  know  that  streams  running 
over  and  through  solid  and  quasi-solid  inorganic  matter, 
tend  to  excavate  definite  courses.  We  saw  reason  for  con- 
cluding that  the  development  of  sap-channels  in  plants 
conforms  to  this  general  principle.  May  we  not  then 
suspect  that  the  nutritive  liquid  contained  in  the  tissue 
of  a  simple  animal,  made  to  ooze  now  in  this  direction  and 
now  in  that  by  the  changes  of  pressure  which  the  animal's 
movements  cause,  comes  to  have  certain  lines  along  which  it 
is  thrust  backwards  and  forwards  more  than  along  other 
lines;  and  must  by  repeated  passings  make  these  more  and 
more  permeable  until  they  become  lacuna?  Such  actions 
will  inevitably  go  on;  and  such  actions  appear  competent  to 
produce  some,  at  least,  of  the  observed  effects.  The  leading 
facts  which  indicate  that  this  is  a  part  cause  of  vascular 
development  are  these. 

Growths  normally  recurring  in  certain  places  at  certain 
intervals,  are  accompanied  by  local  formations  of  blood-ves- 
sels. The  periodic  maturation  of  ova  among  the  Mammalia 
supplies  an  instance.  Through  the  stroma  of  an  ovarium  are 
distributed  innumerable  minute  vesicles,  which,  in  their  early 
stages,  are  microscopic.  Of  these,  severally  contained  in  their 
minute  ovi-sacs,  any  one  may  develop:  the  determining 


THE  INNER  TISSUES  OF  ANIMALS.  343 

cause  being  probably  some  slight  excess  of  nutrition.  When 
the  development  is  becoming  rapid,  the  capillaries  of  the 
neighbouring  stroma  increase  and  form  a  plexus  on  the  walls 
of  the  ovi-sac.  N"ow  since  there  is  no  typical  distribution  of 
the  developing  ova;  and  since  the  increase  of  an  ovum  to 
a  certain  size  precedes  the  increase  of  vascularity  round  it; 
we  can  scarcely  help  concluding  that  the  setting  up  of  cur- 
rents towards  the  point  of  growth  determines  the  forma- 
tion of  the  blood-vessels.  It  may  be  that  having  once  com- 
menced, this  local  vascular  structure  completes  itself  in  a 
typical  manner;  but  it  seems  clear  that  this  greater  develop- 
ment of  blood-vessels  around  the  growing  ovum  is  initiated 
by  the  draught  towards  it.  Abnormal  growths  show 

still  better  this  relation  of  cause  and  effect.  The  false  mem- 
branes sometimes  found  in  the  bronchial  tubes  in  inflam- 
matory diseases,  may  perhaps  fairly  be  held  abnormal  in  but 
a  partial  sense:  it  may  be  said  that  their  vascular  systems 
are  formed  after  the  type  of  the  membranes  to  which  they  are 
akin.  But  this  can  scarcely  be  said  of  the  morbid  growths 
classed  as  malignant.  The  blood-vessels  in  an  encephaloid 
cancer,  are  led  to  enlarge  and  ramify,  often  to  an  immense 
extent,  by  the  unfolding  of  the  morbid  mass  to  which  they 
carry  blood.  Alien  as  is  the  structure  as  a  whole  to  the  type 
of  the  organism ;  and  alien  in  great  measure  as  is  its  tissue 
to  the  tissue  on  which  it  is  seated;  it  nevertheless  happens 
that  the  growth  of  the  alien  tissue  and  accompanying  ab- 
straction of  materials  from  the  blood-vessels,  determine  a 
corresponding  growth  of  these  blood-vessels.  Unless,  then, 
we  say  that  there  is  a  providentially-created  type  of  vascular 
structure  for  each  kind  of  morbid  growth  (and  even  this 
would  not  much  help  us,  since  the  vascular  structure  has 
no  constancy  within  the  limits  of  each  kind),  we  are  com- 
pelled to  admit  that  in  some  way  or  other  the  currents  of 
blood  are  here  directly  instrumental  in  forming  their  own 
channels.  One  more  piece  of  evidence,  before  cited 

as  exemplifying  adaptation  (§67),  may  be  called  to  mind. 


344  PHYSIOLOGICAL  DEVELOPMENT. 

When  any  main  channel  for  blood,  leading  to  or  from  a 
certain  part  of  the  body,  has  been  rendered  impervious, 
others  among  the  channels  leading  to  or  from  this  same  part, 
enlarge  to  the  extent  requisite  for  fulfilling  the  extra  func- 
tion that  falls  upon  them:  the  enlargement  being  caused,  as 
we  must  infer,  by  the  increase  of  the  currents  carried. 

Here,  then,  are  facts  warranting  inductively  the  deduction 
above  drawn.  It  is  true  that  we  are  left  in  the  dark  respecting 
the  complexities  of  the  process.  How  the  channels  for  blood 
come  to  have  limiting  membranes,  and  many  of  them  mus- 
cular coats,  the  hypothesis  does  not  help  us  to  say.  But  the 
evidence  assigned  goes  far  to  warrant  the  belief  that  vascular 
development  is  initiated  by  direct  equilibration;  though  in- 
direct equilibration  may  have  had  the  larger  share  in  establish- 
ing the  structures  which  distinguish  finished  vascular  systems. 

§  301.  Of  the  inner  tissues  which  remain  let  us  next  take 
bone.  In  what  manner  is  differentiated  this  dense  substance 
serving  in  most  cases  for  internal  support? 

When  considering  the  vertebrate  skeleton  under  its 
morphological  aspect  (§256),  it  was  pointed  out  that  the 
formation  of  dense  tissues,  internal  as  well  as  external,  is,  in 
some  cases  at  least,  brought  about  by  the  mechanical  forces 
to  be  resisted.  Through  what  process  it  is  brought  about  we 
could  not  then  stay  to  inquire:  this  question  being  not 
morphological  but  physiological.  Answers  to  some  kindred 
questions  have  since  been  attempted.  Certain  actions  to 
which  the  internal  dense  tissues  of  plants  may  be  ascribed, 
have  been  indicated;  and  more  recently,  analogous  actions 
have  been  assigned  as  causes  of  some  external  dense  tissues 
of  animals.  We  have  now  to  ask  whether  actions  of  the 
same  nature  have  produced  these  internal  dense  tissues  of 
animals. 

The  problem  is  an  involved  one.  Bones  have  more  than 
one  stage.  They  are  membranous  or  cartilaginous  before  they 
become  osseous;  and  their  successive  component  substances 


THE  INNER  TISSUES  OF  ANIMALS.  345 

BO  far  differ  that  the  effects  of  mechanical  actions  upon  them 
differ.  And  having  to  deal  with  transitional  states  in  which 
bone  is  formed  of  mixed  tissues,  having  unlike  physical 
properties  and  unlike  minute  structures,  the  effects  of 
strains  become  too  complicated  to  follow  with  precision. 
Anything  in  the  way  of  interpretation  must  therefore  be 
regarded  as  tentative.  If  analysis  and  comparison  show  that 
the  phenomena  are  not  inconsistent  with  the  hypothesis  of 
mechanical  genesis,  it  is  as  much  as  can  be  expected.  Let  us 
first  observe  more  nearly  the  mechanical  conditions  to  which 
bones  are  subject. 

The  endo-skeleton  of  a  mammal  with  the  muscles  and 
ligaments  holding  it  together,  may  be  rudely  compared  to  a 
structure  built  up  of  struts  and  ties;  of  which,  speaking 
generally,  the  struts  bear  the  pressures  and  the  ties  bear  the 
tensions.  The  framework  of  an  ordinary  iron  roof  will  give 
an  idea  of  the  functions  of  these  two  elements,  and  of  the 
mechanical  characters  required  by  them.  Such  a  framework 
consists  partly  of  pieces  which  have  each  to  bear  a  thrust  in 
the  direction  of  its  length,  and  partly  of  pieces  which  have 
each  to  bear  a  pull  in  the  direction  of  its  length;  and  these 
struts  and  ties  are  differently  formed  to  adapt  them  to  these 
different  strains.  Further,  it  should  be  remarked  that  though 
the  rigidity  of  the  framework  depends  on  the  ties  which  are 
flexible,  as  much  as  on  the  struts  which  are  stiff,  yet  the  ties 
help  to  give  the  rigidity  simply  by  so  holding  the  struts  in 
position  that  they  cannot  escape  from  the  thrusts  which  fall 
on  them.  Now  the  like  relation  holds  with  a  difference 
among  the  bones  and  muscles :  the  difference  being  that  here 
the  ties  admit  of  being  lengthened  or  shortened  and  the 
struts  of  being  moved  about  upon  their  joints.  The  mecha- 
nical relations  are  not  altered  by  this,  however.  The  actions 
are  of  essentially  the  same  kind  in  an  animal  that  is  stand- 
ing, or  keeping  itself  in  a  strained  attitude,  as  in  one  that 
is  changing  its  attitude — the  same  in  so  far  that  we  have 
in  each  a  set  of  flexible  parts  that  are  pulling  and  a  set  of 


34:6  PHYSIOLOGICAL  DEVELOPMENT. 

rigid  parts  that  are  resisting.  It  needs  but  to  remember  the 
'sudden  collapse  and  fall  which  take  place  when  the  muscles 
are  paralyzed,  or  to  remember  the  inability  of  a  bare  skeleton 
to  support  itself,  to  see  that  the  struts  without  the  ties  can- 
not suffice.  And  we  have  but  to  think  of  the  formless  mass 
into  which  a  man  would  sink  when  deprived  of  his  bones,  to 
see  that  the  ties  without  the  struts  cannot  suffice.  To  trace 
the  way  in  which  a  particular  bone  has  its  particular  thrust 
thrown  upon  it,  may  not  always  be  practicable.  Though  it 
is  easy  to  perceive  how  a  flexor  or  extensor  of  the  arm  causes 
by  its  tension  a  reactive  pressure  along  the  line  of  the 
humerus,  and  is  enabled  to  produce  its  effect  only  by  the 
rigidity  of  the  humerus ;  yet  it  is  not  so  easy  to  perceive  how 
such  bones  as  those  of  a  horse's  pelvis  are  similarly  acted 
upon.  Still,  as  the  weight  of  the  hind  quarters  has  to  be 
transferred  from  the  back  to  the  feet,  and  must  be  so  trans- 
ferred through  the  bones,  it  is  manifest  that  though  these 
bones  form  a  very  crooked  line,  the  weight  must  produce  a 
pressure  along  the  axis  of  each:  the  muscles  and  ligaments 
concerned  serving  here,  as  in  other  cases,  so  to  hold  the 
bones  that  they  bear  the  pressure  instead  of  being  displaced 
by  it.  Not  forgetting  that  many  processes  of  the  bones  have 
to  bear  tensions,  we  may  then  say  that  generally,  though  by 
no  means  universally,  bones  are  internal  dense  masses  that 
have  to  bear  pressures — pressures  which  in  the  cylindrical 
bones  become  longitudinal  thrusts.  Leaving  out  exceptional 
cases,  let  us  consider  bones  as  masses  thus  circumstanced. 

When  giving  reasons  for  the  belief  that  the  vertebrate 
skeleton  is  mechanically  originated,  one  of  the  facts  put  in 
evidence  was,  that  in  the  vertebrate  series  the  transition  from 
the  cartilaginous  to  the  osseous  spine  begins  peripherally 
(§  257)  :  each  vertebra  being  at  first  a  ring  of  bone  sur- 
rounding a  mass  of  cartilage.  And  it  was  pointed  out  that 
this  peripheral  ossification  is  ossification  at  the  region  of 
greatest  pressures.  Now  it  is  not  vertebra  only  that  follow 
this  course  of  development.  In  a  cylindrical  bone,  though 


THE  INNER  TISSUES  OF  ANIMALS.  347 

it  is  differently  circumstanced,  the  places  of  commencing  ossi- 
fication are  still  the  places  on  which  the  severest  stress  falls. 
Let  us  consider  how  such  a  bone  that  has  to  bear  a  longitu- 
dinal pressure  is  mechanically  affected.  If  the  end 
of  a  walking-cane  be  thrust  with  force  against  the  ground,  the 
cane  bends ;  and  partially  resuming  its  straightness  when  re- 
lieved, again  bends,  usually  towards  the  same  side,  when  the 
thrust  is  renewed.  A  bend  so  caused  acts  on  the  fibres  of  the 
cane  in  nearly  the  same  way  as  does  a  bend  caused  by  sup- 
porting the  cane  horizontally  at  its  two  ends  and  suspending 
a  weight  from  its  middle.  In  either  case  the  fibres  on  the 
convex  side  are  extended  and  the  fibres  on  the  concave  side 
compressed.  Kindred  actions  occur  in  a  rod  that  is  so  thick 
as  not  to  yield  visibly  under  the  force  applied.  In  the  absence 
of  complete  homogeneity  of  its  substance,  complete  symmetry 
in  its  form,  and  an  application  of  a  force  exactly  along  its 
axis,  there  must  be  some  lateral  deflection;  and  therefore 
some  distribution  of  tensions  and  pressures  of  the  kind  indi- 
cated. And  then,  as  the  fact  which  here  specially  concerns  us, 
we  have  to  note  that  the  strongest  tensions  and  pressures  are 
borne  by  the  outer  layers  of  fibres.  Now  the  shaft  of  a  long 
bone,  subject  to  mechanical  actions  of  this  kind,  similarly  has 
its  outer  layer  most  strained.  In  this  layer,  therefore,  on  the 
mechanical  hypothesis,  ossification  should  commence,  and  here 
it  does  commence — commences,  too,  midway  between  the  ends, 
where  the  bends  produce  on  the  superficial  parts  their  most 
intense  effects.  But  we  have  not  in  this  place  simply 
to  observe  that  ossification  commences  at  the  places  of  greatest 
stress,  but  to  ask  what  causes  it  to  do  this.  Can  we  trace  the 
physical  actions  which  set  up  this  deposit  of  dense  tissue  ?  It 
is,  I  think,  possible  to  indicate  a  "true  cause"  that  is  at  work; 
though  whether  it  is  a  sufficient  cause  may  be  questioned. 
We  concluded  that  in  certain  other  cases,  the  formation  of 
dense  tissue  indirectly  results  from  the  alternate  squeezing 
and  relaxation  of  the  vessels  running  through  the  part;  and 
the  inquiry  now  to  be  made  is,  whether,  in  developing  bone, 


348  PHYSIOLOGICAL  DEVELOPMENT. 

the  same  actions  go  on  in  such  ways  as  to  produce  the  ob- 
served effects.  At  the  outset  we  are  met  by  what  seems  a 
fatal  difficulty — cartilage  is  a  non- vascular  tissue:  this  sub- 
stance of  which  unossified  bones  consist  is  not  permeated 
by  minute  canals  carrying  nutritive  liquid,  and  cannot, 
therefore,  be  a  seat  of  actions  such  as  those  assigned. 
This  apparent  difficulty,  however,  furnishes  a  confirmation. 
For  cartilage  that  is  wholly  without  permeating  canals  does 
not  ossify:  ossification  takes  place  only  at  those  parts  of 
it  into  which  the  canals  penetrate.  Hence,  we  get  ad- 
ditional reason  for  suspecting  that  bone-formation  is  due 
to  the  alleged  cause;  since  it  occurs  where  mechanical 
strains  can  produce  the  actions  described,  but  does  not  occur 
where  mechanical  strains  cannot  produce  them.  Let  us 
consider  more  closely  what  the  several  factors  are.  It  will 
suffice  for  the  argument  if  we  commence  with  the  external 
vascular  layer  as  already  existing,  and  consider  what 
will  take  place  in  it.  Cartilage  is  elastic — is  some- 

what extensible,  and  spreads  out  laterally  under  pressure, 
but  resumes  its  form  when  relieved.  How,  then,  will  the 
minute  channels  traversing  it  in  all  directions  be  affected  at 
the  places  where  it  is  strained  by  a  bend?  Those  on  the 
convex  side  will  be  laterally  squeezed,  in  the  same  way  that 
we  saw  the  sap-vessels  on  the  convex  side  of  a  bent  branch 
are  squeezed;  and  as  exudation  of  the  sap  into  the  adjacent 
prosenchyma  will  be  caused  in  the  one  case,  so,  in  the  other, 
there  will  be  caused  exudation  of  serum  into  the  adjacent 
cartilage:  extra  nutrition  and  increase  of  strength  resulting 
in  both  cases.  The  parallel  ceases  here,  however.  In  the 
shoot  of  a  plant,  bent  in  various  directions  by  the  wind,  the 
side  which  was  lately  compressed  is  now  extended;  and 
hence  that  squeezing  of  the  sap-vessels  which  results  from 
extension,  suffices  to  feed  and  harden  the  tissue  on  all  sides 
of  the  shoot.  But  it  is  not  so  with  a  bone.  Having  yielded 
on  one  side  under  longitudinal  pressure,  and  resumed  as 
nearly  as  may  be  its  previous  shape  when  the  pressure  is 


THE  INNER  TISSUES  OF  ANIMALS.  349 

taken  off,  the  bone  yields  again  towards  the  same  side  when 
again  longitudinally  pressed.  Hence  the  substance  of  its 
concave  side,  never  rendered  convex  by  a  bend  in  the  oppo- 
site direction,  would  not  receive  any  extra  nutrition  did  no 
other  action  come  into  play.  But  if  we  consider  how  inter- 
mittent pressures  must  act  on  cartilage,  we  shall  see  that 
there  will  result  extra  nutrition  of  the  concave  side  also. 
Squeeze  between  two  pieces  of  glass  a  thin  bit  of  caoutchouc 
which  has  a  hole  through  it.  While  the  caoutchouc  spreads 
out  away  from  the  centre,  it  also  spreads  inwards,  so  as 
partially  to  close  the  hole.  Everywhere  its  molecules  move 
away  in  directions  of  least  resistance;  and  for  those  near 
the  hole,  the  direction  of  least  resistance  is  towards  the  hole. 
Let  this  hole  stand  for  the  transverse  section  of  one  of  the 
minute  canals  or  channels  passing  through  cartilage,  and  it 
will  be  manifest  that  on  the  side  of  the  unossified  bone  made 
concave  in  the  way  described,  the  compressed  cartilage  will 
squeeze  the  canals  traversing  it;  and,  in  the  absence  of 
perfect  homogeneity  in  the  cartilage,  the  squeeze  will  cause 
extra  exudation  from  the  canals  into  the  cartilage.  Thus 
every  additional  strain  will  give  to  the  cartilage  it  falls  upon, 
an  additional  supply  of  the  materials  for  growth.  So  that 
presently  the  side  which,  by  yielding  more  than  any  other, 
proves  itself  to  be  the  weakest,  will  cease  to  be  the  weakest. 
What  further  will  happen?  Some  other  side  will  yield  a 
little — the  bends  will  take  place  in  some  other  plane;  and 
the  portions  of  cartilage  on  which  repeated  tensions  and 
pressures  now  fall  will  be  strengthened.  Thus  the  rate  of 
nutrition,  greatest  at  the  place  where  the  bending  is  greatest, 
and  changing  as  the  incidence  of  forces  changes,  will  bring 
about  at  every  point  a  balance  between  the  resistances  and 
the  strains.  Thus,  too,  there  will  be  determined  that  peri- 
pheral induration  which  we  see  in  bones  so  circumstanced. 
As  in  a  shoot  we  saw  that  the  woody  deposit  takes  place 
towards  the  outside  of  the  cylinder,  where,  according  to  the 
hypothesis,  it  ought  to  take  place;  so,  here,  we  see  that  the 


350  PHYSIOLOGICAL  DEVELOPMENT. 

excess  of  exudation  and  hardening,  occurring  where  the 
strains  are  most  intense,  will  form  a  cylinder  having  a  dense 
outside  and  a  porous  or  hollow  inside.  These  pro- 

cesses will  be  essentially  the  same  in  bones  subject  to  more 
complex  mechanical  actions,  such  as  sundry  of  the  flat  bones 
and  others  that  serve  as  internal  fulcra.  Be  the  strains 
transverse  or  longitudinal,  be  they  torsion  strains  or  mixed 
strains,  the  outer  parts  of  the  bone  will  be  more  affected  by 
them  than  its  inner  parts.  They  will  therefore  tend  every- 
where to  produce  resisting  masses  having  outer  parts  more 
dense  than  their  inner  parts.  And  by  causing  most  growth 
where  they  are  most  intense,  they  will  call  out  reactive  forces 
adequate  to  balance  them.  There  are  doubtless 

obstacles  in  the  way  of  this  interpretation.  It  may  be  said 
that  the  forces  acting  on  the  outer  layers  in  the  manner 
described,  would  compress  the  canals  too  little  to  produce 
the  alleged  effects;  and  if  evenly  distributed  along  the  whole 
lengths  of  the  layers,  they  would  probably  do  so.  But  it 
needs  only  to  bend  a  flexible  mass  and  observe  the  tendency 
to  form  creases  on  the  concave  surface,  to  feel  assured  that 
along  the  surface  of  an  ossifying  bone,  the  yielding  of  the 
tissue  when  bent  will  not  be  uniform.  In  the  absence  of 
complete  homogeneity,  the  interstitial  yielding  will  take 
place  at  some  points  more  than  others,  and  at  one  point 
above  all  others.  When,  at  the  weakest  point — the  centre 
of  commencing  ossification — an  extra  amount  of  deposit  has 
been  caused,  it  will  cease  to  be  the  weakest;  and  adjacent 
points,  now  the  weakest,  will  become  the  places  of  yielding 
and  induration.  It  may  be  further  objected  that  the  hypo- 
thesis is  incompatible  with  the  persistence  of  cartilage  for  so 
long  a  time  between  the  epiphysis  of  bones  and  the  bony 
masses  which  they  terminate.  But  there  is  the  reply  that 
the  places  occupied  by  this  cartilage  being  places  at  which 
the  bone  lengthens,  the  non-ossification  is  in  part  apparent 
only — it  is  rather  that  new  cartilage  is  formed  as  fast  as  the 
pre-existing  cartilage  ossifies ;  and  there  is  the  further  reply 


THE  INNER  TISSUES  OF  ANIMALS.  351 

that  the  slowness  of  the  ultimate  ossification  of  this  part,  is 
due  to  its  non-vascularity,  and  to  mechanical  conditions 
which  are  unfavourable  to  its  acquirement  of  vascularity. 
Once  more,  there  is  the  demurrer  that  in  the  epiphyses  ossifi- 
cation does  not  begin  at  the  surface  but  within  the  mass  of 
the  cartilage.  Explanation  of  this  implies  ability  to  follow 
out  the  mechanical  actions  in  a  resilient  substance  which,  like 
india-rubber,  admits  of  being  distorted  in  all  ways  by  pressure 
and  recovering  its  form,  and  it  seems  impossible  to  say  how 
the  more  superficial  and  more  deep-seated  canals  traversing 
it  will  be  respectively  affected. 

Of  course  it  is  not  meant  that  this  osseous  development 
by  direct  equilibration  takes  place  in  the  individual.  Though 
it  is  a  corollary  from  the  argument  that  in  each  individual 
the  process  must  be  furthered  and  modified  by  the  particular 
actions  to  which  the  particular  bones  are  exposed;  yet  the 
leading  traits  of  structure  assumed  by  the  bones  are  assumed 
in  conformity  with  the  inherited  type.  This,  however,  is  no 
difficulty.  The  type  itself  is  to  be  regarded  as  the  accumu- 
lated result  of  such  modifications,  transmitted  and  increased 
from  generation  to  generation.  The  actions  above  described 
as  taking  place  in  the  bone  of  an  individual,  must  be  under- 
stood as  producing  their  total  effect  little  by  little  in  the 
corresponding  bones  of  a  long  series  of  individuals.  Even  if 
but  a  small  modification  can  be  so  wrought  in  the  individual, 
yet  if  such  modification,  or  a  part  of  it,  is  inheritable,  we 
may  readily  understand  how,  in  the  course  of  geologic  epochs, 
the  observed  structures  may  arise  in  the  assigned  way. 

Here  may  fitly  be  added  a  strong  confirmation.  If  we  find 
cases  where  individual  bones,  subject  in  exceptional  degrees 
to  the  actions  described,  present  in  exceptional  amounts  the 
modifications  attributed  to  them,  we  are  greatly  helped  in 
understanding  how  there  may  be  produced  in  the  race  that 
aggregate  of  modifications  which  the  hypothesis  implies. 
Such  cases  occur  in  ricketty  children.  I  am  indebted  to  Mr. 
Busk  for  pointing  out  these  abnormal  formations  of  dense 
tissue,  that  are  not  apparently  explicable  as  results  of 


352  PHYSIOLOGICAL  DEVELOPMENT. 

mechanical  actions  and  re-actions.  It  was  only  on  tracing 
out  the  processes  here  at  work,  that  there  suggested  itself  the 
specific  interpretation  of  the  normal  process,  as  above  set 
forth.  When,  from  constitutional  defect,  bones  do 

not  ossify  with  due  rapidity,  and  are  meanwhile  subject  to 
the  ordinary  strains,  they  become  distorted.  Remembering 
how  a  mass  which  has  been  made  to  yield  in  any  direction 
by  a  force  it  cannot  withstand,  is  some  little  time  before  it 
recovers  completely  its  previous  form,  and  usually,  indeed, 
undergoes  what  is  called  a  "  permanent  set ; "  it  is  inferable 
that  when  a  bone  is  repeatedly  bent  at  the  same  time  that 
the  liquid  contained  in  its  canals  is  poor  in  the  materials 
for  forming  dense  tissue,  there  will  not  take  place  a  propor- 
tionate strengthening  of  the  parts  most  strained;  and  these 
parts  will  give  way.  This  happens  in  rickets.  But  this 
having  happened,  there  goes  on  what,  in  teleological  language, 
we  call  a  remedial  process.  Supposing  the  bone  to  be  one 
commonly  affected — a  femur;  and  supposing  a  permanent 
bend  to  have  been  caused  in  it  by  the  weight  of  the  body; 
the  subsequent  result  is  an  unusual  deposition  of  cartilagin- 
ous and  osseous  matter  on  the  concave  side  of  the  bone.  If 
the  bone  is  represented  by  a  strung  bow,  then  the  deposit 
occurs  at  the  part  represented  by  the  space  between  the  bow 
and  the  string.  And  thus  occurring  where  its  resistance  is 
most  effective,  it  increases  until  the  approximately-straight 
piece  of  bone  formed  within  the  arc,  has  become  strong  enough 
to  bear  the  pressure  without  appreciably  yielding.  Now 

this  direct  adaptation,  seeming  so  like  a  special  provision, 
and  furnishing  so  remarkable  an  instance  of  what,  in  medical 
but  unscientific  language,  is  called  the  vis  medicatrix  natures, 
is  simply  a  result  of  the  above-described  mechanical  actions 
and  re-actions,  going  on  under  the  exceptional  conditions. 
Each  time  such  a  bent  bone  is  subject  to  a  force  which  again 
bends  it,  the  severest  compression  falls  on  the  substance  of 
its  concave  side.  Each  time,  then,  the  canals  running 
through  this  part  of  its  substance  are  violently  squeezed — 


THE  INNER  TISSUES  OP  ANIMALS.  353 

far  more  squeezed  than  they  or  any  other  of  the  canals 
would  have  been,  had  the  bone  remained  straight.  Hence, 
on  every  repetition  of  the  strain,  these  canals  near  the  con- 
cave surface  have  their  contents  forced  out  in  more  than 
normal  abundance.  The  materials  for  the  formation  of  tissue 
are  supplied  in  quantity  greater  than  can  be  assimilated  by 
the  tissue  already  formed;  and  from  the  excess  of  exuded 
plasma,  new  tissue  arises.*  A  layer  of  organizable  material 
accumulates  between  the  concave  surface  and  the  peri- 
osteum; in  this,  according  to  the  ordinary  course  of  tissue- 
growth,  new  vessels  appear;  and  the  added  layer  presently 
assumes  the  histological  character  of  the  layer  from  which 
it  has  grown.  What  next  happens?  This  added  layer, 
further  from  the  neutral  axis  than  that  which  has  thrown 
it  out,  is  now  the  most  severely  compressed,  and  its  vessels 
are  the  most  severely  squeezed.  The  place  of  greatest  exuda- 
tion and  most  rapid  deposit  of  matter,  is  therefore  transferred 
to  this  new  layer ;  and  at  the  same  time  that  active  nutrition 
increases  its  density,  the  excess  of  organizable  material 
forms  another  layer  external  to  it:  the  successive  layers  so 
added,  encroaching  on  the  space  between  the  concave  surface 
of  the  bone  and  the  chord  of  its  arc.  What  limits 

the  encroachment  on  this  space? — what  stops  the  process  of 
filling  it  up?  The  answer  to  this  question  will  be  manifest 
when  observing  that  there  comes  into  play  a  cause  which 
gradually  diminishes  the  forces  falling  on  each  new  layer. 
For  the  transverse  sectional  area  is  step  by  step  increased; 
and  an  increase  of  the  area  over  which  the  weight  borne  is 
distributed,  implies  a  relatively  smaller  pressure  upon  each 
part  of  it.  Further,  as  the  transverse  dimensions  of  the  bone 
increase,  the  materials  composing  its  convex  and  concave 
layers,  becoming  further  from  the  neutral  axis,  become  better 

*  To  this  implied  inference  it  is  objected  that  "excess  of  nutritive  mate- 
rial does  not  necessarily  lead  to  correspondingly  increased  growth."     My  reply 
is  that  a  concomitant  factor  is  activity  of  the  tissue,  and  that  in  its  absence 
growth  is  not  to  be  expected. 
C9 


354:  PHYSIOLOGICAL  DEVELOPMENT. 

placed  for  resisting  the  strains  to  be  borne.  So  that  both  by 
the  increased  quantity  of  dense  matter  and  by  its  mechanically 
more-advantageous  position,  the  bendings  of  the  bone  are 
progressively  decreased.  But  as  they  are  decreased,  each 
new  layer  formed  on  the  concave  surface  has  its  substance 
and  its  vessels  less  compressed;  and  the  resulting  growth 
and  induration  are  rendered  less  rapid.  Evidently,  then,  the 
additions,  slowly  diminishing,  will  eventually  cease;  and  this 
will  happen  when  the  bone  no  longer  bends.  That  is  to  say, 
the  thickening  of  the  bone  will  reach  its  limit  when  there  is 
equilibrium  between  the  incident  forces  and  the  forces  which 
resist  them.  Here,  indeed,  we  may  trace  with  great  clearness 
the  process  of  direct  equilibration — may  see  how  an  unusual 
force,  falling  on  the  moving  equilibrium  of  an  organism  and 
not  overthrowing  it,  goes  on  working  modifications  until  the 
re-action  balances  the  action. 

That,  however,  which  now  chiefly  concerns  us,  is  to  note 
how  this  marked  adaptation  supports  the  general  argument. 
Unquestionably  bone  is  in  this  case  formed  under  the  influ- 
ence of  mechanical  stress,  and  formed  just  where  it  most 
effectually  meets  the  stress.  This  result,  not  otherwise 
explained,  is  explained  by  the  hypothesis  above  set  forth. 
And  when  we  see  that  this  special  deposit  of  bone  is  ac- 
counted for  by  actions  like  those  to  which  bone-formation  in 
general  is  ascribed,  the  probability  that  these  are  the  actions 
at  work  becomes  very  great.* 

*  In  recent  years  (since  1890)  Prof.  Wilhelm  Roux,  in  essays  on  func- 
tional adaptation,  has  set  forth  some  views  akin  to  the  foregoing  in  respect 
to  the  general  belief  they  imply,  though  differing  in  respect  of  the  physio- 
logical processes  ho  indicates.  The  following  relevant  passage  has  been 
translated  for  me  from  an  article  of  his  in  the  Real-Enciidnpadie  der  ge- 
gnmmten  Heilkunde: — "A  more  complete  theory  of  functional  adaptation 
by  the  author  is  founded  on  the  assumption  that  the  '  functional '  stimulus, 
or  '  the  act  of  exercisine  the  function '  (in  muscles  and  glands),  and  espe- 
cially, in  the  case  of  bones,  the  concussion  and  tension  caused  by  stress  and 
strain,  exert  a  '  trophic '  stimulus  on  the  cells,  in  consequence  of  which,  and 
along  with  an  increased  absorption  of  nutriment,  they  grow  and  eventually 
increase  (or  the  osteoblasts  at  the  point  of  greater  stimulus  form  more  bone) ; 
while,  conversely,  with  continued  inactivity,  by  absence  of  these  stimuli  the 


THE  INNER  TISSUES  OF  ANIMALS.  355 

Of  course  it  is  not  alleged  that  osseous  structures  arise  in 
this  way  alone.  The  bones  of  the  skull  and  various  dermal 
bones  cannot  be  thus  interpreted.  Here  the  natural  selec- 
tion of  favourable  variations  appears  the  only  assignable 
cause — the  equilibration  is  indirect.  We  know  that  ossific 
deposits  now  and  then  occur  in  tissues  where  they  are  not 
usually  found;  and  such  deposits,  originally  abnormal,  if 
they  occurred  in  places  where  advantages  arose  from  them, 
might  readily  be  established  and  increased  by  survival  of  the 
fittest.  Especially  might  we  expect  this  to  happen  when  a 
constitutional  tendency  to  form  bone  had  been  established  by 
actions  of  the  kind  described;  for  it  is  a  familiar  fact  that 
differentiated  types  of  tissue,  having  once  become  elements 
of  an  organism,  are  apt  occasionally  to  arise  in  unusual 
places,  and  there  to  repeat  all  their  peculiar  histological 
characters.  And  this  may  possibly  be  the  reason  why  the 
bones  of  the  skull,  though  not  exposed  to  forces  such  as 
those  which  produce,  in  other  bones,  dense  outer  layers  in- 
cluding less  dense  interiors,  nevertheless  repeat  this  general 
trait  of  bony  structure.  While,  however,  it  is  beyond  doubt 
that  some  bones  are  not  due  to  the  direct  influence  of 
mechanical  stress,  we  may,  I  think,  conclude  that  mechanical 
stress  initiates  bone-formation. 

§  302.  What  is  the  origin  of  nerve  ?  In  what  way  do  its 
properties  stand  related  to  the  properties  of  that  protoplasm 
whence  the  tissues  in  general  arise?  and  in  what  way  is  it 
differentiated  from  protoplasm  simultaneously  with  the  other 
tissues?  These  are  profoundly  interesting  questions;  but 
questions  to  which  positive  answers  cannot  be  expected. 
All  that  can  be  done  is  to  indicate  answers  which  seem 
feasible. 

That  the  property  specially  displayed  by  nerve,  is  a  pro- 
nourishment  of  the  cell  declines  so  thnt  the  waste  is  insufficiently  replaced  for 
otherwise  that  the  bone-substance  jrradually  loses  its  power  of  resistance  to 
the  osteoblasts  formed  as  a  result  of  inactivity  "). 


356  PHYSIOLOGICAL  DEVELOPMENT. 

perty  which  protoplasm  possesses  in  a  lower  degree,  is  mani- 
fest. The  sarcode  of  a  Ehizopod  and  the  substance  of  an 
unimpregnated  ovum,  exhibit  movements  that  imply  a  pro- 
pagation of  stimulus  from  one  part  of  the  mass  to  another. 
We  have  not  far  to  seek  for  a  probable  origin  of  this  pheno- 
menon. There  is  good  reason  for  ascribing  it  to  the  extreme 
instability  of  the  organic  colloids  of  which  protoplasm  con- 
sists. These,  in  common  with  colloids  in  general,  assume  dif- 
ferent isomeric  forms  with  great  facility ;  and  they  display  not 
simply  isomerism  but  polymerism.  Further,  this  readiness 
to  undergo  molecular  re-arrangement,  habitually  shows  itself 
in  colloids  by  the  rapid  propagation  of  the  re-arrangement 
from  part  to  part.  As  Prof.  Graham  has  shown,  matter  in 
this  state  often  "  pectizes  "  almost  instantaneously — a  touch 
will  transform  an  entire  mass.  That  is  to  say,  the  change  of 
molecular  state  once  set  up  at  one  end,  spreads  to  the  other 
end — there  is  a  progress  of  a  stimulus  to  change;  and  this  is 
what  we  see  in  a  nerve.  So  much  being  understood,  let  us 
re-state  the  case  more  completely. 

Molecular  change,  implying  as  it  does  motion  of  molecules, 
communicates  motion  to  adjacent  molecules;  be  they  of  the 
same  kind  or  of  a  different  kind.  If  the  adjacent  molecules, 
either  of  the  same  kind  or  of  a  different  kind,  be  stable  in 
composition,  a  temporary  increase  of  oscillation  in  them  as 
wholes,  or  in  their  parts,  may  be  the  only  result ;  but  if  they 
are  unstable  there  are  apt  to  arise  changes  of  arrangement 
among  them,  or  among  their  parts,  of  more  or  less  permanent 
kinds.  Especially  is  this  so  with  the  complex  molecules 
which  form  colloidal  matter,  and  with  the  organic  colloids 
above  all.  Hence  it  is  to  be  inferred  that  a  molecular  dis- 
turbance in  any  part  of  a  living  animal,  set  up  by  either  an 
external  or  internal  agency,  will  almost  certainly  disturb  and 
change  some  of  the  surrounding  colloids  not  originally  im- 
plicated— will  diffuse  a  wave  of  change  towards  other  parts 
of  the  organism:  a  wave  which  will,  in  the  absence  of  per- 
fect homogeneity,  travel  further  in  some  directions  than  in 


THE  INNER  TISSUES  OF  ANIMALS.  357 

others.  Let  us  ask  next  what  will  determine  the 

differences  of  distance  travelled  in  different  directions.  Ob- 
viously any  molecular  agitation  spreading  from  a  centre,  will 
go  furthest  along  routes  that  offer  least  resistance.  What 
routes  will  these  be  ?  Those  along  which  there  lie  most  mole- 
cules that  are  easily  changed  by  the  diffused  molecular  motion, 
and  which  yet  do  not  take  up  much  molecular  motion  in  as- 
suming their  new  states.  Molecules  which  are  tolerably  stable 
will  not  readily  propagate  the  agitation ;  for  they  will  absorb 
it  in  the  increase  of  their  own  oscillations,  instead  of  passing 
it  on.  Molecules  which  are  unstable  but  which,  in  assuming 
isomeric  forms,  absorb  motion,  will  not  readily  propagate  it ; 
since  it  will  disappear  in  working  the  changes  in  them.  But 
unstable  molecules  which,  in  being  isomerically  transformed, 
do  not  absorb  motion,  and  still  more  those  which,  in  being 
so  transformed,  give  out  motion,  will  readily  propagate  any 
molecular  agitation ;  since  they  will  pass  on  the  impulse  either 
undiminished,  or  increased,  to  adjacent  molecules.  If 

then  we  assume,  as  we  are  not  only  warranted  in  doing  but 
are  obliged  to  do,  that  protoplasm  contains  two  or  more 
colloids,  either  mingled  or  feebly  combined  (since  it  cannot 
consist  of  simple  albumen  or  fibrin  or  casein,  or  any  allied 
proximate  principle)  ;  it  may  be  concluded  that  any  mole- 
cular agitation  set  up  by  what  we  call  a  stimulus,  will  diffuse 
itself  further  along  some  lines  than  along  others,  if  the  com- 
ponents of  the  protoplasm  are  not  quite  homogeneously  dis- 
persed, and  if  some  of  them  are  isomerically  transformed 
more  easity,  or  with  less  expenditure  of  motion,  than 
others;  and  it  will  especially  travel  along  spaces  occupied 
chiefly  by  those  molecules  which  give  out  molecular  mo- 
tion during  their  metamorphoses,  if  there  should  be  any 
such.  But  now  let  us  ask  what  structural  effects 

will  be  wrought  along  a  tract  traversed  by  this  wave  of 
molecular  disturbance.  As  is  shown  by  those  transforma- 
tions which  so  rapidly  propagate  themselves  through  colloids, 
molecules  that  have  undergone  a  certain  change  of  form, 


358  PHYSIOLOGICAL  DEVELOPMENT. 

are  apt  to  communicate  a  like  change  of  form  to  ad- 
jacent molecules  of  the  same  kind — the  impact  of  each 
overthrow  is  passed  on  and  produces  another  overthrow. 
Probably  the  proneness  towards  isochronism  of  molecular 
movements  necessitates  this.  If  any  molecule  has  had 
its  components  re-arranged,  and  their  oscillations  conse- 
quently altered,  there  result  movements  not  concordant  with 
the  movements  in  adjacent  untransformed  molecules,  but 
which,  impressing  themselves  on  the  parts  of  such  untrans- 
formed molecules,  tend  to  generate  in  them  concordant  move- 
ments— tend,  that  is,  to  produce  the  re-arrangements  involved 
by  these  concordant  movements.  Is  this  action  limited  to 
strictly  isomeric  substances?  or  may  it  extend  to  substances 
that  are  closely  allied?  If  along  with  the  molecules  of  a 
compound  colloid  there  are  mingled  those  of  some  kindred 
colloid;  or  if  with  the  molecules  of  this  compound  colloid 
there  are  mingled  the  components  out  of  which  other  such 
molecules  may  be  formed;  then  there  arises  the  question — 
does  the  same  influence  which  tends  to  propagate  the  iso- 
meric transformations,  tend  also  to  form  new  molecules  of 
the  same  kind  out  of  the  adjacent  components?  There  is 
reason  to  suspect  that  it  does.  Already  when  treating  of  the 
nutrition  of  parts  (§64),  it  was  pointed  out  that  we  are 
obliged  to  recognize  a  power  possessed  by  each  tissue  to  build 
up,  out  of  the  materials  brought  to  it,  molecules  of  the  same 
type  as  those  of  which  it  is  formed.  This  building  up  of  like 
molecules  seems  explicable  as  caused  by  the  tendency  of  the 
new  components  which  the  blood  supplies,  to  acquire  move- 
ments isochronous  with  those  of  the  like  components  in  the 
tissue;  which  they  can  do  only  by  uniting  into  like  com- 
pound molecules.  Necessarily  they  must  gravitate  towards 
a  state  of  equilibrium;  such  state  of  equilibrium — moving 
equilibrium  of  course — must  be  one  in  which  they  oscillate 
in  the  same  times  with  neighbouring  molecules;  and  so 
to  oscillate  they  must  fall  into  groups  identical  with  the 
groups  around  them.  If  this  be  a  general  principle  of 


THE  INNER  TISSUES  OF  ANIMALS.  359 

tissue-growth  and  repair,  we  may  conclude  that  it  will  apply 
in  the  case  before  us.  A  wave  of  molecular  disturbance 
passing  along  a  tract  of  mingled  colloids  closely  allied  in  com- 
position, and  isomerically  transforming  the  molecules  of  one 
of  them,  will  be  apt  at  the  same  time  to  form  some  new  mole- 
cules of  the  same  type,  at  any  place  where  there  exist  the 
proximate  components,  either  uncombined  or  feebly  combined 
in  some  not  very  different  way.  And  this  will  be  most  likely 
to  occur  where  the  molecules  of  the  colloid  that  are  under- 
going the  isomeric  change,  predominate,  but  have  scattered 
through  them  the  other  molecules  out  of  which  they  may  be 
formed,  either  by  composition  or  modification.  That  is  to 
say,  a  wave  of  molecular  disturbance  diffused  from  a  centre, 
and  travelling  furthest  along  a  line  where  lie  most  molecules 
that  can  be  isomerically  transformed  with  facility,  will  be 
likely  at  the  same  time  to  further  differentiate  this  line,  and 
make  it  more  characterized  than  before  by  the  easy-trans- 
formability  of  its  molecules.  One  additional  step, 

and  the  interpretation  is  reached.  Analogy  shows  it  to  be 
not  improbable  that  these  organic  colloids,  isomerically  trans- 
formed by  slight  molecular  impact  or  increase  of  molecular 
motion,  will  some  of  them  resume  their  previous  molecular 
structures  after  the  disturbance  has  passed.  We  know  that 
what  are  stable  molecular  arrangements  under  one  degree  of 
molecular  agitation,  are  not  stable  under  another  degree ;  and 
there  is  evidence  that  re-arrangements  of  an  inconspicuous 
kind  are  occasionally  brought  about  by  very  slight  changes 
of  molecular  agitation.  Water  supplies  a  clear  case.  Prof. 
Graham  infers  that  water  undergoes  a  molecular  re-arrange- 
ment at  about  32° — that  ice  has  a  colloid  form  as  well  as  a 
crystalloid  form,  dependent  on  temperature.  Send  through 
it  an  extra  wave  of  the  molecular  agitation  we  call  heat,  and 
its  molecules  aggregate  in  one  way.  Let  the  wave  die  away, 
and  its  molecules  resume  their  previous  mode  of  aggregation. 
And  obviously  such  transformations  may  be  repeated  back- 
wards and  forwards  within  narrow  limits  of  temperature. 


360  PHYSIOLOGICAL  DEVELOPMENT. 

Now  among  the  extremely  unstable  organic  colloids,  such  a 
phenomenon  is  far  more  likely  to  happen.  Suppose,  then,  that 
the  nerve-colloid  is  one  of  which  the  molecules  are  changed  in 
form  by  a  passing  wave  of  extra  agitation,  but  resume  their 
previous  form  when  the  wave  has  passed:  the  previous  form 
being  the  most  stable  under  the  conditions  which  then  recur. 
What  follows?  It  follows  that  these  molecules  will  be  ready 
again  to  undergo  isomeric  transformation  when  there  again 
occurs  the  stimulus ;  will,  as  before,  propagate  the  transforma- 
tion most  along  the  tract  where  such  molecules  are  most 
abundant ;  will,  as  before,  tend  to  form  new  molecules  of  their 
own  type;  will,  as  before,  make  the  line  along  which  they  lie 
one  of  easier  transfer  for  the  molecular  agitation.  Every 
repetition  will  help  to  increase,  to  integrate,  to  define  more 
completely,  the  course  of  the  escaping  molecular  motion — 
extending  its  remoter  part  while  it  makes  its  nearer  part 
more  permeable — will  help,  that  is,  to  form  a  line  of  dis- 
charge, a  line  for  conducting  impressions,  a  nerve. 

Such  seems  to  me  a  not  unfair  series  of  deductions  from 
the  known  habitudes  of  colloids  in  general  and  the  organic 
colloids  in  particular.  And  I  think  that  the  implied  nature 
and  properties  of  nerve  correspond  better  with  the  observed 
phenomena  than  do  the  nature  and  properties  implied  by 
other  hypotheses.  Of  course  the  speculation  as  it  here  stands 
is  but  tentative,  and  leaves  much  unexplained.  It  gives  no 
obvious  reply  to  the  questions — what  causes  the  formation  of 
nerves  in  directions  adapted  to  the  needs?  what  determines 
their  appropriate  connexions? — questions,  however,  to  which, 
when  we  come  to  deal  with  physiological  integration,  we  may 
find  not  unsatisfactory  answers.  Moreover  it  says  nothing 
about  the  genesis  of  ganglia.  A  ganglion,  it  is  clear,  must 
consist  of  a  colloidal  matter  equally  unstable,  or  still  more 
unstable,  which,  when  disturbed,  falls  into  some  different 
molecular  arrangement,  perhaps  chemically  simpler,  and  gives 
out  in  so  doing  a  large  amount  of  molecular  motion — serves 
as  a  reservoir  of  molecular  motion  which  may  be  suddenly 


THE  INNER  TISSUES  OF  ANIMALS.  361 

discharged  along  an  efferent  nerve  or  nerves,  when  excite- 
ment of  an  afferent  nerve  has  disengaged  it.  How  such  a 
structure  as  this  results,  the  hypothesis  does  not  show.  But 
admitting  these  shortcomings  it  may  still  be  held  that  we 
are,  in  the  way  pointed  out,  enabled  to  form  some  idea  of  the 
actions  by  which  nervous  tissue  is  differentiated. 

§  303.  A  speculation  akin  to,  and  continuous  with,  the  last, 
is  suggested  by  an  inquiry  into  the  origin  of  muscular  tissue. 
Contractility  as  well  as  irritability  is  a  property  of  protoplasm 
or  sarcode;  and,  as  before  suggested  (§  22),  is  not  improbably 
due  to  isomeric  change  in  one  or  more  of  its  component  col- 
loids. It  is  a  feasible  supposition  that  of  the  several  isomeric 
changes  simultaneously  set  up  among  these  component  col- 
loids, some  may  be  accompanied  by  change  of  bulk  and  some 
not.  Clearly  the  isomeric  change  undergone  by  the  colloid 
which  we  suppose  to  form  nerve,  must  be  one  not  accompanied 
by  appreciable  change  of  bulk;  since  change  of  bulk  implies 
"  internal  work,"  as  physicists  term  it,  and  therefore  ex- 
penditure of  force.  Conversely,  the  colloid  out  of  which 
muscle  originates,  may  be  one  that  readily  passes  into  an 
isomeric  state  in  which  it  occupies  less  space:  the  molecular 
disturbance  causing  this  contraction  being  communicated  to 
it  from  adjacent  portions  of  nerve-substance  that  are  mole- 
cularly  disturbed;  or  being  otherwise  communicated  to  it 
by  direct  mechanical  or  chemical  stimuli:  as  happens  where 
nerves  do  not  exist,  or  where  their  influence  has  been  cut 
off.  This  interpretation  seems,  indeed,  to  be  directly  at 
variance  with  the  fact  that  muscle  does  not  diminish  in  bulk 
during  contraction  but  merely  changes  its  shape.  That  which 
we  see  take  place  with  the  muscle  as  a  whole,  is  said  also  to 
take  place  with  each  fibre — while  it  shortens  it  also  broadens. 
There  is,  however,  a  possible  solution  of  this  difficulty.  A 
contracting  colloid  yields  up  its  water;  and  the  contracted 
colloid  plus  the  free  water,  may  have  the  same  bulk  as  before 
though  the  colloid  has  less.  If  it  be  replied  that  in  this 


362  PHYSIOLOGICAL  DEVELOPMENT. 

case  the  water  should  become  visible  between  the  substance 
of  the  fibre  and  its  sarcolemma  or  sheath,  it  may  be  rejoined 
that  this  is  not  necessary — it  may  be  deposited  interstitially. 
Possibly  the  striated  structure  is  one  that  facilitates  its 
exudation  and  subsequent  re-absorption;  and  to  this  may  be 
due  the  superiority  of  striated  muscle  in  rapidity  of  contrac- 
tion. Granting  the  speculative  character  of  this 
interpretation,  let  us  see  how  far  it  agrees  with  the  facts.  If 
the  actions  are  as  here  supposed,  the  contracted  or  more  inte- 
grated state  of  the  muscular  colloid  will  be  that  which  it 
tends  continually  to  assume — that  into  which  it  has  an  in- 
creasing aptitude  to  pass  when  artificial  paralysis  has  been 
produced,  as  shown  by  Dr.  Norris — that  into  which  it  lapses 
completely  in  rigor  mortis.  The  sensible  motion  generated 
by  the  contraction  can  arise  only  from  the  transformation 
of  insensible  motion.  This  insensible  motion  suddenly 
yielded  up  by  a  contracting  mass,  implies  the  fall  of  its 
component  molecules  into  more  stable  arrangements.  And 
there  can  be  no  such  fall  unless  the  previous  arrangement  is 
unstable.  From  this  point  of  view,  too,  it  is  possible 
to  see  how  the  hydro-carbons  and  carbo-hydrates  consumed 
in  muscular  action,  may  produce  their  effects.  For  these 
non-nitrogenous  elements  of  food,  when  consumed  in  the 
tissues,  give  out  large  amounts  of  molecular  motion.  They 
do  this  in  presence  of  the  muscular  colloids  which  have  lost 
molecular  motion  during  their  fall  in  the  stable  or  contracted 
state.  From  the  molecular  motion  they  give  out,  may  be 
restored  the  molecular  motion  lost  by  the  contracted  colloids ; 
and  these  contracted  colloids  may  thus  have  their  molecules 
raised  to  that  unstable  state  from  which,  again  falling,  they 
can  again  generate  mechanical  motion. 

This  conception  of  the  nature  and  mode  of  action  of  mus- 
cle, while  it  is  suggested  by  known  properties  of  colloidal 
matter  and  conforms  to  the  recent  conclusions  of  organic 
chemistry  and  molecular  physics,  establishes  a  comprehensible 
relation  between  the  vital  actions  of  the  lower  and  the  higher 


THE  INNER  TISSUES  OF  ANIMALS.  363 

animals.  If  we  contemplate  the  movements  of  cilia,  of  a 
Ehizopod's  pseudopodia,  of  a  Polype's  body,  or  of  the  long 
pendant  tentacles  of  a  Medusa,  we  shall  see  great  congruity 
between  them  and  this  hypothesis.  Bearing  in  mind  that  the 
contractile  substance  of  developed  muscle  is  affected  not  by 
nervous  influence  only,  but,  where  nervous  influence  is  de- 
stroyed, is  made  to  contract  by  mechanical  disturbance  and 
chemical  action,  we  may  infer  that  it  does  not  differ  intrin- 
sically from  the  primordial  contractile  substance  which,  in 
the  lowest  animals,  changes  its  bulk  under  other  stimuli  than 
the  nervous.  We  shall  see  significance  in  the  fact  ascer- 
tained by  Dr.  Ransom,  that  various  agents  which  excite 
and  arrest  nervo-muscular  movements  in  developed  animals, 
excite  and  arrest  the  protoplasmic  movements  in  ova.  We 
shall  understand  how  tissues  not  yet  differentiated  into  mus- 
cle and  nerve,  have  this  joint  irritability  and  contractility; 
how  muscle  and  nerve  may  arise  by  the  segregation  of  their 
mingled  colloids,  the  one  of  which,  not  appreciably  altering 
its  bulk  during  isomeric  change,  readily  propagates  molecular 
disturbance,  while  the  other,  contracting  when  isomerically 
changed,  less  readily  passes  on  the  molecular  disturbance; 
and  how,  by  this  differentiation  and  integration  of  the  con- 
ducting and  the  contracting  colloids,  the  one  ramifying 
through  the  other,  it  becomes  possible  for  a  whole  mass  to 
contract  suddenly,  instead  of  contracting  gradually,  as  it  does 
when  undifferentiated. 

The  question  remaining  to  be  asked  is — What  causes  the 
specialization  of  contractile  substance? — What  causes  the 
growth  of  colloid  masses  which  monopolize  this  contractility, 
and  leave  kindred  colloids  to  monopolize  other  properties? 
Has  natural  selection  gradually  localized  and  increased  the 
primordial  muscular  substance?  or  has  the  frequent  recur- 
rence of  irritations  and  consequent  contractions  at  particular 
parts  done  it?  We  have,  I  think,  reason  to  conclude  that 
direct  equilibration  rather  than  indirect  equilibration  has  been 
chiefly  operative.  The  reasoning  that  was  used  in  the  case 


364:  PHYSIOLOGICAL  DEVELOPMENT. 

of  nerve  applies  equally  in  the  case  of  muscle.  A  portion 
of  undifferentiated  tissue  containing  a  predominance  of  the 
colloid  that  contracts  in  changing,  will,  during  each  change, 
tend  to  form  new  molecules  of  its  own  type  from  the  other 
colloids  diffused  through  it:  the  tendency  of  these  entangled 
colloids  to  fall  into  unity  with  those  around  them,  will  be 
aided  by  every  shock  of  isomeric  transformation.  Hence,  re- 
peated contractions  will  further  the  growth  of  the  contracting 
mass,  and  advance  its  differentiation  and  integration.  If, 

too,  we  remember  that  the  muscular  colloid  is  made  to  con- 
tract by  mechanical  disturbance,  and  that  among  mechanical 
disturbances  one  which  will  most  readily  affect  it  simulta- 
neously throughout  its  mass  is  caused  by  stretching,  we  shall 
be  considerably  helped  towards  understanding  how  the  con- 
tractile tissues  are  developed.  If  extension  of  a  muscular 
colloid  previously  at  rest,  produces  in  it  that  molecular  dis- 
turbance which  leads  to  isomeric  change  and  decrease  of 
bulk,  then  there  is  no  difficulty  in  explaining  the  movements 
of  cilia;  the  formation  of  a  contractile  layer  in  the  vascular 
system  becomes  comprehensible;  each  dilatation  of  a  blood- 
vessel caused  by  a  gush  of  blood,  will  be  followed  by  a  con- 
striction; the  heart  will  pulsate  violently  in  proportion  as 
it  is  violently  distended;  arteries  will  develop  in  power  as 
the  stress  upon  them  becomes  greater;  and  we  shall  simi- 
larly have  an  explanation  of  the  increased  muscularity  of 
the  alimentary  canal  which  is  brought  about  by  increased 
distension  of  it. 

That  the  production  of  contractile  tissue  in  certain  locali- 
ties, is  due  to  the  more  frequent  excitement  in  those  localities 
of  the  contractility  possessed  by  undifferentiated  tissue  in 
general,  is  a  view  harmonizing  with  traits  which  the  diffe- 
rentiated contractile  tissue  exhibits.  These  are  the  rela- 
tions between  muscular  exercise,  muscular  power,  and  mus- 
cular structure;  and  it  is  the  more  needful  for  us  here  to 
notice  them  because  of  certain  anomalies  they  present, 
which,  at  first  sight,  seem  inconsistent  with  the  belief  that 


THE  INNER  TISSUES  OF  ANIMALS.  365 

the  functionally-determined  modifications  of  muscle  are  in- 
heritable. 

Muscles  disagree  greatly  in  their  tints :  all  gradations 
between  white  and  deep  red  being  observable.  Contrasts  are 
visible  between  the  muscles  of  different  animals,  between  the 
muscles  of  the  same  animal  at  different  ages,  and  between 
different  muscles  of  the  same  animal  at  the  same  age. 
We  will  glance  at  the  facts  under  these  heads :  noting  under 
each  of  them  the  connexion  which  here  chiefly  concerns 
us — that  between  the  activity  of  muscle  and  its  depth 
of  colour.  The  cold-blooded  Vertebrata  are,  taken 

as  a  group,  distinguished  from  the  warm-blooded  by  the 
whiteness  of  their  flesh;  and  they  are  also  distinguished  by 
their  comparative  inertness.  Though  a  fish  or  a  reptile  can 
exert  considerable  force  for  a  short  time,  it  is  not  capable 
of  prolonged  exertion.  Birds  and  mammals  show  greater 
endurance  along  with  the  darker-coloured  muscles.  If  among 
birds  themselves  or  mammals  themselves  we  make  compari- 
sons, we  meet  with  kindred  contrasts — especially  between 
wild  and  domestic  creatures  of  allied  kinds.  Barn-door  fowls 
are  lighter-fleshed  than  most  untamed  gallinaceous  birds; 
and  among  these  last  the  pheasant,  moving  about  but  little, 
is  lighter-fleshed  than  the  partridge  and  the  grouse  which  are 
more  nomadic.  The  muscles  of  the  sheep  are  not  on  the 
average  so  dark  as  those  of  the  deer;  and  it  is  said  that 
the  flesh  of  the  wild-boar  is  darker  than  that  of  the  pig. 
Perhaps,  however,  the  contrast  between  the  hare  and  the 
rabbit  affords,  among  familiar  animals,  the  best  example  of 
the  alleged  relation:  the  dark-fleshed  hare  having  no  retreat 
and  making  wide  excursions,  while  the  white-fleshed  rabbit, 
passing  a  great  part  of  its  time  in  its  burrow,  rarely  wanders 
far  from  home.  The  parallel  contrast  between  young 

and  old  animals  has  a  parallel  meaning.  Veal  is  much 
whiter  than  beef,  and  lamb  is  of  lighter  colour  than  mutton. 
Though  at  first  sight  these  facts  may  not  seem  to  furnish 
confirmatory  evidence,  since  lambs  in  their  play  appear  to 


366  PHYSIOLOGICAL  DEVELOPMENT. 

expend  more  muscular  force  than  their  sedate  dams ;  yet  the 
meaning  of  the  contrast  is  really  as  alleged.  For  in  conse- 
quence of  the  law -that  the  strains  which  animals  have  to 
overcome,  increase  as  the  cubes  of  the  dimensions,  while 
their  powers  of  overcoming  them  increase  only  as  the  squares 
(§46),  the  movements  of  an  adult  animal  cost  much  more 
in  muscular  effort  than  do  those  of  a  young  animal:  the 
result  being  that  the  sheep  and  the  cow  exercise  their  muscles 
more  vigorously  in  their  quiet  movements,  than  the  lamb 
and  the  calf  in  their  lively  movements.  It  may  be  added  as 
significant,  that  the  domestic  animal  in  which  no  very  marked 
darkening  of  the  flesh  takes  place  along  with  increasing 
age,  namely  the  pig,  is  one  which,  ordinarily  kept  in  a  sty, 
leads  so  quiescent  a  life  that  the  assigned  cause  of  darkening 
does  not  come  into  action.  But  perhaps  the  most 

conclusive  evidences  are  the  contrasts  which  exist  between 
the  active  and  inactive  muscles  of  the  same  animal.  Between 
the  leg-muscles  of  fowls  and  their  pectoral  muscles,  the  differ- 
ence of  colour  is  familiar;  and  we  know  that  fowls  exercise 
their  leg-muscles  much  more  than  the  muscles  which  move 
their  wings.  Similarly  in  the  turkey,  in  the  guinea  fowl,  in 
the  pheasant.  And  then,  adding  much  to  the  force  of  this 
evidence,  we  see  that  in  partridges  and  grouse,  which  belong 
to  the  same  order  as  our  domestic  fowls  but  use  their  wings 
as  constantly  as  their  legs,  little  or  no  difference  is  visible 
between  the  colour  of  these  two  groups  of  muscles.  Special 
contrasts  like  these  do  not,  however,  exhaust  the  proofs;  for 
there  is  a  still  more  significant  general  contrast.  The 
muscle  of  the  heart,  which  is  the  most  active  of  all  muscles, 
is  the  darkest  of  all  muscles. 

The  connexion  of  phenomena  thus  shown  in  so  many  ways, 
implies  that  the  bulk  of  a  muscle  is  by  no  means  the  sole 
measure  of  the  quantity  of  force  it  can  evolve.  It  would  seem 
that,  other  things  equal,  the  depth  of  colour  varies  with  the 
constancy  of  action ;  while,  other  things  equal,  the  bulk  varies 
with  the  amount  of  force  that  has  to  be  put  forth  upon  occa- 


THE  INNER  TISSUES  OF  ANIMALS.  367 

sion.  These  of  course  are  approximate  relations.  More 
correctly  we  may  say  that  the  actions  of  pale  muscles  are 
either  relatively  feeble  though  frequent  (as  in  the  massive 
flanks  of  a  fish),  or  relatively  infrequent  though  strong  (as  in 
the  pectoral  muscles  of  a  common  fowl) ;  while  the  actions  of 
dark  muscles  are  both  frequent  and  strong.  Some  such  dif- 
ferentiation may  be  anticipated  by  inference  from  the  respec- 
tive physiological  requirements.  A  muscle  which  has  upon 
occasion  to  evolve  considerable  force,  but  which  has  thereafter 
a  long  period  of  rest  during  which  repair  may  restore  it  to 
efficiency,  requires  neither  a  large  reserve  of  the  contractile 
substance  that  is  in  some  way  deteriorated  by  action,  nor 
highly-developed  appliances  for  bringing  it  nutritive  mate- 
rials and  removing  effete  products.  Where,  contrariwise,  an 
exerted  muscle  which  has  undergone  much  molecular  change 
in  evolving  much  mechanical  force,  has  soon  again  to  evolve 
much  mechanical  force,  and  so  on  continually ;  it  is  clear  that 
either  the  quantity  of  contractile  substance  present  must  be 
great,  or  the  apparatus  for  nutrition  and  depuration  must 
be  very  efficient,  or  both.  Hence  we  may  look  for  marked 
unlikenesses  of  minute  structure  between  muscles  which  are 
markedly  contrasted  in  activity.  And  we  may  suspect  that 
these  conspicuous  contrasts  of  colour  between  active  and 
inactive  muscles,  are  due  to  these  implied  differences  of 
minute  structure:  partly  differences  between  the  numbers 
of  blood-vessels  and  partly  differences  between  the  quantities 
or  qualities  of  sarcous  matter. 

Here,  then,  we  have  a  key  to  the  apparent  anomaly  above 
hinted  at — the  maintenance  of  bulk  by  certain  muscles  which 
have  been  rendered  comparatively  inactive  by  changed  habits 
of  life.  That  the  pectoral  muscles  of  those  domestic  birds 
which  fly  but  little,  have  not  dwindled  to  any  great  extent, 
has  been  thought  a  fact  at  variance  with  the  conclusion  that 
functionally-produced  adaptations  are  inheritable.  It  has 
been  argued  that  if  parts  which  are  exercised  increase,  not 
only  in  the  individual  but  in  the  race,  while  parts  which 


368  PHYSIOLOGICAL  DEVELOPMENT. 

become  less  active  decrease;  then  a  notable  difference  of  size 
should  exist  between  the  muscles  used  for  flight  in  birds  that 
fly  much,  and  those  in  birds  of  an  allied  kind  that  fly  little. 
But,  as  we  here  see,  this  is  not  the  true  implication.  The 
change  in  such  cases  must  be  chiefly  in  vascularity  and  abun- 
dance of  contractile  substance;  and  cannot  be,  to  any  great 
extent,  in  bulk.  For  a  bird  to  fly  at  all,  its  pectoral  muscles, 
bones  of  attachment,  and  all  accompanying  appliances,  must 
be  kept  up  to  a  certain  level  of  power.  If  the  parts  dwindle 
much,  the  creature  will  be  unable  to  lift  itself  from  the 
ground.  Bearing  in  mind  that  the  force  which  a  bird  ex- 
pends to  sustain  itself  in  the  air  during  each  successive  instant 
of  a  short  flight  is,  other  things  equal,  the  same  as  it  ex- 
pends in  each  successive  instant  of  a  long  flight,  we  shall  see 
that  the  muscles  employed  in  the  two  cases  must  have  some- 
thing like  equal  intensities  of  contractile  power ;  and  that  the 
structural  differences  between  them  must  have  relation  mainly 
to  the  lengths  of  time  during  which  they  can  continue  to  re- 
peat contractions  of  like  intensity.  That  is  to  say,  while  the 
power  of  flight  is  retained  at  all,  the  muscles  and  bones  can- 
not greatly  dwindle ;  but  the  dwindling,  in  birds  whose  flights 
are  short  or  infrequent  or  both,  will  be  in  the  reserve  stock 
of  the  substance  that  is  incapacitated  by  action,  or  in  the 
appliances  that  keep  the  apparatus  in  repair,  or  in  both. 
Only  where,  as  in  the  struthious  birds,  the  habit  of  flight  is 
lost,  can  we  expect  atrophy  of  all  the  parts  concerned  in 
flight ;  and  here  we  find  it. 

Are  such  differentiations  among  the  muscles  functionally 
produced?  or  are  they  produced  by  the  natural  selection  of 
variations  distinguished  as  spontaneous?  We  have,  I  think, 
good  grounds  for  concluding  that  they  are  functionally  pro- 
duced. We  know  that  in  individual  men  and  animals,  the 
power  of  sustained  action  in  muscles  is  rapidly  adaptable  to 
the  amount  of  sustained  action  required.  We  know  that 
being  "out  of  condition,"  is  usually  less  shown  by  the  inability 
to  put  out  a  violent  effort  than  by  the  inability  to  continue 


THE  INNER  TISSUES  OP  ANIMALS.  369 

making  violent  efforts ;  and  we  know  that  the  result  of  train- 
ing for  prize-fights  and  races,  is  more  shown  in  the  prolonga- 
tion of  energy  than  in  the  intensification  of  energy.  At  the 
same  time,  experience  has  taught  us  that  the  structural  change 
which  accompanies  this  functional  change,  is  not  so  much  a 
change  in  the  bulk  of  the  muscles  as  a  change  in  their  inter- 
nal state :  instead  of  being  soft  and  flabby  they  become  hard. 
We  have  inductive  proof,  then,  that  exercise  of  a  muscle  causes 
some  interstitial  growth  along  with  the  power  of  more  sus- 
tained action;  and  there  can  be  no  doubt  that  the  one  is  a 
condition  to  the  other.  What  is  this  interstitial  growth? 
There  is  reason  to  suspect  that  it  is  in  part  an  increased 
deposit  of  the  sarcous  substance  and  in  part  a  development  of 
blood-vessels.  Microscopic  observation  tends  to  confirm  the 
conclusions  before  drawn,  that  repetition  of  contractions  fur- 
thers the  formation  of  the  matter  which  contracts,  and  that 
greater  draughts  of  blood  determine  greater  vascularity. 
And  if  the  contrasts  of  molecular  structure  and  the  contrasts 
of  vascularity,  directly  caused  in  muscles  by  contrasts  in  their 
activities,  are  to  any  degree  inheritable;  there  results  an 
explanation  of  those  constitutional  differences  in  the  colours 
and  textures  of  muscles,  which  accompany  constitutional 
differences  in  their  degrees  of  activity. 

It  may  be  added  that  if  we  are  warranted  in  so  ascribing 
the  differentiations  of  muscles  from  one  another  to  direct 
equilibration,  then  we  have  the  more  reason  for  thinking 
that  the  differentiation  of  muscles  in  general  from  other 
structures  is  also  due  to  direct  equilibration.  That  unlike- 
nesses  between  parts  of  the  contractile  tissues  having  unlike 
functions,  are  caused  by  the  unlikenesses  of  their  functions, 
renders  it  the  more  probable  that  the  unlikenesses  between 
contractile  tissue  and  other  tissues,  have  been  caused  by  ana- 
logous unlikenesses. 

§  304.  These  interpretations,  which  have  already  occupied 
too  large  a  space,  must  here  be  closed.      Of  course  out  of 
70 


370  PHYSIOLOGICAL  DEVELOPMENT. 

phenomena  so  multitudinous  and  varied,  it  has  been  imprac- 
ticable to  deal  with  any  but  the  most  important;  and  it  has 
been  practicable  to  deal  with  these  only  in  a  general  way. 
Much,  however,  as  remains  to  be  explained,  I  think  the  possi- 
bility of  tracing,  in  so  many  cases,  the  actions  to  which  these 
internal  differentiations  may  rationally  be  ascribed,  makes  it 
likely  that  the  remaining  internal  differentiations  are  due  to 
kindred  actions.  We  find  evidence  that,  in  more  cases 
than  seemed  probable,  these  actions  produce  their  effects 
directly  on  the  individual;  and  that  the  unlikenesses  are 
produced  by  accumulation  of  such  effects  from  generation  to 
generation.  While  for  all  the  other  unlikenesses,  we  have, 
as  an  adequate  cause,  the  indirect  effects  wrought  by  the  sur- 
vival, generation  after  generation,  of  the  individuals  in  which 
favourable  variations  have  occurred — variations  such  as  those 
of  which  human  anatomy  furnishes  endless  instances.  Thus 
accounting  for  so  much,  we  may  not  unreasonably  presume 
that  these  co-operative  processes  of  direct  and  indirect  equili- 
bration will  account  for  what  remains. 

[NOTE. — After  having  dismissed  this  revised  chapter  as 
done  with,  and  sent  it  to  the  printer,  further  thought  con- 
cerning those  differentiations  which  produce  bone,  has  re- 
minded me  of  a  fact  of  extreme  and  varied  significance 
named  in  the  first  volume.  I  refer  to  the  formation  of 
adaptive  structures  round  the  ends  of  dislocated  bones,  and 
to  the  formation  of  "  false  joints." 

These  are  ontogenetic  changes  of  which  phylogeny  yields 
no  explanation.  They  do  not  repeat  the  traits  of  ancestral 
organisms,  and  they  cannot  be  ascribed  to  either  of  the 
recognized  evolutionary  factors.  If  a  humerus  be  broken 
across  and,  failing  to  set,  presently  comes  to  have  its  two 
loose  ends  so  modified  as  in  a  measure  to  simulate  the  parts 
of  a  normal  joint — the  ends  becoming  smooth,  covered  with 
periosteum  and  supplied  with  fibrous  tissue,  and  attached 
by  ligaments  in  such  ways  as  to  allow  of  restrained  move- 


THE  INNER  TISSUES  OP  ANIMALS.  371 

merits — it  is  impossible  to  think  that  natural  selection  has 
had  anything  to  do  with  the  power  of  adjustment  thus 
shown.  No  survival  of  individuals  in  which  adaptations 
of  this  kind,  now  in  one  place  and  now  in  another,  were 
better  and  better  effected,  could  account  for  acquirement  of 
the  ability.  Nor  can  it  be  supposed  that  the  ability  might 
result  from  a  functionally-produced  habit;  since  it  is 
scarcely  conceivable  that  the  number  of  cases  in  which  indi- 
viduals profited  by  it  (at  first  a  little  and  gradually  more) 
could  be  such  (even  did  they  survive)  as  to  affect  the 
constitution  of  the  species.  Both  of  the  alleged  causes  of 
structural  modifications  are  out  of  court.  It  is  manifest, 
too,  that  the  foregoing  hypothesis  respecting  bone-formation 
yields  us  not  the  slightest  help. 

But  on  carefully  considering  the  facts,  certain  phenomena 
of  profound  meaning  may  strike  us.  Here,  in  a  part  of  the 
body  where  no  such  tissues  ordinarily  exist  and  to  which  no 
such  structures  are  ordinarily  appropriate,  there  arise  tissues 
and  structures  adapted  to  the  physical  circumstances  imposed 
on  that  part.  Out  of  what  do  these  abnormal  but  appropriate 
tissues  arise?  The  substances  around — osseous,  cartilagin- 
ous, membranous — consist  of  differentiated  elements  too  far 
specialized  to  allow  of  transformation.  These  new  tissues, 
then,  must  originate  from  the  undifferentiated  protoplasm 
pervading  the  part.  The  units  of  this  protoplasm,  subject  to 
the  actions  proper  to  an  articulation,  begin  to  assume  the  ap- 
propriate histological  traits — are  determined  by  local  stimuli 
to  form  tissues  ordinarily  associated  with  such  stimuli.  What 
is  the  inevitable  implication?  These  units — physiological  or 
constitutional,  as  we  may  call  them — must  have  possessed 
latent  potentialities  of  falling  into  these  special  arrangements 
under  stress  of  such  conditions.  At  one  point  there  arises 
periosteum  and  at  another  ligamentous  tissue,  while  for  the 
shaping  of  the  ends  of  the  bones — here  into  a  rude  hinged 
form  and  there  into  a  rude  ball-and-socket  form,  according  to 
the  habitual  movements — there  goes  on  some  appropriate  de- 


372  PHYSIOLOGICAL  DEVELOPMENT. 

posit  of  bone.  Hence  we  must  conclude  that  in  the  units  of 
protoplasm  which  have  not  yet  been  organized  into  special 
tissues,  there  resides  the  ability  to  take  on  one  or  other  type 
of  histological  structure  according  to  circumstances;  and, 
further,  that  there  resides  in  each  of  them  the  still  more 
marvellous  ability  to  cooperate  with  kindred  units  dispersed 
around  in  developing  that  arrangement  of  the  parts  required 
to  constitute  a  "  false  joint."  So  that  while  these  units 
have  a  general  proclivity  towards  the  structure  of  the 
organism  as  a  whole,  they  have  also  proclivities  towards 
structures  proper  to  the  local  conditions  into  which  they 
fall.  There  is  latent  in  each  unit  the  constitution  of  the 
entire  organism  and  by  implication  the  constitution  of  every 
organ ;  and  each  unit  while  cooperating  with  the  aggregate  is 
ready  to  take  part  in  that  particular  arrangement  proper  to 
the  position  it  has  fallen  into.  If  the  reader  will  refer  back 
to  §§  97 d,  97e,  in  which  it  is  shown  that  each  member  of  a 
human  society  possesses  a  combination  of  potentialities  like 
these,  he  will  be  the  better  enabled  to  believe  that  this  thing 
may  be  so  while  he  is  unable  to  conceive  how  it  is  so. 

And  here,  indeed,  let  it  be  pointed  out  how  completely 
irrelevant  is  the  test  of  conceivableness  as  applied  to  these 
ultimate  physiological  actions.  For  as  here,  from  the  un- 
united  ends  of  the  broken  bone,  there  presently  arises  a  rude 
joint  with  fit  membranes,  ligaments,  and  even  synovial  fluid, 
though  we  are  absolutely  unable  to  imagine  the  process  by 
which  the  adjacent  tissues  produce  this  structure;  so  there 
may  be  from  an  organ  enlarged  by  function,  such  reactive 
effect  upon  the  system  at  large  as  eventually  to  influence  the 
reproductive  cells,  though  we  may  be  absolutely  unable  to 
imagine  how  this  can  be  done.] 


CHAPTER  IX. 

PHYSIOLOGICAL   INTEGRATION   IN   ANIMALS. 

§  305.  PHYSIOLOGICAL  differentiation  and  physiological 
integration,  are  correlatives  that  vary  together.  We  have  but 
to  recollect  the  familiar  parallel  between  the  division  of  labour 
in  a  society  and  the  physiological  division  of  labour,  to  see 
that  as  fast  as  the  kinds  of  work  performed  by  the  com- 
ponent parts  of  an  organism  become  more  numerous,  and  as 
fast  as  each  part  becomes  more  restricted  to  its  own  work,  so 
fast  must  the  parts  have  their  actions  combined  in  such  ways 
that  no  one  can  go  on  without  the  rest  and  the  rest  cannot 
go  on  without  each  one. 

Here  our  inquiry  must  be,  how  the  relationship  of  these 
two  processes  is  established — what  causes  the  integration  to 
advance  pari  passu  with  the  differentiation.  Though  it  is 
manifest,  a  priori,  that  the  mutual  dependence  of  functions 
must  be  proportionate  to  the  specialization  of  functions ;  yet 
it  remains  to  find  the  mode  in  which  the  increasing  co-ordi- 
nation is  determined. 

Already,  among  the  Inductions  of  Biology,  this  relation 
between  differentiation  and  integration  has  been  specified 
and  illustrated  (§59).  Before  dealing  with  it  deductively, 
a  few  further  examples,  grouped  so  as  to  exhibit  its  several 
aspects,  will  be  advantageous. 

§  306.  If  the  lowly-organized  Planaria  has  its  body  broken 
up  and  its  gullet  detached,  this  will,  for  a  while,  continue 

373 


374:  PHYSIOLOGICAL  DEVELOPMENT. 

to  perform  its  function  when  called  upon,  just  as  though  it 
were  in  its  place:  a  fragment  of  the  creature's  own  body 
placed  in  the  gullet,  will  be  propelled  through  it,  or  swal- 
lowed by  it.  But,  as  the  seeming  strangeness  of  this  fact 
implies,  we  find  no  such  independent  actions  of  analogous 
parts  in  the  higher  animals.  Again,  a  piece  cut  out  of  the 
disc  of  a  Medusa  continues  with  great  persistence  repeating 
those  rhythmical  contractions  which  we  see  in  the  disc  as  a 
whole;  and  thus  proves  to  us  that  the  contractile  function 
in  each  portion  of  the  disc,  is  in  great  measure  independent. 
But  it  is  not  so  with  the  locomotive  organs  of  more  differen- 
tiated types.  When  separated  from  the  rest  these  lose  their 
powers  of  movement.  The  only  member  of  a  vertebrate 
animal  which  continues  to  act  after  detachment,  is  the  heart ; 
and  the  heart  has  motor  powers  complete  within  itself. 

Where  there  is  this  small  dependence  of  each  part  upon 
the  whole,  there  is  but  small  dependence  of  the  whole  upon 
each  part.  The  longer  time  which  it  takes  for  the  arrest  of 
a  function  to  produce  death  in  a  less-differentiated  animal 
than  in  a  more-differentiated  animal,  may  be  illustrated  by 
the  case  of  respiration.  Suffocation  in  a  man  speedily 
causes  resistance  to  the  passage  of  the  blood  through  the 
capillaries,  followed  by  congestion  and  stoppage  of  the  heart : 
great  disturbance  throughout  the  system  results  in  a  few 
seconds,  and  in  a  minute  or  two  all  the  functions  cease. 
But  in  a  frog,  with  its  undeveloped  respiratory  organ,  and  a 
skin  through  which  a  considerable  aeration  of  the  blood  is 
carried  on,  breathing  may  be  suspended  for  a  long  time  with- 
out injury.  Doubtless  this  difference  is  proximately  due  to 
the  greater  functional  activity  in  the  one  case  than  in  the 
other,  and  the  more  pressing  need  for  discharging  the  pro- 
duced carbon  dioxide;  but  the  greater  functional  activity  being 
itself  made  possible  by  the  higher  specialization  of  functions, 
this  remains  the  primary  cause  of  the  greater  dependence  of 
the  other  functions  on  respiration,  where  the  respiratory 
apparatus  has  become  highly  specialized.  Here 


PHYSIOLOGICAL  INTEGRATION  IN  ANIMALS.       3^5 

indeed,  we  see  the  relation  under  another  aspect.  This  more 
rapid  rhythm  of  the  functions  which  increased  heterogeneity 
of  structure  makes  possible,  is  itself  a  means  of  integrating 
the  functions.  Watch,  when  it  is  running  down,  a  compli- 
cated machine  of  which  the  parts  are  not  accurately  adjusted, 
or  are  so  worn  as  to  be  somewhat  loose.  There  will  be 
observed  certain  irregularities  of  movement  just  before  it 
comes  to  rest — certain  of  the  parts  which  stop  first,  are 
again  made  to  move  a  little  by  the  continued  movement  of 
the  rest,  and  then  become  themselves,  in  turn,  the  causes  of 
renewed  motion  in  other  parts  which  have  ceased  to  move. 
That  is  to  say,  while  the  connected  rhythmical  changes  of 
the  machine  are  quick,  their  actions  and  reactions  on  one 
another  are  regular — all  the  motions  are  well  integrated ;  but 
as  the  velocity  diminishes  irregularities  arise — the  motions 
become  somewhat  disintegrated.  Similarly  with  organic 
functions :  increase  of  their  rapidity  involves  increase  of  a 
joint  momentum  which  controls  each  and  co-ordinates  all. 
Thus  if  we  compare  a  snake  with  a  mammal,  we  see  that 
its  functions  are  not  tied  together  so  closely.  The. mammal, 
and  especially  the  superior  mammal,  requires  food  with  con- 
siderable regularity;  keeps  up  a  respiration  which  varies 
within  but  moderate  limits;  and  has  periods  of  activity  and 
rest  that  alternate  evenly  and  frequently.  But  the  snake, 
taking  food  at  long  intervals,  may  have  these  intervals 
greatly  extended  without  fatal  results ;  its  dormant  and  its 
active  states  recur  less  uniformly ;  and  its  rate  of  respiration 
varies  within  much  wider  limits — now  being  scarcely  per- 
ceptible and  now,  as  you  may  prove  by  exciting  it,  becoming 
conspicuous.  So  that  here,  where  the  rhythms  are  very  slow, 
they  are  individually  less  regular,  and  are  united  into  a  less 
regular  compound  rhythm — are  less  integrated. 

Perhaps  the  clearest  general  idea  of  the  co-ordination  of 
functions  that  accompanies  their  specialization,  is  obtained 
by  observing  the  slowness  with  which  a  little-differentiated 
animal  responds  to  a  stimulus  applied  to  one  of  its  parts, 


376  PHYSIOLOGICAL  DEVELOPMENT. 

and  the  rapidity  with  which  such  a  local  stimulus  is  re- 
sponded to  by  a  more-differentiated  animal.  A  sea-anemone 
and  a  fly  will  serve  for  the  comparison.  A  tentacle  of  a  sea- 
anemone,  when  touched,  slowly  contracts;  and  if  the  touch 
has  been  rude,  the  contraction  presently  extends  to  the  other 
tentacles  and  eventually  to  the  entire  body:  the  stimulus  to 
movement  is  gradually  diffused  throughout  the  organism. 
But  if  you  touch  a  fly,  or  rather  if  you  come  near  enough 
to  threaten  a  touch,  the  entire  apparatus  of  flight  is  instantly 
brought  into  combined  action.  Whence  arises  this  contrast? 
The  one  creature  has  but  faintly  specialized  contractile 
organs,  and  fibres  for  conveying  impressions.  The  other  has 
definite  muscles  and  nerves  and  a  co-ordinating  centre.  The 
parts  of  the  little-differentiated  sea-anemone  have  their 
functions  so  feebly  co-ordinated,  that  one  may  be  strongly 
affected  for  some  time  before  any  effect  is  felt  by  another  at 
a  distance  from  it ;  but  in  the  much-differentiated  fly,  various 
remote  parts  instantly  have  changes  propagated  to  them  from 
the  affected  part,  and  by  their  united  actions  thus  set  up,  the 
whole  organism  adjusts  itself  so  as  to  avoid  the  danger. 

These  few  added  illustrations  will  make  the  nature  of  this 
general  relation  sufficiently  clear.  Let  us  now  pass  to  the 
interpretation  of  it. 

§  307.  If  a  Hydra  is  cut  in  two,  the  nutritive  liquids 
diffused  through  its  substance  cannot  escape  rapidly,  since 
there  are  no  open  channels  for  them;  and  hence  the  con- 
ditions of  the  parts  at  a  distance  from  the  cut  is  but  little 
affected.  But  where,  as  in  the  more-differentiated  animals, 
the  nutritive  liquid  is  contained  in  vessels  which  have  con- 
tinuous communications,  cutting  the  body  in  two,  or  cutting 
off  any  considerable  portion  of  it,  is  followed  by  escape  of 
the  liquid  from  these  vessels  to  a  large  extent;  and  this 
affects  the  nutrition  and  efficiency  of  organs  remote  from 
the  place  of  injury.  Then  where,  as  in  further-developed 
creatures,  there  exists  an  apparatus  for  propelling  the  blood 


PHYSIOLOGICAL  INTEGRATION  IN  ANIMALS.       377 

through  these  ramifying  channels,  injury  of  a  single  one 
will  cause  a  loss  of  blood  that  quickly  prostrates  the  entire 
organism.  Hence  the  rise  of  a  completely-differentiated  vas- 
cular system,  is  the  rise  of  a  system  which  integrates  all 
members  of  the  body,  by  making  each  dependent  on  the  in- 
tegrity of  the  vascular  system,  and  therefore  on  the  integrity 
of  each  member  through  which  it  ramifies.  In 

another  mode,  too,  the  establishment  of  a  distributing  appa- 
ratus produces  a  physiological  union  that  is  great  in  propor- 
tion as  this  distributing  apparatus  is  efficient.  As  fast  as  it 
assumes  a  function  unlike  the  rest,  each  part  of  an  animal 
modifies  the  blood  in  a  way  more  or  less  unlike  the  rest,  both 
by  the  materials  it  abstracts  and  by  the  products  it  adds; 
and  hence  the  more  differentiated  the  vascular  system  be- 
comes, the  more  does  it  integrate  all  parts  by  making  each 
of  them  feel  the  qualitative  modification  of  the  blood  which 
every  other  has  produced.  This  is  simply  and  conspicuously 
exemplified  by  the  lungs.  In  the  absence  of  a  vascular 
system,  or  in  the  absence  of  one  that  is  well  marked  off 
from  the  imbedding  tissues,  the  nutritive  plasma  or  the  crude 
blood,  gets  what  small  aeration  it  can,  only  by  coming  near 
the  creature's  outer  surface,  or  those  inner  surfaces  which  are 
bathed  by  water.  But  where  there  have  been  formed  definite 
channels  branching  throughout  the  body,  and  particularly 
where  there  exist  specialized  organs  for  pumping  the  blood 
through  these  channels,  it  manifestly  becomes  possible  for 
the  aeration  to  be  carried  on  in  one  part  peculiarly  modified 
to  further  it,  while  all  other  parts  have  the  aerated  blood 
brought  to  them.  And  how  greatly  the  differentiation  of  the 
vascular  system  thus  becomes  a  means  of  integrating  the 
various  organs,  is  shown  by  the  fatal  result  that  follows  when 
the  current  of  aerated  blood  is  interrupted. 

Here,  indeed,  it  becomes  obvious  both  that  certain  physio- 
logical differentiations  make  possible  certain  physiological 
integrations ;  and  that,  conversely,  these  integrations  make 
possible  other  differentiations.  Besides  the  waste  products 


378  PHYSIOLOGICAL  DEVELOPMENT. 

which  escape  through  the  lungs,  there  are  waste  products 
which  escape  through  the  skin,  the  kidneys,  the  liver.  The 
blood  has  separated  from  it  in  each  of  these  structures,  the 
particular  product  which  this  structure  has  become  adapted 
to  separate ;  leaving  the  other  products  to  be  separated  by  the 
other  adapted  structures.  How  have  these  special  adapta- 
tions been  made  possible?  By  union  of  the  organs  as 
recipients  of  one  circulating  mass  of  blood.  While  there  is  no 
efficient  apparatus  for  transfer  of  materials  through  the  body, 
the  waste  products  of  each  part  have  to  make  their  escape 
locally;  and  the  local  channels  of  escape  must  be  competent 
to  take  off  indifferently  all  the  waste  products.  But  it 
becomes  practicable  and  advantageous  for  the  differently- 
localized  excreting  structures  to  become  fitted  to  separate 
different  waste  products,  as  soon  as  the  common  circulation 
through  them  grows  so  efficient  that  the  product  left  unex- 
creted  by  one  is  quickly  carried  to  another  better  fitted  to 
excrete  it.  So  that  the  integration  of  them  through  a 
common  vascular  system,  is  the  condition  under  which  only 
they  can  become  differentiated.  Perhaps  the  clearest 

idea  of  the  way  in  which  differentiation  leads  to  integration, 
and  how,  again,  increased  integration  makes  possible  still 
further  differentiation,  will  be  obtained  by  contemplating  the 
analogous  dependence  in  the  social  organism.  While  it  has 
no  roads,  a  country  cannot  have  its  industries  much  special- 
ized: each  locality  must  produce,  as  best  it  can,  the  various 
commodities  it  consumes,  so  long  as  it  has  no  facilities  for 
barter  with  other  localities.  But  the  localities  being  unlike  in 
their  natural  fitnesses  for  the  various  industries,  there  tends 
ever  to  arise  some  exchange  of  the  commodities  they  can 
respectively  produce  with  least  labour.  This  exchange  leads 
to  the  formation  of  channels  of  communication.  The  cur- 
rents of  commodities  once  set  up,  make  their  foot-paths  and 
horse-tracks  more  permeable;  and  as  fast  as  the  resistance 
to  exchange  becomes  less,  the  currents  of  commodities 
become  greater.  Each  locality  takes  more  of  the  products 


PHYSIOLOGICAL  INTEGRATION  IN  ANIMALS.        379 

of  adjacent  ones,  and  each  locality  devotes  itself  more  to  the 
particular  industry  for  which  it  is  naturally  best  fitted:  the 
functional  integration  makes  possible  a  further  functional 
differentiation.  This  further  functional  differentiation  reacts. 
The  greater  demand  for  the  special  product  of  each  locality, 
excites  improvements  in  production — leads  to  the  use  of 
methods  which  both  cheapen  and  perfect  the  commodity. 
Hence  results  a  still  more  active  exchange;  a  still  clearer 
opening  of  the  channels  of  communication;  a  still  closer 
mutual  dependence.  Yet  another  influence  comes  into  play. 
As  fast  as  the  intercourse,  at  first  only  between  neighbouring 
localities,  makes  for  itself  better  roads — as  fast  as  rivers  are 
bridged  and  marshes  made  easily  passable,  the  resistance  to 
distribution  becomes  so  far  diminished,  that  the  things  grown 
or  made  in  each  district  can  be  profitably  carried  to  a  greater 
distance ;  and  as  the  economical  integration  is  thus  extended 
over  a  wider  area,  the  economical  differentiation  is  again 
increased;  since  each  district,  having  a  larger  market  for  its 
commodity,  is  led  to  devote  itself  more  exclusively  to  pro- 
ducing this  commodity.  These  actions  and  reactions  con- 
tinue until  the  various  localities,  becoming  greatly  developed 
and  highly  specialized  in  their  industries,  are  at  the  same 
time  functionally  integrated  by  a  network  of  roads,  and 
finally  railways,  along  which  rapidly  circulate  the  currents 
severally  sent  out  and  received  by  the  localities.  And  it  will 
be  manifest  that  in  individual  organisms  a  like  correlative 
progress  must  have  been  caused  in  an  analogous  way. 

§  308.  Another  and  higher  form  of  physiological  integra- 
tion in  animals,  is  that  which  the  nervous  system  effects. 
Each  part  as  it  becomes  specialized,  begins  to  act  upon  the 
rest  not  only  indirectly  through  the  matters  it  takes  from 
and  adds  to  the  blood,  but  also  directly  through  the  molecular 
disturbances  it  sets  up  and  diffuses.  Whether  nerves  them- 
selves are  differentiated  by  the  molecular  disturbances  thus 
propagated  in  certain  directions,  or  whether  they  are  other- 


380  PHYSIOLOGICAL  DEVELOPMENT. 

wise  differentiated,  it  must  equally  happen  that  as  fast  as 
they  become  channels  along  which  molecular  disturbances 
travel,  the  parts  they  connect  become  physiologically  inte- 
grated, in  so  far  that  a  change  in  one  initiates  a  change  in 
the  other.  We  may  dimly  perceive  that  if  portions  of  what 
was  originally  a  uniform  mass  having  a  common  function, 
undertake  subdivisions  of  the  function,  the  molecular 
changes  going  on  in  them  will  be  in  some  way  complemen- 
tary to  one  another:  that  peculiar  form  of  molecular  motion 
which  the  one  has  lost  in  becoming  specialized,  the  other  has 
gained  in  becoming  specialized.  And  if  the  molecular  motion 
that  was  common  to  the  two  portions  while  they  were  undiffer- 
entiated,  becomes  divided  into  two  complementary  kinds  of 
molecular  motion;  then  between  these  portions  there  will  be 
a  contrast  of  molecular  motions  such  that  whatever  is  plus 
in  the  one  will  be  minus  in  the  other ;  and  hence  there  will  be 
a  special  tendency  towards  a  restoration  of  the  molecular  equi- 
librium between  the  two:  the  molecular  motion  continually 
propagated  away  from  either  will  have  its  line  of  least  resist- 
ance in  the  direction  of  the  other.  If,  as  argued 
in  the  last  chapter,  repeated  restorations  of  molecular  equili- 
brium, always  following  the  line  of  least  resistance,  tend  ever 
to  make  it  a  line  of  diminished  resistance;  then,  in  propor- 
tion as  any  parts  become  more  physiologically  integrated  by 
the  establishment  of  this  channel  for  the  easy  transmission 
of  molecular  motion  between  them,  they  may  become  more 
physiologically  differentiated.  The  contrast  between  their 
molecular  motions  leads  to  the  line  of  discharge;  the  line  of 
discharge,  once  formed,  permits  a  greater  contrast  of  their 
molecular  motions  to  arise;  thereupon  the  quantities  of 
molecular  motion  transferred  to  restore  equilibrium,  being 
increased,  the  channel  of  transfer  is  made  more  permeable; 
and  its  further  permeability,  so  caused,  renders  possible  a  still 
more  marked  unlikeness  of  action  between  the  parts.  Thus 
the  differentiation  and  the  integration  progress  hand  in  hand 
as  before.  How  the  same  principle  holds  through- 


PHYSIOLOGICAL  INTEGRATION  IN  ANIMALS.       381 

out  the  higher  stages  of  nervous  development,  can  be  seen 
only  still  more  vaguely.  Nevertheless,  it  is  comprehensible 
that  as  functions  become  further  divided,  there  will  arise  the 
need  for  sub-connexions  along  which  there  may  take  place 
secondary  equilibrations  subordinate  to  the  main  ones.  It  is 
manifest,  too,  that  whereas  the  differentiation  of  functions 
proceeds,  not  necessarily  by  division  into  two,  but  often  by 
division  into  several,  and  usually  in  such  ways  as  not  to  leave 
any  two  functions  that  are  just  complementary  to  one  an- 
other, the  restorations  of  equilibrium  cannot  be  so  simple  as 
above  supposed.  And  especially  when  we  bear  in  mind  that 
many  differentiated  functions,  as  those  of  the  senses,  cannot 
be  held  complementary  to  any  other  functions  in  particular; 
it  becomes  manifest  that  the  equilibrations  that  have  to  be 
made  in  an  organism  of  much  heterogeneity,  are  extremely 
complex,  and  do  not  take  place  between  each  organ  and  some 
other,  but  between  each  organ  and  all  the  others.  The  pecu- 
liarity of  the  molecular  motion  propagated  from  each  organ, 
has  to  be  neutralized  by  some  counter-peculiarity  in  the 
average  of  the  molecular  motions  with  which  it  is  brought 
into  relation.  All  the  variously-modified  molecular  motions 
from  the  various  parts,  must  have  their  pluses  and  minuses 
mutually  cancelled:  if  not  locally,  then  at  some  centre  to 
which  each  unbalanced  motion  travels  until  it  meets  with 
some  opposite  unbalanced  motion  to  destroy  it.  Still,  involved 
as  these  actions  must  become,  it  is  possible  to  see  how  the 
general  principle  illustrated  by  the  simple  case  above  sup- 
posed, will  continue  to  hold.  For  always  the  molecular 
motion  proceeding  from  any  one  differentiated  part,  will 
travel  most  readily  towards  that  place  where  a  molecular 
motion  most  complementary  to  it  in  kind  exists — no  matter 
whether  this  complementary  molecular  motion  be  that  pro- 
ceeding from  any  one  other  organ,  or  the  resultant  of  the 
molecular  motions  proceeding  from  many  other  organs.  So 
that  the  tendency  will  be  for  each  channel  of  communication 
or  nerve,  to  unite  itself  with  some  centre  or  ganglion,  where  it 


382  PHYSIOLOGICAL  DEVELOPMENT. 

comes  into  relation  with  other  nerves.  And  if  there  be  any 
parts  of  its  peculiar  molecular  motion  uncancelled  by  the 
molecular  motions  it  meets  at  this  centre ;  or  if,  as  will  prob- 
ably happen,  the  average  molecular  motion  which  it  there 
unites  to  produce,  differs  from  the  average  molecular  motion 
elsewhere;  then,  as  before,  there  will  arise  a  discharge  along 
another  channel  or  nerve  to  another  centre  or  ganglion, 
where  the  residuary  difference  may  be  cancelled  by  the 
differences  it  meets;  or  whence  it  may  be  still  further 
propagated  till  it  is  so  cancelled.  Thus  there  will  be  a  ten- 
dency to  a  general  nervous  integration  keeping  pace  with 
the  differentiation. 

Of  course  this  must  be  taken  as  nothing  more  than  the 
indication  of  initial  tendencies — not  as  an  hypothesis  suffi- 
cient to  account  for  all  the  facts.  It  leaves  out  of  sight  the 
origin  and  functions  of  ganglia,  considered  as  something  more 
than  nerve-junctions.  Were  there  only  these  lines  of  easy 
transmission  of  molecular  disturbance,  a  change  set  up  in 
one  organ  could  never  do  more  than  produce  its  equivalent 
of  change  in  some  other  or  others;  and  there  could  be  none 
of  that  large  amount  of  motion  initiated  by  a  small  sensation, 
which  we  habitually  see.  The  facts  show,  unmistakably,  that 
the  slight  disturbance  communicated  to  a  ganglion,  causes  an 
overthrow  of  that  highly-unstable  nervous  matter  contained 
in  it,  and  a  discharge  from  it  of  the  greatly-increased  quantity 
of  molecular  motion  so  generated.  This,  however,  is  beyond 
our  immediate  topic.  All  we  have  here  to  note  is  the  inter- 
dependence and  unification  of  functions  that  naturally  follow 
the  differentiation  of  them. 

§  309.  Something  might  be  added  concerning  the  further 
class  of  integrations  by  which  organisms  are  constituted 
mechanically-coherent  wholes.  Carrying  further  certain  of 
the  arguments  contained  in  the  last  chapter,  it  might  be 
not  unreasonably  inferred  that  the  binding  together  of  parts 
by  bones,  muscles,  and  ligaments,  is  a  secondary  result  of 


PHYSIOLOGICAL  INTEGRATION  IN  ANIMALS.        383 

those  same  actions  by  which  bones,  muscles,  and  ligaments 
are  specialized.  But  adequate  treatment  of  this  division  of 
the  subject  is  at  present  scarcely  possible. 

What  little  of  fact  and  inference  has  been  above  set  down, 
will,  however,  serve  to  make  comprehensible  the  general  truths 
respecting  which,  in  their  main  outlines,  there  can  be  no 
question.  Beginning  with  the  feebly-differentiated  sponge, 
of  which  the  integration  is  also  so  feeble  that  cutting  off  a 
piece  interferes  in  no  appreciable  degree  with  the  activity 
and  growth  of  the  rest,  it  is  undeniable  that  the  advance  is 
through  stages  in  which  the  multiplication  of  unlike  parts 
having  unlike  actions,  is  accompanied  by  an  increasing  inter- 
dependence of  the  parts  and  their  actions ;  until  we  come  to 
structures  like  our  own,  in  which  a  slight  change  initiated  in 
one  part  will  instantly  and  powerfully  affect  all  other  parts — 
will  convulse  an  immense  number  of  muscles,  send  a  wave  of 
contraction  through  all  the  blood-vessels,  awaken  a  crowd  of 
ideas  with  an  accompanying  gush  of  emotions,  affect  the 
action  of  the  lungs,  of  the  stomach,  and  of  all  the  secreting 
organs.  And  while  it  is  a  manifest  necessity  that  along  with 
this  subdivision  of  functions  which  the  higher  organisms  show 
us,  there  must  be  this  close  co-ordination  of  them,  the  fore- 
going paragraphs  suggest  how  this  necessary  correlation  is 
brought  about.  For  a  great  part  of  the  physiological  union 
that  accompanies  the  physiological  specialization,  there  ap- 
pears to  be  a  sufficient  cause  in  the  process  of  direct  equili- 
bration; and  indirect  equilibration  may  be  fairly  presumed  a 
sufficient  cause  for  that  which  remains. 


CHAPTER  X. 

SUMMARY   OF   PHYSIOLOGICAL   DEVELOPMENT. 

§  310.  INTERCOURSE  between  each  part  and  the  particular 
conditions  to  which  it  is  exposed,  either  habitually  in  the 
individual  or  occasionally  in  the  race,  thus  appears  to  be  the 
origin  of  physiological  development ;  as  we  found  it  to  be  the 
origin  of  morphological  development.  The  unlikenesses  of 
form  that  arise  among  members  of  an  aggregate  that  were 
originally  alike,  we  traced  to  unlikenesses  in  the  incident 
forces.  And  in  the  foregoing  chapters  we  have  traced  to 
unlikenesses  in  the  incident  forces,  these  unlikenesses  of 
minute  structure  and  chemical  composition  that  simulta- 
neously arise  among  the  parts. 

In  summing  up  the  special  truths  illustrative  of  this 
general  truth,  it  will  be  proper  here  to  contemplate  more 
especially  their  dependence  on  first  principles.  Dealing  with 
biological  phenomena  as  phenomena  of  evolution,  we  have  to 
interpret  not  only  the  increasing  morphological  heterogeneity 
of  organisms,  but  also  their  increasing  physiological  hetero- 
geneity, in  terms  of  the  re-distribution  of  matter  and  motion. 
While  we  make  our  rapid  re-survey  of  the  facts,  let  us  then 
more  particularly  observe  how  they  are  subordinate  to  the 
universal  course  of  this  re-distribution. 

§  311.  The  instability  of  the  homogeneous,  or,  strictly 
speaking,  the  inevitable  lapse  of  the  more  homogeneous  into 
the  less  homogeneous,  which  we  before  saw  endlessly  exem- 


SUMMARY  OF  PHYSIOLOGICAL  DEVELOPMENT.     385 

plified  by  the  morphological  differentiations  of  the  parts  of 
organisms,  we  have  here  seen  afresh  exemplified  in  ways  also 
countless,  by  the  physiological  differentiations  of  their  parts. 
And  in  the  one  case  as  in  the  other,  this  change  from  uni- 
formity to  multiformity  in  organic  aggregates,  is  caused,  as 
it  is  in  all  inorganic  aggregates,  by  the  necessary  exposure 
of  their  component  parts  to  actions  unlike  in  kind  or  quan- 
tity or  both.  General  proof  of  this  is  furnished  by  the  order 
in  which  the  differences  appear.  If  parts  are  rendered 
physiologically  heterogeneous  by  the  heterogeneity  of  the  in- 
cident forces,  then  the  earliest  contrasts  should  be  between 
parts  that  are  the  most  strongly  contrasted  in  their  relations 
to  incident  forces;  the  next  earliest  contrasts  should  occur 
where  there  are  the  next  strongest  contrasts  in  these  relations ; 
and  so  on.  It  turns  out  that  they  do  so. 

Everywhere  the  differentiation  of  outside  from  inside 
comes  first.  In  the  simplest  plants  the  unlikeness  of  the 
cell- wall  to  the  cell-contents  is  the  conspicuous  trait  of 
structure.  The  contrasts  seen  in  the  simplest  animals  are 
of  the  same  kind:  the  film  that  covers  a  Ehizopod  and  the 
more  indurated  coat  of  an  Infusorian,  are  more  unlike  the 
contained  sarcode  than  the  other  parts  of  this  are  from  one 
another;  and  the  tendency  during  the  life  of  the  animal  is 
for  the  unlikeness  to  become  greater.  What  is  true 

of  Protophyta  and  Protozoa,  is  true  of  the  germs  of  all  organ- 
isms up  to  the  highest :  the  differentiation  of  outer  from  inner 
is  the  first  step.  When  the  protoplasm  of  an  Alga-cell  has 
broken  up  into  the  clusters  of  granules  which  are  eventually 
to  become  spores,  each  of  these  quickly  acquires  a  mem- 
branous coating;  constituting  an  unlikeness  between  surface 
and  centre.  Similarly  with  the  ovule  of  every  higher  plant : 
the  mass  of  cells  forming  it,  early  exhibits  an  outside  layer  of 
cells  distinguished  from  the  cells  within.  With  animal-germs 
it  is  the  same.  Be  it  in  a  ciliated  gemmule,  be  it  in  the 
unfertilized  ova  of  Aphides  and  of  the  Cecidomyia,  or  be  it  in 
true  ova,  the  primary  differentiation  conforms  to  the  rela- 


386  PHYSIOLOGICAL  DEVELOPMENT. 

tions  of  exterior  and  interior.  If  we  turn  to  adult 

organisms,  vegetal  or  animal,  we  see  that  whether  they  do  or 
do  not  display  other  contrasts  of  parts,  they  always  display 
this  contrast.  Though  otherwise  almost  homogeneous,  such 
Fungi  as  the  puff-ball,  or,  among  Algce,  all  which  have  a 
thallus  of  any  thickness,  present  marked  differences  between 
those  of  their  cells  which  are  in  immediate  contact  with  the 
environment  and  those  which  are  not.  Such  differences  they 
present  in  common  with  every  higher  plant;  which,  here  in 
the  shape  of  bark  and  there  in  the  shape  of  cuticle,  has  an 
envelope  inclosing  it  even  up  to  its  petals  and  stamens.  In 
like  manner  among  animals,  there  is  always  either  a  true 
skin  or  an  outer  coat  analogous  to  one.  Wherever  aggregates 
of  the  first  order  have  united  into  aggregates  of  the  second 
and  third  orders — wherever  they  have  become  the  morpho- 
logical units  of  such  higher  aggregates — the  outermost  of 
them  have  grown  unlike  those  lying  within.  Even  the 
Sponge  is  not  without  a  layer  that  may  by  analogy  be  called 
dermal. 

This  lapse  of  the  relatively  homogeneous  into  the  rela- 
tively heterogeneous,  first  showing  itself,  as  on  the  hypothesis 
of  evolution  it  must  do,  by  the  rise  of  an  unlikeness  between 
outside  and  inside,  goes  on  next  to  show  itself,  as  we  infer 
that  it  must  do,  by  the  establishment  of  secondary  contrasts 
among  the  outer  parts  answering  to  secondary  contrasts 
among  the  forces  falling  on  them.  So  long  as  the  whole  sur- 
face of  a  plant  remains  similarly  related  to  the  environment, 
as  in  a  Protococcus,  it  remains  uniform ;  but  when  there  come 
to  be  an  attached  surface  and  a  free  surface,  these,  being  sub- 
ject to  unlike  actions,  are  rendered  unlike.  This  is  visible 
even  in  a  unicellular  Alga  when  it  becomes  fixed;  it  is 
shown  in  the  distinction  between  the  under  and  upper  parts 
of  ordinary  Fungi;  and  we  see  it  in  the  universal  difference 
between  the  imbedded  ends  and  the  exposed  ends  of  the 
higher  plants.  And  then  among  the  less  marked  contrasts 
of  surface  answering  to  the  less  marked  contrasts  in  the  inci- 


SUMMARY  OP  PHYSIOLOGICAL  DEVELOPMENT.     387 

dent  forces,  come  those  between  the  upper  and  under  sides  of 
leaves;  which,  as  we  have  seen,  vary  in  degree  as  the  con- 
trasts of  forces  vary  in  degree,  and  disappear  where  these  con- 
trasts disappear.  Equally  clear  proof  is  furnished 
by  animals,  that  the  original  uniformity  of  surface  lapses  into 
multiformity,  in  proportion  as  the  actions  of  the  environment 
upon  the  surface  become  multiform.  In  a  Worm,  burrowing 
through  damp  soil  which  acts  equally  on  all  its  sides,  or  in  a 
Tcenia,  uniformly  bathed  by  the  contents  of  the  intestine  it 
inhabits,  the  parts  of  the  integument  do  not  appreciably 
differ  from  one  another;  but  in  creatures  not  surrounded  by 
the  same  agencies,  as  those  that  crawl  and  those  that  have 
their  bodies  partially  inclosed,  there  are  unlikenesses  of  in- 
tegument corresponding  to  unlikenesses  of  the  conditions.  A 
snail's  foot  has  an  under  surface  not  uniform  with  the 
exposed  surface  of  its  body,  and  this  again  is  not  uniform 
with  the  protected  surface.  Among  articulate  animals  there 
is  usually  a  distinction  between  the  ventral  and  the  dorsal 
aspects;  and  in  those  of  the  Arthropoda  which  subject  their 
anterior  and  posterior  ends  to  different  environing  agencies, 
as  do  the  ant-lion  and  the  hermit-crab,  these  become  super- 
ficially differentiated.  Analogous  general  contrasts 
occur  among  the  Vertebrata.  Fishes,  though  their  outsides  are 
uniformly  bathed  by  water,  have  their  backs  more  exposed 
to  light  than  their  bellies,  and  the  two  are  commonly  distinct 
in  colour.  When  it  is  not  the  back  and  belly  which  are  thus 
dissimilarly  conditioned,  but  the  sides,  as  in  the  Pleuronectidce, 
then  it  is  the  sides  which  become  contrasted;  and  there  may 
be  significance  in  the  fact  that  those  abnormal  individuals  of 
this  order  which  revert  to  the  ancestral  undistorted  type,  and 
swim  vertically,  have  the  two  sides  alike.  In  such  higher 
vertebrates  as  reptiles,  we  see  repeated  this  differentiation 
of  the  upper  and  under  surfaces :  especially  in  those  of  them 
which,  like  snakes,  expose  these  surfaces  to  the  most  diverse 
actions.  Even  in  birds  and  mammals  which  usually,  by 
raising  the  under  surface  considerably  above  the  ground, 


388  PHYSIOLOGICAL  DEVELOPMENT. 

greatly  diminish  the  contrast  between  its  conditions  and  the 
conditions  to  which  the  upper  surface  is  subject,  there  still 
remains  some  unlikeness  of  clothing  answering  to  the  remain- 
ing unlikeness  between  the  conditions.  Thus,  with- 
out by  any  means  saying  that  all  such  differentiations  are 
directly  caused  by  differences  in  the  actions  of  incident  forces, 
which,  as  before  shown  (§  294),  they  cannot  be,  it  is  clear 
that  many  of  them  are  so  caused.  It  is  clear  that  parts  of 
the  surface  exposed  to  very  unlike  environing  agencies,  become 
very  unlike ;  and  this  is  all  that  needs  to  be  shown. 

Complex  as  are  the  transformations  of  the  inner  parts 
of  organisms  from  the -relatively  homogeneous  into  the  rela- 
tively heterogeneous,  we  still  see  among  them  a  conformity 
to  the  same  general  order.  In  both  plants  and  animals  the 
earlier  internal  differentiations  answer  to  the  stronger  con- 
trasts of  conditions.  Plants,  absorbing  all  their 
nutriment  through  their  outer  surfaces,  are  internally  modi- 
fied mainly  by  the  transfer  of  materials  and  by  mechanical 
stress.  Such  of  them  as  do  not  raise  their  fronds  above  the 
surface,  have  their  inner  tissues  subject  to  no  marked  con- 
trasts save  those  caused  by  currents  of  sap;  and  the  lines 
of  lengthened  and  otherwise  changed  cells  which  are  formed 
where  these  currents  run,  and  are  most  conspicuous  where 
these  currents  must  obviously  be  the  strongest,  are  the  only 
decided  differentiations  of  the  interior.  But  where,  as  in 
the  higher  Cryptogams  and  in  Phaenogams,  the  leaves  are 
upheld,  and  the  supporting  stem  is  transversely  bent  by 
the  wind,  the  inner  tissues,  subject  to  different  amounts  of 
mechanical  strain,  differentiate  accordingly:  the  deposit  of 
dense  substance  commences  in  that  region  where  the  sap- 
containing  cells  and  canals  suffer  the  greatest  intermittent 
compressions.  Animals,  or  at  least  such  of  them 
as  take  food  into  their  interiors,  are  subject  to  forces  of 
another  class  tending  to  destroy  their  original  homogeneity. 
Food  is  a  foreign  substance  which  acts  on  the  interior  as  an 
environing  object  which  touches  it  acts  on  the  exterior — is 


SUMMARY  OF  PHYSIOLOGICAL  DEVELOPMENT.     389 

literally  a  portion  of  the  environment  which,  when  swal- 
lowed, becomes  a  cause  of  internal  differentiations  as  the  rest 
of  the  environment  continues  a  cause  of  external  differentia- 
tions. How  essentially  parallel  are  the  two  sets  of  actions 
and  reactions,  we  have  seen  implied  by  the  primordial  identity 
of  the  endoderm  and  ectoderm  in  simple  animals,  and  of  the 
skin  and  mucous  membrane  in  complex  animals  (§§288,  289). 
Here  we  have  further  to  observe  that  as  food  is  the  original 
source  of  internal  differentiations,  these  may  be  expected  to 
show  themselves  first  where  the  influence  of  the  food  is 
greatest;  and  to  appear  later  in  proportion  as  the  parts  are 
more  removed  from  the  influence  of  the  food.  They  do  this. 
In  animals  of  low  type,  the  coats  of  the  alimentary  cavity  or 
canal  are  more  differentiated  than  the  tissue  which  lies  be- 
tween the  alimentary  canal  and  the  wall  of  the  body.  This  tis- 
suein  the  higher  Ccelenterata, is  a  feebly-organized  parenchyma 
traversed  by  canals  lined  with  simple  ciliated  cells ;  and  in  the 
lower  Mollusca  the  structures  bounding  the  perivisceral  cavity 
and  its  ramifying  sinuses,  are  similarly  imperfect.  Further, 
it  is  observable  that  the  differentiation  of  this  perivisceral 
sac  and  its  sinuses  into  a  vascular  system,  proceeds  centri- 
fugally  from  the  region  where  the  absorbed  nutriment  enters 
the  mass  of  circulating  liquid,  and  where  this  liquid  is  quali- 
tatively more  unlike  the  tissues  than  it  is  at  the  remoter 
parts  of  the  body. 

Physiological  development,  then,  is  initiated  by  that  in- 
stability of  the  homogeneous  which  we  have  seen  to  be  every- 
where a  cause  of  evolution  (First  Principles,  §§  149 — 155). 
That  the  passage  from  comparative  uniformity  of  composi- 
tion and  minute  structure  to  comparative  multiformity,  is  set 
up  in  organic  aggregates,  as  in  all  other  aggregates,  by  the 
necessary  unlikenesses  of  the  actions  to  which  the  parts  are 
subject,  is  shown  by  the  universal  rise  of  the  primary  differen- 
tiation into  the  parts  that  are  universally  most  contrasted  in 
their  circumstances,  and  by  the  rise  of  secondary  differen- 


390  PHYSIOLOGICAL  DEVELOPMENT. 

tiations  obviously  related  in  their  order  to  secondary  contrasts 
of  conditions. 


§  312.  How  physiological  development  has  all  along  been 
aided  by  the  multiplication  of  effects — how  each  differen- 
tiation has  ever  tended  to  become  the  parent  of  new  differen- 
tiations, we  have  had,  incidentally,  various  illustrations.  Let 
us  here  review  the  working  of  this  cause. 

Among  plants  we  see  it  in  the  production  of  progressively- 
multiplying  heterogeneities  of  tissue  by  progressive  increase 
of  bulk.  The  integration  of  fronds  into  axes  and  of  axes  into 
groups  of  axes,  sets  up  unlikenesses  of  action  among  the 
integrated  units,  followed  by  unlikenesses  of  minute  struc- 
ture. Each  gust  transversely  strains  the  various  parts  of  the 
stem  in  various  degrees,  and  longitudinally  strains  in  various 
degrees  the  roots;  and  while  there  is  inequality  of  stress  at 
every  place  in  stem  and  branch,  so,  at  every  place  in  stem 
and  branch,  the  outer  layers  and  the  successively  inner  layers 
are  severally  extended  and  compressed  to  unequal  amounts, 
and  have  unequal  modifications  wrought  in  them.  Let  the 
tree  add  to  its  periphery  another  generation  of  the  units 
composing  it,  and  immediately  the  mechanical  strains  on  the 
supporting  parts  are  all  changed  in  different  degrees,  initiat- 
ing new  differences  internally.  Externally,  too,  new  differ- 
ences are  initiated.  Shaded  by  the  leaf-bearing  outer  stratum 
of  shoots,  the  inner  structures  cease  to  bear  leaves,  or  to  put 
out  shoots  which  bear  leaves ;  and  instead  of  that  green  cover- 
ing which  they  originally  had,  become  covered  with  bark  of 
increasing  thickness.  Manifestly,  then,  the  larger  integration 
of  units  that  are  originally  simple  and  uniform,  entails 
physiological  changes  of  various  orders,  varying  in  their 
degrees  at  all  parts  of  the  aggregate.  Each  branch  which, 
favourably  circumstanced,  flourishes  more  than  its  neigh- 
bours, becomes  a  cause  of  physiological  differentiations,  not 
only  in  its  neighbours  from  which  it  abstracts  sap  and  pres- 


SUMMARY  OF  PHYSIOLOGICAL  DEVELOPMENT.     391 

ently  turns  from  leaf-bearers  into  fruit-bearers,  but  also  in 
the  remoter  parts. 

That  among  animals  physiological  development  is  fur- 
thered by  the  multiplication  of  effects,  we  have  lately  seen 
proved  by  the  many  changes  in  other  organs,  which  the 
growth  or  modification  of  each  excreting  and  secreting  organ 
initiates.  By  the  abstracted  as  well  as  by  the  added  mate- 
rials, it  alters  the  quality  of  the  blood  passing  through  all 
members  of  the  body;  or  by  the  liquid  it  pours  into  the 
alimentary  canal,  it  acts  on  the  food,  and  through  it  on  the 
blood,  and  through  it  on  the  system  as  a  whole:  an  addi- 
tional differentiation  in  one  part  thus  setting  up  additional 
differentiations  in  many  other  parts;  from  each  of  which, 
again,  secondary  differentiating  forces  reverberate  through 
the  organism.  Or,  to  take  an  influence  of  another  order,  we 
have  seen  how  the  modified  mechanical  action  of  any  member 
not  only  modifies  that  member,  but  becomes,  by  its  reactions, 
a  cause  of  secondary  modifications — how,  for  example,  the 
burrowing  habits  of  the  common  mole,  leading  to  an  almost 
exclusive  use  of  the  fore  limbs,  have  entailed  a  dwindling 
of  the  hind  limbs,  and  a  concomitant  dwindling  of  the 
pelvis,  which,  becoming  too  small  for  the  passage  of  the 
young,  has  initiated  still  more  anomalous  modifications. 

So  that  throughout  physiological  development,  as  in  evo- 
lution at  large,  the  multiplication  of  effects  has  been  a  factor 
constantly  at  work,  and  working  more  actively  as  the  develop- 
ment, has  advanced.  The  secondary  changes  wrought  by 
each  primary  change,  have  necessarily  become  more  numerous 
in  proportion  as  organisms  have  become  more  complex.  And 
every  increased  multiplication  of  effects,  further  differen- 
tiating the  organism  and,  by  consequence,  further  integrating 
it,  has  prepared  the  way  for  still  higher  differentiations  and 
integrations  similarly  caused. 

§  313.  The  general  truth  next  to  be  resumed,  is  that  these 
processes  have  for  their  limit  a  state  of  equilibrium — proxi- 


392  PHYSIOLOGICAL  DEVELOPMENT. 

mately  a  moving  equilibrium  and  ultimately  a  complete 
equilibrium.  The  changes  we  have  contemplated  are  but  the 
concomitants  of  a  progressing  equilibration.  In  every  aggre- 
gate which  we  call  living,  as  well  as  in  all  other  aggregates, 
the  instability  of  the  homogeneous  is  but  another  name  for 
the  absence  of  balance  between  the  incident  forces  and  the 
forces  which  the  aggregate  opposes  to  them ;  and  the  passage 
into  heterogeneity  is  the  passage  towards  a  state  of  balance. 
And  to  say  that  in  every  aggregate,  organic  or  other,  there 
goes  on  a  multiplication  of  effects,  is  but  to  say  that  one  part 
which  has  a  fresh  force  impressed  on  it,  must  go  on  changing 
and  communicating  secondary  changes,  until  the  whole  of  the 
impressed  force  has  been  used  up  in  generating  equivalent 
reactive  forces. 

The  principle  that  whatever  new  action  an  organism  is 
subject  to,  must  either  overthrow  the  moving  equilibrium  of 
its  functions  and  cause  the  sudden  equilibration  called  death, 
or  else  must  progressively  alter  the  organic  rhythms  until, 
by  the  establishment  of  a  new  reaction  balancing  the  new 
action  a  new  moving  equilibrium  is  produced,  applies  as 
much  to  each  member  of  an  organism  as  to  the  organism  in 
its  totality.  Any  force  falling  on  any  part  not  adapted  to 
bear  it,  must  either  cause  local  destruction  of  tissue,  or  must, 
without  destroying  the  tissue,  continue  to  change  it  until  it 
can  change  it  no  further ;  that  is — until  the  modified  reaction 
of  the  part  has  become  equal  to  the  modified  action.  What- 
ever the  nature  of  the  force  this  must  happen.  If  it  is  a 
mechanical  force,  then  the  immediate  effect  is  some  distortion 
of  the  part — a  distortion  having  for  its  limit  that  attitude 
in  which  the  resistance  of  the  structures  to  further  change  of 
position,  balances  the  force  tending  to  produce  the  further 
change ;  and  the  ultimate  effect,  supposing  the  force  to  be  con- 
tinuous or  recurrent,  is  such  a  permanent  alteration  of  form, 
or  alteration  of  structure,  or  both,  as  establishes  a  permanent 
balance.  If  the  force  is  physico-chemical,  or  chemical,  the 
general  result  is  still  the  same:  the  component  molecules  of 


SUMMARY  OF  PHYSIOLOGICAL  DEVELOPMENT.     393 

the  tissue  must  have  their  molecular  arrangements  changed, 
and  the  change  in  their  molecular  arrangements  must  go  on 
until  their  molecular  motions  are  so  re-adjusted  as  to  equili- 
brate the  molecular  motions  of  the  new  physico-chemical  or 
chemical  agent.  In  other  words,  the  organic  matter  com- 
posing the  part,  if  it  continues  to  be  organic  matter  at  all, 
must  assume  that  molecular  composition  which  enables  it  to 
bear,  or  as  we  say  adapts  it  to,  the  incident  forces. 

NOT  is  it  less  certain  that  throughout  the  organism  as  a 
whole,  equilibration  is  alike  the  proximate  limit  of  the  changes 
wrought  by  each  action,  as  well  as  the  ultimate  limit  of  the 
changes  wrought  by  any  recurrent  actions  or  continuous 
action.  The  movements  every  instant  going  on,  are  move- 
ments towards  a  new  state  of  equilibrium.  Eaising  a  limb 
causes  a  simultaneous  shifting  of  the  centre  of  gravity,  and 
such  altered  tensions  and  pressures  throughout  the  body  as 
re-adjust  the  disturbed  balance.  Passage  of  liquid  into  or 
out  of  a  tissue,  implies  some  excess  of  force  in  one  direction 
there  at  work ;  and  ceases  only  when  the  force  so  diminishes  or 
the  counter-forces  so  increase  that  the  excess  disappears.  A 
nervous  discharge  is  reflected  and  re-reflected  from  part  to 
part,  until  it  has  all  been  used  up  in  the  re-arrangements  pro- 
duced— equilibrated  by  the  reactions  called  out.  And  what 
is  thus  obviously  true  of  every  normal  change,  is  equally  true 
of  every  abnormal  change — every  disturbance  of  the  estab- 
lished rhythm  of  the  functions.  If  such  disturbance  is  a 
single  one,  the  perturbations  set  up  by  it,  reverberating 
throughout  the  system,  leave  its  moving  equilibrium  slightly 
altered.  If  the  disturbance  is  repeated  or  persistent,  its  suc- 
cessive effects  accumulate  until  they  have  produced  a  new 
moving  equilibrium  adjusted  to  the  new  force. 

Each  re-balancing  of  actions,  having  for  its  necessary  con- 
comitant a  modification  of  tissues,  it  is  an  obvious  corollary 
that  organisms  subjected  to  successive  changes  of  conditions, 
must  undergo  successive  differentiations  and  re-differentia- 
tions. Direct  equilibration  in  organisms,  with  all  its  accom- 


394  PHYSIOLOGICAL  DEVELOPMENT. 

panying  structural  alterations,  is  as  certain  as  is  that  uni- 
versal progress  towards  equilibrium  of  which  it  forms  part. 
And  just  as  certain  is  that  indirect  equilibration  in  organisms 
to  which  the  remaining  large  class  of  differentiations  is  due. 
The  development  of  favourable  variations  by  the  killing  of 
individuals  in  which  they  do  not  occur  or  are  least  marked, 
is,  as  before,  a  balancing  between  certain  local  structures  and 
the  forces  they  are  exposed  to ;  and  is  no  less  inevitable  than 
the  other. 

§  314.  In  all  which  universal  laws,  we  find  ourselves  again 
brought  down  to  the  persistence  of  force,  as  the  deepest 
knowable  cause  of  those  modifications  which  constitute 
physiological  development;  as  it  is  the  deepest  knowable 
cause  of  all  other  evolution.  Here,  as  elsewhere,  the  per- 
petual lapse  from  less  to  greater  heterogeneity,  the  perpetual 
begetting  of  secondary  modifications  by  each  primary  modi- 
fication, and  the  perpetual  approach  to  a  temporary  balance 
on  the  way  towards  a  final  balance,  are  necessary  implica- 
tions of  the  ultimate  fact  that  force  cannot  disappear  but  can 
only  change  its  form. 

It  is  an  unquestionable  deduction  from  the  persistence  of 
force,  that  in  every  individual  organism  each  new  incident 
force  must  work  its  equivalent  of  change;  and  that  where  it 
is  a  constant  or  recurrent  force,  the  limit  of  the  change  it 
works  must  be  an  adaptation  of  structure  such  as  opposes  to 
the  new  outer  force  an  equal  inner  force.  The  only  thing 
open  to  question  is,  whether  such  re-adjustment  is  inherit- 
able; and  further  consideration  will,  I  think,  show,  that  to 
say  it  is  not  inheritable  is  indirectly  to  say  that  force  does 
not  persist.  If  all  parts  of  an  organism  have  their  func- 
tions co-ordinated  into  a  moving  equilibrium,  such  that  every 
part  perpetually  influences  all  other  parts,  and  cannot  be 
changed  without  initiating  changes  in  all  other  parts — if  the 
limit  of  change  is  the  establishment  of  a  complete  harmony 
among  the  movements,  molecular  and  other,  of  all  parts ;  then 


SUMMARY  OP  PHYSIOLOGICAL  DEVELOPMENT.     395 

among  other  parts  that  are  modified,  molecularly  or  other- 
wise, must  be  those  which  cast  off  the  germs  of  new  organ- 
isms. The  molecules  of  their  produced  germs  must  tend 
ever  to  conform  the  motions  of  their  components,  and  there- 
fore the  arrangements  of  their  components,  to  the  molecular 
forces  of  the  organism  as  a  whole;  and  if  this  aggregate 
of  molecular  forces  be  modified  in  its  distribution  by  a  local 
change  of  structure,  the  molecules  of  the  germs  must  be 
gradually  changed  in  the  motions  and  arrangements  of  their 
components,  until  they  are  re-adjusted  to  the  aggregate  of 
molecular  forces. 


CHAPTER  XA. 

THE    INTEGRATION    OF   THE   ORGANIC    WORLD. 

§  314a.  THAT  from  the  beginning  of  life  there  has  been 
an  ever-increasing  heterogeneity  in  the  Earth's  Flora  and 
Fauna,  is  a  truth  recognized  by  all  biologists  who  accept  the 
doctrine  of  evolution.  In  discussing  the  origin  of  species 
Mr.  Darwin  and  others  have  been  mainly  occupied  in  ex- 
plaining the  genesis  of  now  this  and  now  that  form  of 
organism,  considered  as  a  member  of  one  or  other  series,  and 
regarded  as  becoming  differentiated  from  its  allies.  But  by 
implication,  if  not  avowedly,  there  has  been  simultaneously 
accepted  the  belief  that  the  forms  continually  produced  by 
divergences  and  re-divergences,  have  constituted  an  assem- 
blage increasingly  multiform  in  its  included  kinds.  And 
this,  which  we  are  shown  by  the  process  of  organic  evolution 
as  followed  out  in  its  details,  is  a  corollary  from  the  doctrine 
of  evolution  at  large,  as  was  pointed  out  in  §  159  of  First 
Principles. 

Meanwhile  there  has  been  little  if  any  recognition  of  an 
accompanying  change,  no  less  fundamental.  In  the  general 
transformation  which  constitutes  Evolution,  differentiation 
and  integration  advance  hand  in  hand;  so  that  along  with 
the  production  of  unlike  parts  there  progresses  the  union  of 
these  unlike  parts  into  a  whole.  Examples  of  various  kinds 
before  given  will  recur  to  the  reader,  and  an  addition  to  them 
has  just  been  set  forth  in  the  chapter  on  "  Physiological 
Integration."  One  more  example,  world-wide  in  its  reach, 
has  still  to  be  named. 


THE  INTEGRATION  OF  THE  ORGANIC  WORLD.     397 

For  here  it  remains  to  point  out  that  along  with  the  in- 
creasing multiplication  of  types  of  organisms  covering  the 
Earth's  surface,  there  has  been  ever  going  on  an  increasing 
mutual  dependence  of  them — an  increasing  integration  of 
the  entire  aggregate  of  living  things. 

Many  facts  which  are  obvious  and  many  which  are  quite 
familiar  will  be  named  as  evidence.  But  I  must  be  excused 
for  reminding  the  reader  of  things  that  he  knows  and  things 
that  he  may  easily  observe,  since,  unless  the  evidence,  trite 
as  it  may  be,  is  gathered  together  and  properly  marshalled, 
the  generalization  enunciated  will  not  be  thought  valid. 

§  3146.  Eespecting  the  physiological  characters  of  the 
earliest  forms  there  is  an  assumption  from  which  no  escape 
seems  possible — the  assumption  that  they  united  animal  and 
vegetal  characters.  Even  among  existing  microscopic  types 
of  the  lowest  classes,  there  is  such  community  of  plant- 
traits  and  animal-traits  that  doubts  respecting  their  proper 
places  in  one  or  the  other  kingdom  are  continually  raised 
— doubts,  too,  whether,  if  regarded  as  vegetal,  they  are  to  be 
grouped  as  algoid  or  fungoid. 

Here,  however,  without  entering  on  moot  questions,  we 
may  draw  the  a  priori  conclusion  that  these  earliest  living 
things  were  double-natured,  in  so  far  that  they  must  have 
had  the  ability  to  assimilate  from  the  inorganic  world  all  the 
materials  of  which  protoplasm  consists — must  therefore, 
along  with  the  power  of  appropriating  carbon  from  its  gase- 
ous compound,  also  have  had  the  power  of  appropriating  ni- 
trogen, either  from  one  of  its  combined  oxides  or  directly  from 
the  air  with  which  water  is  more  or  less  charged.  For  before 
organic  substances  existed  there  could  have  been  none  but 
inorganic  sources  from  which  nitrogen  could  be  obtained. 

This  conclusion  concerns  us  only  because  it  implies 
homogeneity  of  nature  in  these  primordial  forms  of  life. 
There  could  not  at  first  have  existed  among  these  minutest 
of  Protozoa  even  such  vague  distinctions  as  are  now  presented 


398  PHYSIOLOGICAL  DEVELOPMENT. 

in  a  shadowy  way  by  their  modern  representatives.  And  the 
implication  is  that  during  the  period  throughout  which  these 
smallest,  lowest,  and  simplest  living  things  alone  existed, 
there  could  have  been,  in  the  absence  of  kinds,  no  mutual 
dependence. 

Since,  among  various  of  the  lowest  types  now  known  to 
us,  the  same  individual  exhibits  a  life  which  is  now  pre- 
dominantly vegetal  and  now  predominantly  animal,  we 
cannot  err  in  assuming  that  there  eventually  took  place 
differentiations  of  this  original  plant-animal  type  into  types 
permanently  unlike:  some  in  which  the  traits  were  more 
markedly  vegetal  and  others  in  which  they  were  more 
markedly  animal.  As  fast  as  this  differentiation  arose, 
there  came  the  beginnings  of  cooperation  between  the  pre- 
dominantly vegetal  types  which  by  the  aid  of  light  formed 
organic  matter  from  the  inorganic  world,  and  the  predomi- 
nantly animal  types  which,  in  chief  measure,  utilized  the 
matter  so  formed.  Evidently  with  the  rise  of  such  a  dif- 
ferentiation came  an  incipient  mutual  dependence.  If  to 
the  implied  algoid  type  and  the  animal  type  there  be  added 
the  fungoid  type,  somewhat  intermediate  in  character,  which 
in  a  large  proportion  of  cases  lives  on  the  decaying  remnants 
of  the  other  two,  we  are  furnished  with  a  rude  conception  of 
the  primary  differentiations  and  the  accompanying  vague 
mutual  dependences.  ( 

Speculation  aside, it  suffices  to  say  that  early  in  the  history 
of  life  there  must  have  arisen  the  distinction  between  Pro- 
tozoa and  Protophyta,  and  that  this  distinction  foreshadowed 
that  widest  contrast  which  the  higher  organic  world  presents 
— the  contrast  between  plants  and  animals.  It  is  needless 
to  do  more  than  name  the  mutual  dependence  between  these 
two  great  divisions.  That,  as  being  respectively  decomposers 
of  carbon  dioxide  and  exhalers  of  carbon  dioxide,  they  act 
reciprocally,  as  also  in  some  measure  by  interchange  of  nitro- 
genous matters;  and  that  the  implied  general  cooperation 
serves  in  an  indirect  way  to  unite  their  lives,  and  in  that 


THE  INTEGRATION   OF  THE  ORGANIC  WORLD.      399 

sense  to  integrate  the  two  kingdoms ;  needs  not  to  be  insisted 
upon.  Further  complications  of  the  mutual  dependence  will 
be  mentioned  by  and  by.  For  the  present  it  suffices  to 
recognize  this  division  of  organic  functions  as  the  first  which 
arose  and  as  continuing  to  be  that  fundamental  one  which 
more  than  all  others  binds  organisms  at  large  together. 

§  314c.  It  will  be  thought  by  many  readers  that  in  speak- 
ing of  the  contrasted  vital  activities  of  plants  and  animals  as 
constituting  a  "division  of  organic  functions,"  I  am  straining 
words  beyond  their  meanings ;  since  the  conception  of  organic 
functions  postulates  an  organized  whole  in  which  they  exist, 
and  plants  and  animals  constitute  no  such  organized  whole. 
But  there  is  at  hand  an  unexpected  defence  for  this  concep- 
tion— a  defence  not  forthcoming  a  generation  ago,  but  which 
now  all  biologists  will  recognize  as  relevant.  I  refer  to  the 
phenomena  of  symbiosis.  These  present  various  cases  in 
which  the  plant-function  and  the  animal-function  are  carried 
on  in  the  same  body, — cases  in  which  the  cooperation  is  not 
between  separate  vegetal  organisms  which  accumulate  nutri- 
tive matters  and  separate  animal  organisms  which  consume 
them,  but  is  a  cooperation  between  vegetal  elements  and 
animal  elements  forming  parts  of  the  same  organism. 

As  introductory  to  examples  of  these  must  first,  however, 
be  named  an  example  of  such  cooperation  between  the  two 
great  classes  of  vegetal  organisms — the  fungoid  and  the  al- 
goid.  Incredible  as  the  statement  once  seemed,  it  is  a  state- 
ment now  accepted,  that  what  we  know  as  lichens,  and  used  to 
consider  as  plants  forming  a  certain  low  class,  are  now  found 
to  be  not  plants  in  the  ordinary  sense  at  all,  but  compound 
growths  formed  of  minute  algae  and  minute  fungi,  carrying 
on  their  lives  together:  the  algoa  furnishing  to  the  fungi 
certain  constituents  they  need  but  cannot  directly  obtain, 
and  the  fungi  profiting  by  certain  materials  they  obtain  from 
the  algae,  either  while  living  or  while  individually  decaying. 
Whence  it  would  seem  that  after  the  microscopic  vegetal 


400  PHYSIOLOGICAL  DEVELOPMENT. 

type  had  become  in  a  large  degree  differentiated  into  two 
main  types,  in  adaptation  to  different  conditions  of  life,  and 
had  acquired  appropriate  specialities  of  nature,  there  grew  up 
this  communistic  arrangement  between  certain  of  them,  en- 
abling each  to  benefit  by  the  powers  which  the  other  had 
acquired:  evidently  an  exchange  of  services,  a  physiological 
division  of  labour,  a  mutual  dependence  -of  functions  analo- 
gous to  that  which  exists  between  functions  in  an  ordinary 
plant  or  animal. 

Not  differing  in  principle  but  only  in  application,  is  that 
symbiosis  above  referred  to  as  existing  between  Protophyta 
and  many  Protozoa,  as  well  as  between  such  Protophyta 
and  the  lowest  kinds  of  Metazoa.  A  recent  statement  that 
certain  amoebae,  made  green  by  contained  chlorophyll,  con- 
tinue to  grow  and  multiply  after  they  have  consumed  what 
nutritive  matter  may  be  at  hand,  is  in  harmony  with  various 
facts  alleged  of  other  Protozoa — various  other  kinds  of  Khizo- 
pods,  various  Heliozoa,  numerous  ciliated  and  flagellated 
Infusoria.  Among  Metazoa  the  like  association  occurs  in 
one  of  the  sponges,  in  the  Hydra  viridis,  in  various  turbel- 
larians,  in  a  rotifer,  and  even  in  two  molluscs.  In  these 
cases  the  partnership  between  the  vegetal  cells  and  the 
animal  cells  (existing  either  as  units  or  as  an  organized  group 
such  as  a  polype),  is  a  partnership  which,  as  before,  profits 
each  of  the  partners — an  inference  supported  by  the  fact  that 
Metazoa  containing  these  algoid  cells  usually  place  them- 
selves where  the  light  falls  upon  them,  and  can  therefore 
further  the  production  of  the  carbo-hydrates  which  event- 
ually become  useful  to  the  animal-cells,  while  these  in  some 
way  reciprocate  the  benefit. 

Here,  then,  we  have  exchange  of  services  between  asso- 
ciated plant-elements  and  animal-elements — a  performance 
by  them  of  different  organic  functions  for  the  benefit  of  the 
aggregate  which  they  unite  to  form.  Hence,  when  these 
vegetal  elements  and  animal  elements  are  separately  em- 
bodied in  plants  and  animals,  which  profit  by  one  another, 


THE  INTEGRATION  OF  THE  ORGANIC  WORLD.      401 

we  may  still  properly  regard  their  respective  lives  as  mutually- 
dependent  organic  functions,  as  said  in  the  preceding  section. 
We  are  enabled  the  better  to  see  how  the  Earth's  Flora  and 
Fauna,  which  are  respectively  accumulators  of  motion  and 
expenders  of  motion,  form  mutually-dependent  parts  of  a 
whole,  and  are  in  that  sense  integrated.  And  we  shall  be 
prepared  to  see  how  all  other  relations  between  organisms 
which  make  them  subservient  one  to  another,  similarly  con- 
stitute elements  in  a  general  integration  of  the  organic  world. 

§  314J.  Another  form  of  mutual  dependence  and  conse- 
quently of  integration  is  conspicuous — that  which  accom- 
panied the  progressive  increase  of  size  in  organisms  of  the 
higher  classes.  We  have  but  to  contemplate  the  possibilities 
to  see  that  life  must  necessarily  have  commenced  with  minute 
forms,  and  that  the  progress  to  larger  ones  must  have  been 
by  small  steps. 

For  had  creatures  of  appreciable  sizes  been  the  first  to 
exist  they  would  inevitably  have  disappeared  from  lack  of 
food.  Having  no  resource  but  to  devour  one  another,  they 
would  quickly  have  brought  life  to  an  end.  There  must  have 
been  smaller  types  serving  as  prey  for  larger  ones  before 
these  could  continue  to  exist  and  to  multiply:  microbes 
affording  food  to  infusoria,  infusoria  affording  food  to  such 
sized  creatures  as  the  Entomostraca,  these  again  supplying 
food  to  small  fishes,  such  as  loch-trout,  and  these  last  yielding 
to  larger  fishes  masses  sufficiently  great  for  their  needs :  each 
higher  grade  requiring  lower  grades  of  appropriate  bulk.  It 
needs  but  to  ask  what  would  become  of  tigers  if  there  were 
no  mammals  larger  than  mice,  to  see  that  the  animal  world  is 
a  linked  assemblage,  of  which  the  connected  members  stand 
within  certain  ratios  of  mass ;  and  that  during  the  evolution 
of  higher  and  larger  types  the  linking  of  grades  has  become 
closer. 

That  among  plants  considered  as  an  aggregate  relations 
of  like  kind,  though  far  less  distinct  ones,  have  all  along 

72 


402  PHYSIOLOGICAL  DEVELOPMENT. 

been  growing  may  be  reasonably  concluded.  In  a  world 
peopled  only  by  microscopic  types  there  could  not  have 
existed  the  conditions  needful  for  large  trees.  Gradual  dis- 
integration of  rock-surfaces,  partly  effected  by  physical 
agencies  and  partly  by  low  forms  of  plants,  had  to  prepare 
the  way  for  superior  plants.  The  production  of  sufficient 
soil  by  mineralogical  decay  as  well  as  by  the  decay  of 
organisms,  plant  and  animal,  may  be  regarded  as  having 
been  a  preliminary  to  larger  plant-growth;  and  though  at 
present  the  dependence  is  far  less  close  than  that  among 
animals,  yet  the  benefits  yielded  to  metaphytes  by  the  de- 
composing actions  carried  on  by  protophytes,  as  well  as  those 
carried  on  by  microbes  permeating  the  soil,  imply  a  con- 
tinued general  interdependence  throughout  the  aggregate  of 
plant-forms,  apart  from  more  special  interdependences.  And  . 
then  along  with  this  indebtedness  of  the  greater  plants  to 
the  smaller  during  the  process  of  evolution,  there  must  be 
named  that  indebtedness  of  plant-life  to  animal-life  which 
Mr.  Darwin  has  shown  in  his  book  on  the  agency  of  worms 
as  producers  of  mould. 

§  314e.  Services  of  one  to  another,  and  consequent  unions, 
of  more  special  kinds  are  infinitely  varied,  alike  within  each 
kingdom  and  between  the  two  kingdoms.  I  refer  to  those 
seen  in  parasitism,  commensalism,  and  other  forms  of  asso- 
ciation. While  they  do  not  conduce  to  unions  of  the  kind 
thus  far  considered,  these  nevertheless  constitute  innumer- 
able links  whereby  the  lives  of  organisms,  plant  and  animal, 
are  tied  together;  sometimes  for  the  advantage  of  both  but 
in  most  cases  for  the  benefit  of  one  to  the  injury  of  the  other. 

Among  plants  the  degrees  of  dependence  are  various.  Un- 
able to  raise  themselves  into  the  air  and  light,  some  climb, 
like  the  ivy,  by  modified  rootlets,  or  spirally  coil  themselves, 
or  hang  by  tendrils.  Others  there  are  which  gradually 
strangle  the  trees  they  embrace,  or  which,  like  lichens  in 
damp  climates,  festooning  the  smaller  trees,  by  and  by  cause 


THE  INTEGRATION  OF  THE  ORGANIC  WORLD.      403 

their  decay.  Of  higher  types  of  epiphytes  which  use  trees 
only  to  gain  elevation,  the  orchids  may  be  instanced.  And 
then  we  have  plants  which,  like  the  mistletoe,  fix  themselves 
on  the  bark  of  their  hosts,  utilizing  them  partly  for  purposes 
of  elevation  and  partly  by  appropriation  of  their  juices.  After 
these  may  be  named  those  extreme  cases  in  which  the  para- 
sitic plants,  ceasing  to  have  any  chlorophyll-bearing  leaves, 
live  wholly  on  the  juices  of  the  invaded  plants.  At  home 
the  common  dodder,  and  in  the  tropics  the  Rafflesiacece, 
belong  to  this  group.  There  must  be  added  the  numerous 
forms  of  minute  fungi  which  in  like  manner  thrive  at  the 
expense  of  the  plants  they  infest.  In  all  these  cases  the 
interdependence  is  one-sided,  though,  as  we  shall  presently 
see,  while  detrimental  to  one  of  the  two  concerned,  it  is  not 
always  detrimental  to  the  organic  world  as  a  whole. 

That  utilization  of  one  by  another  among  animals  which 
causes  immediate  death,  is  familiar  enough  in  the  relations 
between  carnivores  and  herbivores.  Almost  as  familiar  are 
those  seen  in  parasitism.  Less  familiar  are  those  seen  in 
commensalism ;  and  the  least  familiar  are  those  which  show 
us  exchange  of  services.  Among  these  last — the  mutually- 
beneficial  relations — that  between  the  crocodile  and  the  bird 
which  picks  parasites  out  of  its  teeth  is  a  striking  one;  and 
no  less  so  is  that  of  the  pique-gouffe,  an  African  bird  which 
pierces  the  tumour  on  a  buffalo's  back  that  incloses  a  para- 
site. Then  of  another  kind  we  have  the  connexion  between 
aphides  and  ants :  the  one  profiting  by  being  carried  to 
better  pastures  and  the  other  by  increased  saccharine  excre- 
tion. Next  comes  the  class  of  messmates,  the  connexions 
between  some  of  which  are  relatively  innocent,  as  witness 
the  Sea-anemone  which  settles  itself  on  the  shell  occupied 
by  a  Hermit-crab,  or  as  witness  the  Remora  fixed  on  a 
shark's  skin.  Less  innocent  is  the  relation  under  which  one 
of  the  two  seizes  a  share  of  the  food  obtained  by  the  other, 
like  the  annelid  which  insinuates  itself  between  the  Hermit- 
crab  and  the  whelk-shell  it  inhabits,  or  like  the  small  fishes 


404  PHYSIOLOGICAL  DEVELOPMENT. 

inhabiting  certain  Medusce,  or  those  which  nestle  in  the 
branchial  sac  of  the  Lophius.  After  these  may  be  named  the 
less  injurious  forms  of  parasites  proper — those  which,  dis- 
tinguished as  Epizoa,  fix  themselves  on  the  skins  of  their 
hosts,  permanently  or  temporarily,  such  as,  of  the  one  kind, 
the  LerncEd  on  fishes,  and  of  the  other  kind  the  Tick  on 
mammals  and  birds.  Then  there  come  the  other  class  of 
parasites,  most  of  them  highly  injurious,  distinguished  as 
Entozoa,  living  within  the  bodies  of  their  hosts,  now  in  parts 
of  their  alimentary  canals,  now  on  other  of  their  mucous 
surfaces,  and  now  in  various  of  their  organs :  these  last  two 
groups  being  so  numerous  in  their  kinds  that  there  are 
commonly  more  species  than  one  proper  to  each  larger  animal. 
One  stage  further  in  the  complication  meets  us  in  the  para- 
sites upon  parasites. 

But  now  the  general  fact,  to  which  these  brief  indications 
are  introductory,  is  that  «the  use  made  of  one  organism  by 
another  has  been  ever  widening  and  becoming  more  involved. 
Among  plants  utilization  of  the  larger  by  the  smaller — of 
trees  by  epiphytes  and  parasites — must  have  arisen  since  the 
times  when  the  larger  came  into  existence — times  relatively 
late  in  the  course  of  organic  evolution.  Moreover  most  of 
the  plants  which  utilize  others,  either  by  climbing  up  them  or 
settling  themselves  high  up  on  their  stems  or  sucking  their 
juices,  are  phaenogams,  and  the  plants  they  utilize  are  also 
phsenogams;  so  that  these  innumerable  interdependences 
must  have  been  established  since  the  phasnogamic  type  has 
become  so  predominant  in  respect  of  both  size  and  kind. 
Similarly  among  animals.  Though  there  are  many  parasites 
belonging,  like  the  Trematodes,  to  very  low  classes,  there  are 
many  which  belong  to  the  Arthropoda,  and,  being  degraded 
forms  of  that  class,  must  have  come  into  existence  after 
Arthropods  of  considerable  structure  had  been  evolved. 
Again,  a  large  part  of  the  animals  infested  by  Epizoa  and 
Entozoa  are  vertebrates — many  of  the  highest  types;  and  as 
these  are  relatively  modern  all  this  parasitism  must  be  of 


THE  INTEGRATION  OP  THE  ORGANIC  WORLD.      405 

late  date.  So,  too,  of  much  commensalism  and  many 
mutually-beneficial  associations.  The  reciprocal  services  of 
ants  and  aphides  must  have  originated  since  the  Hymenoptera 
and  Hemiptera  became  established  types,  and  since  the  days 
when  certain  insects  of  the  ant-type  had  become  social,  and 
since  the  days  when  aphides  had  become  degraded  members 
of  their  order :  both  dates  being  relatively  recent.  And  still 
more  recent  must  have  been  the  commensalism  between  the 
ants  and  the  many  species  of  other  insects  which  inhabit 
their  nests. 

Leaving  out  relations  of  the  kinds  just  named,  it  seems 
that  down  from  those  between  carnivores  and  their  prey  to 
those  between  lice  and  their  hosts,  such  relations  profit  one 
of  the  two  species  concerned  and  injure  the  other,  and  that 
there  the  matter  ends.  But  it  does  not  end  there;  for  that 
multiplication  of  effects  to  which  people  are  usually  blind, 
brings  about  changes  which,  as  hinted  above,  though  inju- 
rious to  the  individual  are  beneficial  to  the  species,  and 
which,  when  not  beneficial  to  the  species,  are  often  beneficial 
to  the  aggregate  of  species. 

Even  where  animals  of  one  class  live  by  devouring  animals 
of  another  class,  we  see,  on  looking  beyond  the  immediate 
results,  certain  remote  results  that  are  advantageous.  In 
the  first  place  the  process  is  one  by  which  inferior  individuals 
— the  least  agile,  swift,  strong,  or  sagacious — are  picked  out 
and  prevented  from  leaving  posterity  and  lowering  the 
average  quality  of  their  kind.  At  the  same  time  individuals 
made  feeble  by  injury  or  old  age,  are  among  those  to  be 
killed  and  saved  from  suffering  prolonged  pains :  the  evils  of 
death  by  disease  and  starvation  being  thus  limited  to  the  pre- 
datory animals,  relatively  small  in  their  numbers.  Mean- 
while a  check  is  put  on  undue  multiplication.  Where  a  tract 
of  country  has  been  overrun  by  rabbits,  weasels,  thriving  on 
the  abundant  supply  of  food,  presently  become  numerous 
enough  to  bring  the  population  of  rabbits  within  moderate 
limits;  and  by  doing  this  benefit  not  only  all  those  kinds  of 


406  PHYSIOLOGICAL  DEVELOPMENT. 

plants  which  are  being  eaten  down,  and  all  those  other  ani- 
mals which  live  on  such  plants,  but  also  the  rabbits  them- 
selves; since,  increasing  beyond  the  means  of  subsistence,  a 
large  part  of  them  would,  if  not  killed,  die  of  hunger.  Be- 
tween aphides  and  lady-birds  we  see  a  connexion  of  like 
nature:  great  increase  of  the  first  yielding  abundant  food  to 
larva?  of  the  second,  ending  after  a  season  or  so  in  swarms  of 
lady-birds,  and  consequently  of  their  larvae,  whereby  the 
aphides,  immensely  diminished,  cease  so  greatly  to  injure 
various  plants  and  the  animals  dependent  on  them.  Even 
minute  parasites,  by  the  evils  they  inflict  on  one  species, 
profit  others :  instance  the  enormous  destruction  of  flies 
which  a  microscopic  fungus  caused  a  few  years  ago — a 
destruction  which  relieved  not  only  man  but  all  the  animals 
which  flies  irritate:  often  so  much  as  to  hinder  them  from 
feeding.  Such  instances  remind  us  how  numerous  are  the 
bonds  by  which  the  lives  of  organisms  are  tied  together. 

§  314/.  I  have  reserved  to  the  last  the  clearest  and  most 
striking  illustration  of  this  progressing  integration  through- 
out the  organic  world.  I  refer  to  the  mutually-beneficial 
relations  established  between  plants  and  animals  through  the 
agency  of  flowers  and  insects. 

Everyone  nowadays  has  been  made  familiar  with  the  pro- 
cess of  plant-fertilization,  and  knows  that  (leaving  out  of 
consideration  plants  fertilized  by  wind-borne  pollen)  the 
ability  to  bear  seed  depends  largely  on  the  aid  given  by  bees, 
butterflies,  and  moths.  The  exchange  of  services  has  been 
growing  ever  more  various  and  complicated  during  long  past 
periods.  We  have  the  acquirement  by  flowers  of  bright 
colours  serving  to  guide  these  insects  to  places  where  honey 
is  to  be  found;  and  we  have  their  perfumes,  also  serving  for 
guidance.  Then  we  have  the  many  different  arrangements, 
often  complicated,  by  which  the  visiting  insects  are  obliged 
to  carry  away  pollen  and  dust  with  it  the  stigmas  of  flowers 
on  which  they  subsequently  settle:  thus  effecting  cross- 


THE  INTEGRATION  OF  THE  ORGANIC  WORLD.     4Q7 

fertilization.  Pari  passu  have  gone  on  insect-developments 
made  possible  by  these  arrangements  and  furthering  them. 
Especially  must  be  named  the  modification  of  certain  Hymen- 
optera  into  honey-storing  bees :  the  implication  being  that 
the  entire  economy  established  by  these  social  insects  has 
been  sequent  on  the  growth  of  this  system  of  reciprocal 
benefits.  And  then,  just  instancing  the  dependence  between 
a  particular  flower  having  a  long  tubular  corolla,  and  a  par- 
ticular moth  having  an  appropriately  long  proboscis,  it 
suffices  to  say  that  innumerable  specialities  of  this  general 
relation  everywhere  multiply  the  links  by  which  the  vegetal 
world  and  the  animal  world  are  here  connected.  That  the 
effects  of  the  connections  tell  largely  on  the  prosperity  of 
both,  is  suggested  by  some  instances  Mr.  Darwin  gives,  and 
by  a  statement  recently  made  in  the  United  States,  by  Dr.  L. 
0.  Howard,  that  the  greater  fostering  of  bees  would  much 
increase  certain  of  the  crops. 

But  now  observe  the  broad  fact  to  which  these  few  details 
concerning  plant-fertilization  are  introductory.  All  these 
general  and  special  relations  between  plants  and  animals 
have  arisen  since  the  phaenogamic  type  came  into  existence — 
have,  indeed,  arisen  since  the  higher  members  of  that  type, 
the  Angiosperms,  have  appeared;  for  the  Gymnosperms  do 
not  play  any  part  in  this  intercommunion.  But  so  far  as  we 
can  judge  of  present  results  of  geologic  explorations,  there 
were  no  Angiosperms  during  the  Eozoic  and  Paleozoic 
periods.  So  that  this  class  of  connexions  between  animals 
and  vegetals  must  have  been  established  since  carboniferous 
times — a  period  long,  indeed,  but  far  shorter  than  that  which 
organic  evolution  at  large  has  occupied. 

§  314<jr.  I  have  but  just  touched  on  some  salient  parts  of  a 
subject,  immense  in  extent  and  extremely  involved,  which  it 
would  take  a  volume  to  set  forth  adequately.  Enough  has 
'been  said,  however,  to  indicate  the  truth  which  it  is  the 
purpose  of  the  chapter  to  bring  into  view  and  emphasize — 


408  PHYSIOLOGICAL  DEVELOPMENT. 

the  truth  that  both  of  the  two  great  laws  of  evolution  are 
exemplified  in  the  organic  world  as  a  whole,  as  they  are 
exemplified  in  every  organism,  and  in  all  other  things. 

The  reader  has  long  since  become  familiar  with  the  gene- 
ralization that  while  Evolution  is  a  change  from  the  homo- 
geneous to  the  heterogeneous,  it  is  also  a  change  from  the 
incoherent  to  the  coherent;  and  this  change  from  the  inco- 
herent to  the  coherent  has  been  above  exhibited  as  going  on 
even  throughout  that  vast  assemblage  of  organisms,  plant 
and  animal,  which  cover  the  Earth's  surface.  In  what  we 
are  obliged  to  conceive  as  the  earliest  stage,  when  the  most 
minute  types  of  life  alone  existed,  the  aggregate  of  living 
things  was  at  once  homogeneous  and  incoherent.  In  the 
course  of  epochs  immeasurable  in  duration,  this  uniform 
aggregate  of  beings  has  been  becoming  more  multiform. 
And  now  we  see  that  instead  of  forms  of  life  everywhere 
without  the  slightest  union  caused  by  mutual  dependence, 
there  have  slowly  arisen  forms  of  life  among  which  mutual 
dependences  have  entailed  vital  connexions  correspondingly 
marked.  Along  with  progressing  differentiation  there  has 
ever  been  progressing  integration.  So  that  we  may  recog- 
nize something  like  a  growing  life  of  the  entire  aggregate  of 
organisms  in  addition  to  the  lives  of  individual  organisms — 
an  exchange  of  services  among  parts  enhancing  the  life  of 
the  whole. 

In  this  final  generalization  the  law  of  Evolution  is  mani- 
fested under  its  most  transcendental  form. 


PART  VI. 
LAWS    OF    MULTIPLICATION 


CHAPTER  I. 

THE   FACTORS.* 

§  315.  IF  organisms  have  been  evolved,  their  respective 
powers  of  multiplication  must  have  been  determined  by 
natural  causes.  Grant  that  the  countless  specialities  of 
structure  and  function  in  plants  and  animals,  have  arisen 
from  the  actions  and  reactions  between  them  and  their 
environments,  continued  from  generation  to  generation;  and 
it  follows  that  from  these  actions  and  reactions  have  also 
arisen  those  countless  degrees  of  fertility  which  we  see 
among  them.  As  in  all  other  respects  an  adaptation  of  each 
species  to  its  conditions  of  existence  is  directly  or  indirectly 
brought  about;  so  must  there  be  directly  or  indirectly 
brought  about  an  adaptation  of  its  reproductive  activity  to 
its  conditions  of  existence. 

We  may  expect  to  find,  too,  that  permanent  and  temporary 

*  An  outline  of  the  doctrine  set  forth  in  the  following  chapters,  was 
originally  published  in  the  Westminster  Review  for  April,  1852,  under  the 
title — A  Theory  of  Population  deduced  from  the  General  Law  of  Animal 
Fertility ;  and  was  shortly  afterwards  republished  with  a  prefatory  note 
stating  that  it  must  be  accepted  as  a  sketch  which  I  hoped  at  some  future 
time  to  elaborate.  In  now  revising  and  completing  it,  I  have  omitted  a 
non-essential  part  of  the  argument,  while  I  have  expanded  the  remainder 
by  adding  to  the  number  of  facts  put  in  evidence,  by  meeting  objections 
which  want  of  space  before  obliged  me  to  pass  over,  and  by  drawing  various 
secondary  conclusions.  The  original  paper,  with  omissions,  will  be  found  in 
Appendix  A  to  Volume  I  of  this  work. 

411 


412  LAWS  OP  MULTIPLICATION. 

differences  of  fertility  have  the  same  general  interpretation. 
If  the  small  variations  of  structure  and  function  that  arise 
within  the  limits  of  each  species,  are  due  to  actions  like  those 
which,  by  their  long-accumulating  effects,  have  produced  the 
immense  contrasts  between  the  various  types;  we  may  con- 
clude that,  similarly,  the  actions  to  which  changes  in  the 
rate  of  multiplication  of  each  species  are  due,  also  produce, 
in  great  periods  of  time,  the  enormous  differences  between 
the  rates  of  multiplication  of  different  species. 

Before  inquiring  in  what  ways  the  rapidities  of  increase 
are  adjusted  to  the  requirements,  both  temporary  and  perma- 
nent, it  will  be  needful  to  look  at  the  factors.  Let  us  set 
down  first  those  which  belong  to  the  environment,  and  then 
those  which  belong  to  the  organism. 

§  316.  Every  living  aggregate  being  one  of  which  the 
inner  actions  are  adjusted  to  balance  outer  actions,  it  follows 
that  the  maintenance  of  its  moving  equilibrium  depends  on 
its  exposure  to  the  right  amounts  of  these  actions.  Its 
moving  equilibrium  may  be  overturned  if  one  of  these  actions 
is  either  too  great  or  too  small  in  amount ;  and  it  may  be  so 
overturned  either  by  excess  or  defect  of  some  inorganic 
agency  in  its  environment,  or  by  excess  or  defect  of  some 
organic  agency. 

Thus  a  plant,  constitutionally  fitted  to  a  certain  warmth 
and  humidity,  is  killed  by  extremes  of  temperature,  as  well 
as  by  extremes  of  drought  and  moisture.  It  may  dwindle 
away  from  want  of  soil,  or  die  from  the  presence  of  too  great 
or  too  small  a  quantity  of  some  mineral  substance  which  the 
soil  supplies  to  it.  In  like  manner,  every  animal  can  main- 
tain the  balance  of  its  functions  so  long  only  as  the  environ- 
ment adds  to  or  deducts  from  its  heat  at  rates  not  exceeding 
definite  limits.  Water,  too,  must  be  accessible  in  amount 
sufficient  to  compensate  loss.  If  the  parched  air  is  rapidly 
abstracting  its  liquid  which  there  is  no  pool  or  river  to 
restore,  its  functions  cease;  and  if  it  is  an  aquatic  creature, 


THE  FACTORS.  413 

drought  may  kill  it  either  by  drying  up  its  medium  or  by 
giving  it  a  medium  inadequately  aerated.  Thus  each  organ- 
ism, adjusted  to  a  certain  average  in  the  actions  of  its 
inorganic  environment,  or  rather,  we  should  say,  adjusted  to 
certain  moderate  deviations  from  this  average,  is  destroyed 
by  extreme  deviations.  So,  too,  is  it  with  the  environ- 

ing organic  agencies.  Among  plants,  only  the  parasitic  kinds 
and  those  united  by  symbiosis  (as  well  as  a  few  innocent 
"lodgers")  depend  for  their  individual  preservation  on  the 
presence  of  certain  other  organisms  (though  the  presence  of 
certain  other  organisms  is  needful  to  most  plants  for  the  pre- 
servation of  the  race  by  aiding  fertilization).  Here,  for  the 
continuance  of  individual  life,  particular  organisms  must 
be  absent  or  not  very  numerous — beasts  that  browse,  cater- 
pillars that  devour  leaves,  aphides  that  suck  juices.  Among 
animals,  however,  the  maintenance  of  the  functional  balance 
is  both  positively  and  negatively  dependent  on  the  amounts 
of  surrounding  organic  agents.  There  must  be  an  accessible 
sufficiency  of  the  plants  or  animals  serving  for  food;  and  of 
organisms  that  are  predatory  or  parasitic  or  otherwise  detri- 
mental, the  number  must  not  pass  a  certain  limit. 

This  dependence  of  the  moving  equilibrium  in  every  indi- 
vidual organism  on  an  adjustment  of  its  forces  to  the  forces 
of  the  environment,  and  the  overthrow  of  this  equilibrium 
by  failure  of  the  adjustment,  is  comprehensive  of  all  cases. 
At  first  sight  it  does  not  seem  to  include  what  we  call  natural 
death;  but  only  death  by  violence,  or  starvation,  or  cold,  or 
drought.  But  in  reality  natural  death,  no  less  than  every 
other  kind  of  death,  is  caused  by  the  failure  to  meet  some 
outer  action  by  a  proportionate  inner  action.  The  apparent 
difference  is  due  to  the  fact  that  in  old  age,  when  the 
quantity  of  force  evolved  in  the  organism  gradually  dimi- 
nishes, the  momentum  of  the  functions  becomes  step  by  step 
less,  and  the  variations  of  the  external  forces  relatively 
greater;  until  there  finally  comes  an  occasion  when  some 
quite  moderate  deviation  from  that  average  to  which  the 


414. 


LAWS  OF  MULTIPLICATION. 


feeble  moving  equilibrium  is  adjusted,  produces  in  it  a  fatal 
perturbation. 

§  317.  The  individuals  of  every  species  being  thus  de- 
pendent on  certain  environing  actions;  and  severally  having 
their  moving  equilibria  sooner  or  later  overthrown  by  one  or 
other  of  these  environing  actions;  we  have  next  to  consider 
in  what  ways  the  environing  actions  are  so  met  as  to  prevent 
extinction  of  the  species.  There  are  two  essentially  different 
ways.  There  may  be  in  each  individual  a  small  or  great 
ability  to  adjust  itself  to  variations  of  the  agencies  around 
it  and  to  a  small  or  great  number  of  such  varying  agencies 
— there  may  be  little  or  much  power  of  preserving  the 
balance  of  the  functions.  And  there  may  be  much  or  little 
power  of  producing  new  individuals  to  replace  those  whose 
moving  equilibria  have  been  overthrown.  A  few  facts  must 
be  set  down  to  enforce  these  abstract  statements. 

There  are  both  active  and  passive  adaptations  by  which 
organisms  are  enabled  to  survive  adverse  influences.  Plants 
show  us  but  few  active  adaptations :  that  of  the  Pitcher-plant 
and  those  of  the  reproductive  parts  of  some  flowers  (which  do 
not,  however,  conduce  to  self-preservation)  are  exceptional 
instances.  But  plants  have  various  passive  adaptations;  as 
thorns,  stinging  hairs,  poisonous  and  acrid  juices,  repugnant 
odours,  and  the  woolliness  or  toughness  that  makes  their 
leaves  uneatable.  Animals  exhibit  far  more  numerous 

adjustments,  both  passive  and  active.  In  some  cases  they 
survive  desiccation,  they  hybernate,  they  acquire  thicker 
clothing,  and  so  are  fitted  to  bear  unfavourable  inorganic 
actions;  and  they  are  in  many  cases  fitted  passively  to  meet 
the  adverse  actions  of  other  organisms,  by  bearing  spines  or 
armour  or  shells,  by  simulating  neighbouring  objects  in  colour 
or  form  or  both,  by  emitting  disagreeable  odours,  or  by  having 
disgusting  tastes.  In  still  more  numerous  ways  they  actively 
contend  with  unfavourable  conditions.  Against  the  seasons 
they  guard  by  storing  up  food,  by  secreting  themselves  in 


THE  FACTORS.  415 

crevices,  or  by  forming  burrows  and  nests.  They  save  them- 
selves from  enemies  by  developed  powers  of  locomotion, 
taking  the  shape  of  swiftness  or  agility  or  aptitude  for 
changing  their  media;  by  their  strength  either  alone  or 
aided  by  weapons;  lastly  by  their  intelligence,  without 
which,  indeed,  their  other  superiorities  would  avail  them 
little.  And  then  these  various  active  powers  serving  for 
defence,  become,  in  other  cases,  the  powers  that  enable  ani- 
mals to  aggress,  and  to  preserve  their  lives  by  the  success 
of  their  aggressions. 

The  second  process  by  which  extinction  is  prevented — the 
formation  of  new  individuals  to  replace  the  individuals 
destroyed — is  carried  on,  as  described  in  the  chapter  on 
"  Genesis,"  by  two  methods,  the  sexual  and  the  asexual. 
Plants  multiply  by  spontaneous  fission,  by  gemmation,  by 
proliferation,  and  by  the  evolution  of  young  ones  from  de- 
tached cells  and  scales  and  leaves;  and  they  also  multiply 
by  the  casting  off  of  spores  and  sporangia  and  seeds.  In  like 
manner  among  animals,  there  are  varied  kinds  of  agamo- 
genesis,  from  spontaneous  fission  up  to  parthenogenesis,  all  of 
them  conducing  to  rapid  increase  of  numbers;  and  we  have 
the  more  familiar  process  of  gamogenesis,  also  carried  on 
in  a  great  variety  of  ways.  This  formation  of  new 

individuals  to  replace  the  old,  is,  however,  inadequately  con- 
ceived if  we  contemplate  only  the  number  born  or  detached 
on  each  occasion.  There  are  four  factors,  all  variable,  on 
which  the  rate  of  multiplication  depends.  The  first  is  the 
age  at  which  reproduction  commences;  the  second  is  the 
frequency  with  which  broods  are  produced;  the  third  is  the 
number  contained  in  each  brood;  and  the  fourth  is  the 
length  of  time  during  which  the  bringing  forth  of  broods 
continues.  There  must  be  taken  into  account  a  further 
element — the  amount  of  aid  given  by  the  parent  to  each 
germ  in  the  shape  of  stored-up  nutriment,  continuous  feed- 
ing, warmth,  protection,  &c. :  on  which  amount  of  aid,  vary- 
ing between  immensely  wide  limits,  depends  the  number  of 


416  LAWS  OF  MULTIPLICATION. 

the  new  individuals  that  survive  long  enough  to  replace  the 
old,  and  perform  the  same  reproductive  process. 

Thus,  regarding  every  living  organism  as  having  a  moving 
equilibrium  dependent  on  environing  forces,  but  ever  liable 
to  be  overthrown  by  irregularities  in  those  forces,  and  always 
so  overthrown  sooner  or  later;  we  see  that  each  species  of 
organism  can  be  maintained  only  by  the  generation  of  new 
individuals  with  a  certain  rapidity,  and  by  helping  them 
more  or  less  fully  to  establish  their  moving  equilibria. 

§  318.  Such  are  the  factors  with  which  we  are  here  con- 
cerned. I  have  presented  them  in  abstract  shapes  for  the 
purpose  of  showing  how  they  are  expressible  in  general 
terms  of  force — how  they  stand  related  to  the  ultimate  laws 
of  redistribution  of  matter  and  motion. 

For  the  purposes  of  the  argument  now  to  follow,  we  may, 
however,  conveniently  deal  with  these  factors  under  a  more 
familiar  guise.  Ignoring  their  other  aspects,  we  may  class 
the  factors  which  affect  each  race  of  organisms  as  forming 
two  conflicting  sets.  On  the  one  hand,  by  what  we  call 
natural  death,  by  enemies,  by  lack  of  food,  by  atmospheric 
changes,  &c.,  the  race  is  constantly  being  destroyed.  On  the 
other  hand,  partly  by  the  endurance,  the  strength,  the  swift- 
ness, and  the  sagacity  of  its  members,  and  partly  by  their 
fertility,  it  is  constantly  being  maintained.  These  conflicting 
sets  of  factors  may  be  generalized  as — the  forces  destructive 
of  race  and  the  forces  preservative  of  race.  So  generalizing 
them,  let  us  ask  what  are  the  necessary  implications. 


CHAPTER  II. 

A   PRIORI  PRINCIPLE. 

§  319.  THE  number  of  a  species  must  at  any  time  be  either 
decreasing  or  stationary  or  increasing.  If,  generation  after 
generation,  its  members  die  faster  than  others  are  born,  the 
species  must  dwindle  and  finally  disappear.  If  its  rate  of 
multiplication  is  equal  to  its  rate  of  mortality,  there  can  be 
no  numerical  change  in  it.  And  if  the  deductions  by  death 
are  fewer  than  the  additions  by  birth,  the  species  must 
become  more  abundant.  These  we  may  safely  set  down  as 
necessities.  The  forces  destructive  of  race  must  be  either 
greater  than  the  forces  preservative  of  race,  or  equal  to  them, 
or  less  than  them;  and  there  cannot  but  result  these  effects 
on  number. 

We  are  here  concerned  only  with  races  that  continue  to 
exist;  and  may  therefore  leave  out  of  consideration  those 
in  which  the  destructive  forces,  remaining  permanently  in 
excess  of  the  preservative  forces,  cause  extinction.  Prac- 
tically, too,  we  may  exclude  the  stationary  condition ;  for  the 
chances  are  infinity  to  one  against  the  maintenance  of  a  per- 
manent equality  between  the  births  and  the  deaths.  Hence, 
our  inquiry  resolves  itself  into  this : — In  races  that  con- 
tinue to  exist,  what  laws  of  numerical  variation  result  from 
these  variable  conflicting  forces,  which  are  respectively  de- 
structive of  race  and  preservative  of  race? 

§  320.  Clearly  if  the  forces  destructive  of  race,  when  once 
73  417 


418  LAWS  OF  MULTIPLICATION. 

in  excess,  had  nothing  to  prevent  them  from  remaining  in 
excess,  the  race  would  disappear;  and  clearly  if  the  forces 
preservative  of  race,  when  once  in  excess,  had  nothing  to 
prevent  them  from  remaining  in  excess,  the  race  would  go  on 
increasing  to  infinity.  In  the  absence  of  any  compensating 
actions,  the  only  possible  avoidance  of  these  opposite  extremes 
would  be  an  unstable  equilibrium  between  the  conflicting 
forces,  resulting  in  a  perfectly  constant  number  of  the 
species:  a  state  which  we  know  does  not  exist,  and  against 
the  existence  of  which  the  probabilities  are,  as  already  said, 
infinite.  It  follows,  then,  that  as  in  every  continuously- 
existing  species,  neither  of  the  two  conflicting  sets  of  forces 
remains  permanently  in  excess;  there  must  be  some  way  of 
stopping  that  excess  of  the  one  or  the  other  which  is  ever 
occurring. 

How  is  this  done?  Should  any  one  allege,  in  conformity 
with  the  old  method  of  interpretation,  that  there  is  in  each 
case  a  providential  interposition  to  rectify  the  disturbed 
balance,  he  commits  himself  to  the  supposition  that  of  the 
millions  of  species  inhabiting  the  Earth,  each  one  is  yearly 
regulated  in  its  degree  of  fertility  by  a  miracle;  since  in  no 
two  years  do  the  forces  which  foster,  or  the  forces  which 
check,  each  species,  remain  the  same;  and  therefore,  in  no 
two  years  is  there  required  the  same  fertility  to  balance 
the  mortality.  Few  if  any  will  say  that  God  continually 
alters  the  reproductive  activity  of  every  parasitic  fungus  and 
every  Tape-worm  or  Trichina,  so  as  to  prevent  its  extinction 
or  undue  multiplication ;  which  they  must  say  if  they  adopt 
the  hypothesis  of  supernatural  adjustment.  And  in  the 
absence  of  this  hypothesis  there  remains  only  one  other. 
The  alternative  possibility  is,  that  the  balance  of  the  pre- 
servative and  destructive  forces  is  self-sustaining — is  of  the 
kind  distinguished  as  a  stable  equilibrium:  an  equilibrium 
such  that  any  excess  of  one  of  the  forces  at  work,  itself 
generates,  by  the  deviation  it  produces,  certain  counter-forces 
which  eventually  out-balance  it,  and  initiate  an  opposite  devi- 


A   PRIORI  PRINCIPLE.  419 

ation.  Let  us  consider  how,  in  the  case  before  us,  such  a 
stable  equilibrium  must  be  constituted. 

§  321.  When  a  season  favourable  to  it,  or  a  diminution  of 
creatures  detrimental  to  it,  causes  any  species  to  become 
more  numerous  than  usual,  an  immediate  increase  of  certain 
destructive  influences  takes  place.  If  it  be  a  plant,  the  sup- 
posed greater  abundance  itself  implies  fuller  occupation  of 
the  places  available  for  growth — an  occupation  which,  leav- 
ing fewer  such  places  as  the  multiplication  goes  on,  becomes 
a  check  on  further  multiplication — itself  causes  a  greater 
mortality  of  seeds  that  fail  to  root  themselves.  And  after- 
wards, in  addition  to  this  passive  resistance  to  continued 
increase,  there  comes  an  active  resistance :  the  creatures  which 
thrive  at  the  expense  of  the  species — the  larvae,  the  birds,  the 
herbivores — increase  too.  If  it  be  an  animal  that  has  grown 
more  numerous,  then,  unless  by  some  exceptional  coincidence 
a  simultaneous  and  proportionate  addition  to  the  animals  or 
plants  serving  for  food  has  occurred,  there  must  result  a 
relative  scarcity  of  food.  Enemies,  too,  be  they  beasts  of- 
prey  or  be  they  parasites,  must  quickly  begin  to  multiply. 
Hence,  each  kind  of  organism,  previously  existing  in  some- 
thing like  its  normal  number,  cannot  have  its  number  raised 
without  a  rise  of  the  destructive  forces,  negative  and  positive, 
quickly  commencing.  Both  negative  and  posi- 

tive destructive  forces  must  augment  until  this  increase  of 
the  species  is  arrested.  The  competition  for  places  on  which 
to  grow,  if  the  species  be  vegetal,  or  for  food  if  it  be  animal, 
must  become  more  intense  as  the  over-peopling  of  the  habitat 
progresses;  until  there  is  reached  the  limit  at  which  the 
mortality  equals  the  reproduction.  And  as,  at  the  same 
time,  enemies  will  multiply  with  a  rapidity  which  soon 
brings  them  abreast  of  the  augmented  supply  of  prey,  the 
positive  restraint  they  exert  will  help  to  bring  about  an 
earlier  arrest  of  the  expansion  than  pressure  of  population 
alone  would  cause.  One  more  inference  mav  be 


420  LAWS  OF  MULTIPLICATION. 

drawn.  Had  the  species  to  meet  no  repressing  influence 
save  that  negative  one  of  relatively-diminished  space  or 
relatively-diminished  food-supply,  the  cause  leading  to  its 
increase  might  carry  it  up  to  the  limit  set  by  this,  and  there 
leave  it:  its  enlarged  number  might  be  permanent.  But 
the  positive  repressing  influence  that  has  been  called  into 
existence,  will  prevent  this.  For  the  increase  of  enemies, 
commencing,  as  it  must,  after  the  increase  of  the  species, 
and  advancing  in  geometrical  progression  until  it  is  itself 
checked  in  like  manner,  will  end  in  an  excess  of  enemies. 
Whereupon  must  result  a  mortality  of  the  species  greater 
than  its  multiplication — a  decrease  which  will  continue 
until  its  habitat  is  under-peopled,  its  unduly-numerous 
enemies  decimated  by  starvation,  and  the  destroying  agencies 
reduced  to  a  minimum.  Whence  will  follow  another  in- 
crease. 

Thus,  as  before  indicated  (First  Prin.  §§  85,  173),  there  is 
here,  as  wherever  antagonistic  forces  are  in  action,  an  alter- 
nate predominance  of  each,  causing  a  rhythmical  movement 
— a  rhythmical  movement  which  constitutes  a  moving  equili- 
brium in  those  cases  where  the  forces  are  not  dissipated  with 
appreciable  rapidity,  or  are  re-supplied  as  fast  as  they  are 
dissipated.  While,  therefore,  on  the  one  hand,  we  see  that 
the  continued  existence  of  a  species  necessarily  implies  some 
action  by  which  the  destructive  and  preservative  forces  are 
self-adjusted;  we  see,  on  the  other  hand,  that  such  an  action 
is  an  inevitable  consequence  of  the  universal  process  of 
equilibration. 

§322.  Is  this  the  sole  equilibration  which  must  exist? 
Clearly  not.  The  temporary  compensating  adjustments  of 
multiplication  to  mortality  in  each  species,  are  but  intro- 
ductory to  the  permanent  compensating  adjustments  of  mul- 
tiplication to  mortality  among  species  in  general.  The  above 
reasoning  would  hold  just  as  it  now  does,  were  all  species 
equally  prolific  and  all  equally  short-lived.  It  yields  no 


A  PRIORI  PRINCIPLE.  421 

answer  to  the  inquiries — why  do  their  fertilities  differ  so 
enormously,  or  why  do  their  mortalities  differ  so  enormously  ? 
and  how  is  the  general  fertility  adapted  to  the  general  mor- 
tality in  each  ?  The  balancing  process  we  have  contemplated 
can  go  on  only  within  moderate. limits — must  fail  entirely  in 
the  absence  of  a  due  proportion  between  the  ordinary  birth- 
rate and  the  ordinary  death-rate.  If  the  reproduction  of 
mice  proceeded  as  slowly  as  the  reproduction  of  men,  mice 
would  be  extinct  before  a  new  generation  could  arise:  even 
did  their  natural  lives  extend  to  fifteen  or  sixteen  years,  it 
would  still  be  extremely  improbable  that  any  would  for  so 
long  survive  all  the  dangers  they  are  exposed  to.  Con- 
versely, did  oxen  propagate  as  fast  as  infusoria,  the  race 
would  die  of  starvation  in  a  week.  Hence,  the  minor  adjust- 
ment of  varying  multiplication  to  varying  mortality  in  each 
species,  implies  some  major  adjustment  of  average  multiplica- 
tion to  average  mortality.  What  must  this  adjustment  be  ? 

We  have  already  seen  that  the  forces  preservative  of  race 
are  two — ability  in  each  member  of  the  race  to  preserve 
itself,  and  ability  to  produce  other  members — power  to  main- 
tain individual  life,  and  power  to  generate  the  species. 
These  must  vary  inversely.  When,  from  lowness  of  organi- 
zation, the  ability  to  contend  with  external  dangers  is  small, 
there  must  be  great  fertility  to  compensate  for  the  conse- 
quent mortality;  otherwise  the  race  must  die  out.  When, 
on  the  contrary,  high  endowments  give  much  capacity  of 
self-preservation,  a  correspondingly-low  degree  of  fertility  is 
requisite.  Given  the  dangers  to  be  met  as  a  constant  quan- 
tity; then  as  the  ability  to  meet  them  must  be  a  constant 
quantity  too;  and  as  this  is  made  up  of  the  two  factors, 
power  to  maintain  individual  life  and  power  to  multiply, 
these  cannot  do  other  than  vary  inversely:  one  must  de- 
crease as  the  other  increases. 

It  needs  but  to  conceive  the  results  of  nonconformity  to 
this  law,  to  see  that  every  species  must  either  conform  to  it 
or  cease  to  exist.  Suppose,  first,  a  species  whose  individuals, 


422  LAWS  OP  MULTIPLICATION. 

having  but  small  self-preservative  powers,  are  rapidly  de- 
stroyed, to  be  at  the  same  time  without  reproductive  powers 
proportionately  great.  The  defect  of  fertility,  if  extreme, 
will  result  in  the  death  of  one  generation  before  another  has 
grown  up.  If  less  extreme,  it  will  entail  a  scarcity  such 
that  in  the  next  generation  sexual  congress  will  be  too  infre- 
quent to  maintain  even  the  small  number  which  remains ;  and 
the  race  will  dwindle  with  increasing  rapidity.  If  still  less 
extreme,  the  consequent  degree  of  sparseness,  while  not  so 
great  as  to  prevent  an  adequate  number  of  procreative  unions, 
will  be  so  great  as  to  render  special  food  abundant  and 
special  enemies  few — will  thus  diminish  the  destructive 
forces  so  much  that  the  self-preservative  forces  will  become 
relatively  great:  so  great,  relatively,  that  when  combined 
with  the  small  ability  to  propagate  the  species,  they  will 
suffice  to  balance  the  small  destructive  forces.  Suppose, 
next,  a  species  whose  individuals  have  high  powers  of  self- 
preservation,  while  they  have  powers  of  multiplication  much 
beyond  what  is  needful.  The  excess  of  fertility,  if  extreme, 
will  cause  sudden  extinction  of  the  species  by  starvation. 
If  less  extreme,  it  must  produce  a  permanent  increase  in 
the  number  of  the  species;  and  this,  followed  by  intenser 
competition  for  food  and  augmented  number  of  enemies,  will 
involve  such  an  increase  of  the  dangers  to  individual  life, 
that  the  great  self-preserving  powers  of  the  individuals  will 
not  be  more  than  sufficient  to  cope  with  them.  That  is  to 
say,  if  the  fertility  is  relatively  too  great,  then  the  ability  to 
maintain  individual  life  inevitably  becomes  smaller,  relatively 
to  the  requirements;  and  the  inverse  proportion  is  thus 
established. 

So  that  when,  from  comparing  the  different  states  of  the 
same  species,  we  go  on  to  compare  the  states  of  different 
species,  we  see  that  there  is  an  analogous  adjustment — ana- 
logous in  the  sense  that  great  mortality  is  associated  with 
great  multiplication,  and  small  mortality  with  small  multi- 
plication. And  we  see  that  the  unlikeness  of  the  cases  con- 


A  PRIORI  PRINCIPLE.  423 

sists  merely  in  this,  that  what  is  a  temporary  relation  in  the 
one  is  a  permanent  relation  in  the  other. 

§  323.  For  the  moment  it  does  not  concern  us  to  inquire 
what  is  the  origin  of  this  permanent  relation.  That  which 
we  have  now  to  note,  is  simply  that  in  some  way  or  other 
there  must  be  established  an  inverse  proportion  between  the 
power  to  sustain  individual  life  and  the  power  to  produce 
new  individuals.  Whether  or  not  this  permanent  relation 
is  self-adjusting  in  long  periods  of  time,  as  the  temporary 
relation  is  self-adjusting  in  short  periods  of  time,  is  a 
separate  question.  The  purpose  of  this  chapter  is  fulfilled  by 
showing  that  such  a  permanent  relation  must  exist. 

But  having  recognized  the  a  priori  principle  that  in  races 
which  continuously  survive,  the  forces  destructive  of  race 
must  be  equilibrated  by  the  forces  preservative  of  race;  and 
that,  supposing  these  are  constant,  there  must  be  an  inverse 
proportion  between  self-preservation  and  race-preservation; 
we  may  go  on  to  inquire  how  this  relation,  necessary  in 
theory,  arises  in  fact.  Leaving  out  the  untenable  hypothesis 
of  a  supernatural  pre-adjustment,  we  have  to  ask  in  what 
way  an  adjustment  comes  about  as  a  result  of  Evolution. 
Is  it  due  to  the  survival  of  varieties  in  which  the  proportion 
of  fertility  to  mortality  happens  to  be  the  best?  Or  is  the 
fertility  adapted  to  the  mortality  in  a  more  direct  way?  To 
these  questions  let  us  now  address  ourselves. 


CHAPTER  III. 

OBVERSE  A   PRIORI  PRINCIPLE. 

§  324.  WHEN  dealing  with  its  phenomena  inductively,  we 
saw  that  however  it  may  be  carried  on,  Genesis  "  is  a  process 
of  negative  or  positive  disintegration ;  and  is  thus  essentially 
opposed  to  that  process  of  integration  which  is  the  primary 
process  in  individual  evolution."  (§  76.)  Each  new  indivi- 
dual, whether  separated  as  a  germ  or  in  some  more-developed 
form,  is  a  deduction  from  the  mass  of  a  pre-existing  indivi- 
dual or  of  two  pre-existing  individuals.  Whatever  nutritive 
matter  is  stored  up  along  with  the  germ,  if  it  be  deposited  in 
the  shape  of  an  egg,  is  so  much  nutritive  matter  lost  to  the 
parent.  No  drop  of  blood  can  be  absorbed  by  the  foetus,  nor 
any  draught  of  milk  sucked  by  the  young  when  born,  without 
taking  from  the  mother  tissue-forming  and  force-evolving 
materials  to  an  equivalent  amount.  And  all  subsequent  sup- 
plies given  to  progeny,  if  they  are  nurtured,  involve,  to  a 
parent  or  parents,  so  much  waste  in  exertion  which  does  not 
bring  its  return  in  assimilated  food. 

Conversely,  the  continued  aggregation  of  materials  into 
one  organism,  renders  impossible  the  formation  of  other  or- 
ganisms out  of  those  materials.  As  much  assimilated  food 
as  is  united  into  a  single  whole,  is  so  much  assimilated  food 
withheld  from  a  plurality  of  wholes  which  might  else  have 
been  produced.  Given  the  absorbed  nutriment  as  a  constant 
quantity,  and  the  longer  the  building  of  it  up  into  a  con- 
crete shape  goes  on,  the  longer  must  be  postponed  any  build- 
424 


OBVERSE  A  PRIORI  PRINCIPLE.  425 

ing  of  it  up  into  discrete  shapes.  And,  similarly,  the  larger 
the  proportion  of  matter  consumed  in  the  functional  actions 
of  parents,  the  smaller  must  be  the  proportion  of  matter 
which  can  remain  to  establish  and  support  the  functional 
actions  of  offspring. 

Though  the  necessity  of  these  universal  relations  is  toler- 
ably obvious  as  thus  stated  generally,  it  will  be  useful  to 
dwell  for  a  brief  space  on  their  leading  aspects. 

§  325.  That  disintegration  which  constitutes  genesis,  may 
be  such  as  to  disperse  entirely  the  aggregate  which  integra- 
tion has  previously  produced — the  parent  may  dissolve  wholly 
into  progeny.  This  dissolution  of  each  aggregate  into  two 
or  many  aggregates,  may  occur  at  very  short  intervals,  in 
which  case  the  bulk  attained  can  be  but  extremely  small;  or 
it  may  occur  at  longer  intervals,  in  which  case  a  larger  bulk 
may  be  attained. 

Instead  of  quickly  losing  its  own  individuality  in  the 
individualities  of  its  offspring,  each  member  of  the  race  may, 
after  growing  for  a  time,  have  portions  of  its  substance  begin 
to  develop  into  the  parental  shape  and  presently  detach 
themselves;  and  the  parent,  maintaining  its  own  identity, 
may  continue  indefinitely  so  to  produce  young  ones.  But 
clearly,  the  earlier  it  commences  doing  this,  and  the  more 
rapidly  it  does  it,  the  sooner  must  the  increase  of  its  own 
bulk  be  stopped. 

Or  again,  growth  and  development  continuing  for  a  long 
period  without  any  deduction  of  materials,  an  individual  of 
considerable  size  and  organization  may  result;  and  then  the 
abstraction  of  substance  for  the  formation  of  new  individuals, 
or  rather  the  eggs  of  them,  may  be  so  great  that  as  soon  as 
the  eggs  are  laid  the  parent  dies  of  exhaustion — dies,  that  is, 
from  an  excessive  loss  of  the  nutritive  matters  needed  for  its 
own  activities.* 

*  I  was  here  thinking  only  of  the  cases  which  are  general  among  insects, 
but  it  seems  that  vertebrate  animals,  too,  furnish  cases.  Mr.  Cunningham 
writes : — "  There  is  a  curious  instance  of  this  in  the  Conger  :  the  female 


426  LAWS  OF  MULTIPLICATION. 

Once  more,  the  deduction  of  materials  for  the  propagation 
of  the  species  may  be  postponed  long  enough  to  allow  of  great 
bulk  and  complex  structure  being  attained.  The  procreative 
subtraction  then  setting  in,  while  it  checks  and  presently 
stops  growth,  may  be  so  moderate  as  to  leave  vital  capital 
sufficient  to  carry  on  the  activities  of  the  parent;  may  go 
on  as  long  as  parental  vigour  suffices  to  furnish,  without  fatal 
result,  the  materials  needed  to  produce  young  ones ;  and  may 
cease  when  such  a  surplus  cannot  be  supplied,  leaving  the 
parental  life  to  continue. 

§  326.  The  opposite  side  of  this  antagonism  has  also 
several  aspects.  Progress  of  organic  evolution  may  be  shown 
in  increased  bulk,  in  increased  structure,  in  increased  amount 
or  variety  of  action,  or  in  combinations  of  these;  and  under 
any  of  its  forms  this  carrying  higher  of  each  individuality, 
implies  a  correlative  retardation  in  the  establishment  of  new 
individualities. 

Other  things  equal,  every  normal  addition  to  the  bulk  of 
an  organism  is  an  augmentation  of  its  life.*  Besides  being  an 
advance  in  integration,  it  implies  a  greater  total  of  activities 
gone  through  in  the  assimilation  of  materials ;  and  it  implies, 
thereafter,  a  greater  total  of  the  vital  changes  taking  place 
from  moment  to  moment  in  all  parts  of  the  enlarged  mass. 
Moreover,  while  increased  size  is  thus,  in  so  far,  the  expres- 
sion of  increased  life,  it  is  also,  where  the  organism  is  active, 
the  expression  of  increased  ability  to  maintain  life — increased 
strength.  Aggregation  of  substance  is  almost  the  only  mode 
in  which  self-preserving  power  is  shown  among  the  lowest 
types;  and  even  among  the  highest,  sustaining  the  body  in 
its  integrity  is  that  in  which  self-preservation  fundamentally 
consists — is  the  end  which  the  widest  intelligence  is  indi- 

prows  to  6  or  7  feet  long  and  a  weight  of  60  Ibs.  and  upwards  and  then 
ceases  to  feed  for  6  months  while  the  eggs  develop,  and  when  the  eggs  are 
shed  dies." 

*  I  say  "  normal "  for  the  purpose  of  excluding  not  only  morbid  growths 
but  excess  of  fat. 


OBVERSE  A  PRIORI  PRINCIPLE.  427 

rectly  made  to  subserve.  While,  on  the  one  hand,  the  in- 
crease of  tissue  constituting  growth  is  conservative  both  in 
essence  and  in  result;  on  the  other  hand,  decrease  of  tissue, 
either  from  injury,  disease,  or  old  age,  is  in  both  essence  and 
result  the  reverse.  And  if  so,  every  addition  to  individual 
life  thus  implied,  necessarily  delays  or  diminishes  the  casting 
off  of  matter  to  form  new  individuals. 

Other  things  equal,  too,  a  greater  degree  of  organization 
involves  a  smaller  degree  of  that  disorganization  shown  by 
the  separation  of  reproductive  gemma  and  germs.  Detach- 
ment of  a  living  portion  or  portions  from  what  was  previously 
a  living  whole,  is  a  ceasing  of  co-ordination;  and  is  therefore 
essentially  at  variance  with  that  establishment  of  greater  co- 
ordination which  is  achieved  by  structural  development.  In 
the  extreme  cases  where  a  living  mass  is  continually  dividing 
and  subdividing,  it  is  manifest  that  there  cannot  arise  much 
physiological  division  of  labour;  since  progress  towards 
mutual  dependence  of  parts  is  prevented  by  the  parts  be- 
coming independent.  Contrariwise,  it  is  equally  clear  that 
in  proportion  as  the  physiological  division  of  labour  is  carried 
far,  the  separative  process  must  be  localized  in  some  com- 
paratively small  portion  of  the  organism,  where  it  may  go 
on  without  affecting  the  general  structure — must  become 
relatively  subordinate.  The  advance  that  is  shown  by 
greater  heterogeneity,  must  be  a  hindrance  to  multiplication 
in  another  way.  For  organization  entails  cost.  That  trans- 
fer and  transformation  of  materials  implied  by  differentia- 
tion, can  be  effected  only  by  expenditure  of  force;  and  this 
supposes  consumption  of  digested  and  absorbed  food,  which 
might  otherwise  have  gone  to  make  new  organisms,  or  the 
germs  of  them.  Hence,  that  individual  evolution  which 
consists  in  progressive  differentiation,  as  well  as  that  which 
consists  in  progressive  integration,  necessarily  diminishes 
that  species  of  dissolution,  general  or  local,  which  propaga- 
tion of  the  race  exhibits. 

In  active  organisms  we  have  yet  a  further  opposition 


428  LAWS  OF  MULTIPLICATION. 

between  self-maintenance  and  maintenance  of  the  race.  All 
motion,  sensible  and  insensible,  generated  by  an  animal  for 
the  preservation  of  its  life,  is  motion  liberated  from  decom- 
posed nutriment — nutriment  which,  if  not  thus  decomposed, 
would  have  been  available  for  reproduction;  or  rather — 
might  have  been  replaced  by  nutriment  fitted  for  reproductive 
purposes,  absorbed  from  other  kinds  of  food.  Hence,  in  pro- 
portion as  the  activities  increase — in  proportion  as,  by  its 
more  varied,  complex,  rapid,  and  vigorous  actions,  an  animal 
gains  power  to  support  itself  and  to  cope  with  surrounding 
dangers,  it  must  lose  power  to  propagate. 

§  327.  How  may  this  antagonism  be  best  expressed  in  a 
brief  way?  If  self-preservation  displayed  itself  in  the 
highest  organisms,  as  it  does  in  the  lowest,  in  little  else  but 
continuous  growth ;  and  if  race-preservation  consisted  always, 
as  it  does  often,  of  nothing  beyond  detachment  of  portions 
from  the  parental  mass;  then  the  antagonism  would  be, 
throughout,  the  obviously-necessary  one  of  integration  and 
disintegration.  Maintenance  of  the  individual  and  propaga- 
tion of  the  species,  being  respectively  aggregative  and  sepa- 
rative, it  would  be  as  self-evident  that  they  vary  inversely, 
as  it  is  self-evident  that  addition  and  subtraction  undo  one 
another.  But  though  the  simplest  types  show  us  the  opposi- 
tion of  self-maintenance  and  race-maintenance  almost  wholly 
under  this  form;  and  though  higher  types,  up  to  the  most 
complex,  exhibit  it  to  a  great  extent  under  this  form ;  yet,  as 
we  have  just  seen,  this  is  not  its  only  form.  The  total 
material  monopolized  by  the  individual  and  withheld  from 
the  race,  must  be  stated  as  the  quantity  united  to  form  its 
fabric,  plus  the  quantity  expended  in  differentiating  its 
fabric,  plus  the  quantity  expended  in  its  self-conserving 
actions.  Similarly,  the  total  material  devoted  to  the  race  at 
the  expense  of  the  individual,  includes  that  which  is  directly 
subtracted  from  the  parent  in  the  shape  of  egg  or  foetus,  plus 
that  which  is  directly  subtracted  in  the  shape  of  milk,  plus 


OBVERSE  1  PRIORI  PRINCIPLE.  429 

that  which  is  indirectly  subtracted  in  the  shape  of  matter 
consumed  in  exertions  for  fostering  the  young.  Hence 
this  inverse  variation  is  not  expressible  in  simple  terms  of 
aggregation  and  separation.  As  we  advance  to  more  highly- 
evolved  organisms,  the  total  cost  of  an  individual  becomes 
very  much  greater  than  is  implied  by  the  amount  of  tissue 
composing  it.  So,  too,  the  total  cost  of  producing  each  new 
individual  becomes  very  much  greater  than  that  of  its  mere 
substance.  And  it  is  between  these  two  total  costs  that  the 
antagonism  exists. 

We  may,  indeed,  reduce  the  antagonism  to  a  form  com- 
prehensive of  all  cases,  if  we  consider  it  as  existing  between 
the  sums  of  the  forces,  latent  and  active,  used  for  the  two 
purposes.  The  molecules  which  make  up  a  plant  or  animal, 
have  been  formed  by  the  absorption  of  forces  directly  or 
indirectly  derived  from  the  Sun;  and  hence  the  quantity  of 
matter  raised  to  the  form  called  organic,  which  a  plant  or 
animal  presents,  is  equivalent  to  a  certain  amount  of  force. 
Another  amount  of  force  is  expressed  by  the  totality  of  its 
differentiations.  A  further  amount  of  force  is  that  dissipated 
in  its  actions.  And  in  these  three  amounts  added  together, 
we  have  the  whole  expense  of  the  individual  life.  So,  too, 
the  whole  expense  of  establishing  each  new  individual  in- 
cludes— first  the  forces  latent  in  the  substance  composing 
it  when  born  or  hatched;  second  the  forces  latent  in  the 
prepared  nutriment  afterwards  supplied ;  and  third  the  forces 
expended  in  feeding  and  protecting  it.  These  two  sets  of 
forces  being  taken  from  a  common  fund,  it  is  manifest  that 
either  set  can  increase  only  by  decrease  of  the  other.  If,  of 
the  force  which  the  parent  obtains  from  the  environment, 
much  is  consumed  in  its  own  life,  little  remains  to  be  con- 
sumed in  producing  other  lives;  and,  conversely,  if  there  is 
a  great  consumption  in  producing  other  lives,  it  can  only  be 
where  comparatively  little  is  reserved  for  parental  life. 

Hence,  then,  Individuation  and  Genesis  are  necessarily 
antagonistic.  Grouping  under  the  word  Individuation  all 


430  LAWS  OF  MULTIPLICATION. 

processes  by  which  individual  life  is  completed  and  main- 
tained; and  enlarging  the  meaning  of  the  word  Genesis  so 
as  to  include  all  processes  aiding  the  formation  and  perfecting 
of  new  individuals;  we  see  that  the  two  are  fundamentally 
opposed.  Assuming  other  things  to  remain  the  same — 
assuming  that  environing  conditions  as  to  climate,  food, 
enemies,  &c.,  continue  constant;  then,  inevitably,  every 
higher  degree  of  individual  evolution  is  followed  by  a  lower 
degree  of  race-multiplication,  and  vice  versa.  Progress  in 
bulk,  complexity,  or  activity,  involves  retrogress  in  fertility; 
and  progress  in  fertility  involves  retrogress  in  bulk,  com- 
plexity, or  activity. 

This  statement  needs  a  slight  qualification.  For  reasons 
to  be  hereafter  assigned,  the  relation  described  is  never  com- 
pletely maintained;  and  in  the  small  departure  from  it,  we 
shall  find  a  remarkable  self-acting  tendency  to  further  the 
supremacy  of  the  most-developed  types.  Here,  however,  this 
hint  must  suffice:  explanation  would  carry  us  too  far  out  of 
our  line  of  argument.  For  the  present  it  will  not  lead  us 
astray  if  we  regard  this  inverse  variation  of  Individuation 
and  Genesis  as  exact. 

§  328.  Thus,  then,  the  condition  which  each  race  must 
fulfil  if  it  is  to  survive,  is  a  condition  which,  in  the  nature  of 
things,  it  ever  tends  to  fulfil.  In  the  last  chapter  we  saw 
that  a  species  cannot  be  maintained  unless  the  power  to 
preserve  individual  life  and  the  power  to  propagate  other 
individuals  vary  inversely.  And  here  we  have  seen  that, 
irrespective  of  an  end  to  be  subserved,  these  powers  cannot 
do  other  than  vary  inversely.  On  the  one  hand,  given  a 
certain  totality  of  destroying  forces  with  which  the  species 
has  to  contend;  and  in  proportion  as  its  members  have 
severally  but  small  ability  to  resist  these  forces,  it  is  requisite 
that  they  should  have  great  ability  to  form  new  individuals, 
and  vice  versa.  On  the  other  hand,  given  the  quantity  of 
force,  absorbed  as  food  or  otherwise,  which  the  species  can 


OBVERSE  A  PRIORI  PRINCIPLE.  431 

use  to  counterbalance  these  destroying  forces ;  and  in  propor- 
tion as  much  of  it  is  expended  in  preserving  the  individual, 
little  of  it  can  be  reserved  for  producing  new  individuals, 
and  vice  versa.  There  is  thus  complete  accordance  between 
the  requirements  considered  under  each  aspect.  The  two 
necessities  correspond. 

We  might  rest  on  these  deductions  and  their  several  corol- 
laries. Without  going  further  we  might  with  safety  assert 
the  general  truths  that,  other  things  equal,  advancing  evolu- 
tion must  be  accompanied  by  declining  fertility;  and  that,  in 
the  highest  types,  fertility  must  still  further  decrease  if  evolu- 
tion still  further  increases.  We  might  be  sure  that  if,  other 
things  equal,  the  relations  between  an  organism  and  its  en- 
vironment become  so  changed  as  permanently  to  diminish 
the  difficulties  of  self-preservation,  there  will  be  a  permanent 
increase  in  the  rate  of  multiplication;  and,  conversely,  that 
a  decrease  of  fertility  will  result  where  altered  circumstances 
make  self-preservation  more  laborious. 

But  we  need  not  content  ourselves  with  these  a  priori 
inferences.  If  they  are  true,  there  must  be  an  agreement 
between  them  and  the  observed  facts.  Let  us  see  how  far 
such  an  agreement  is  traceable. 


CHAPTER  IV. 

DIFFICULTIES   OF   INDUCTIVE   VERIFICATION. 

§  329.  WERE  all  species  subject  to  the  same  kinds  and 
amounts  of  destructive  forces,  it  would  be  easy,  by  comparing 
different  species,  to  test  the  inverse  variation  of  Individuation 
and  Genesis.  Or  if  either  the  power  of  self-preservation  or 
the  power  of  multiplication  were  constant,  there  would  be 
little  difficulty  in  seeing  how  the  other  changed  as  the 
destroying  forces  changed.  But  comparisons  are  nearly 
always  partially  vitiated  by  some  want  of  parity.  Each 
factor,  besides  being  variable  as  a  whole,  is  compounded  of 
factors  that  are  severally  variable.  Not  simply  is  the  sum 
of  the  forces  destructive  of  race  different  in  every  case;  and 
not  simply  are  both  sets  of  forces  preservative  of  race  unlike 
in  their  totalities  in  every  case ;  but  each  is  made  up  of  actions 
that  bear  such  changing  proportions  to  one  another  as  to 
prevent  any  positive  estimation  of  its  amount. 

Before  dealing  with  the  facts  as  well  as  we  can,  it  will  be 
best  to  glance  at  the  chief  difficulties;  so  that  we  may  see 
the  kind  of  verification  which  is  alone  possible. 

§  330.  Either  absolutely,  or  relatively  to  any  species,  every 
environment  differs  more  or  less  from  every  other. 

There  are  the  unlikenesses  of  media — air,  water,  earth, 
organic  matter;  severally  involving  special  resistances  to 
movement,  and  special  losses  of  heat.  There  are  the  con- 
432 


DIFFICULTIES  OF  INDUCTIVE  VERIFICATION.      433 

trasts  of  climate :  here  great  expenditure  for  the  maintenance 
of  temperature  is  needed,  and  there  very  little;  in  one 
zone  an  organism  is  supplied  with  abundant  light  all  the 
year  round,  and  in  another  only  for  a  few  months;  this 
region  yields  an  almost  unfailing  supply  of  water,  while  that 
entails  the  exertion  of  travelling  many  miles  every  night  for  a 
draught. 

Permanent  differences  in  the  natures  and  distributions  of 
aliment  greatly  interfere  with  the  comparisons.  The  Swal- 
low goes  through  more  exertion  than  the  Sparrow  in  securing 
a  given  weight  of  food;  but  then  their  foods  are  dissimilar 
in  nutritive  qualities.  There  is  a  want  of  parallelism  between 
the  circumstances  of  those  herbivores  which  live  where  the 
plains  are  annually  covered  for  a  time  with  rich  herbage,  but 
afterwards  become  parched  up,  and  of  those  inhabiting  more 
temperate  regions.  Insects  whose  larvae  feed  on  an  abundant 
plant,  as  do  several  of  the  genus  Vanessa  on  the  Nettle,  have 
practically  an  environment  very  unlike  that  of  insects  such 
as  Deilephila  Euphorbia,  whose  larvae  feed  on  a  comparatively 
rare  plant — the  Sea-Spurge. 

Again,  comparisons  between  creatures  otherwise  akin  in 
their  constitutions  and  circumstances,  are  hindered  by  ine- 
qualities in  their  relations  to  enemies.  Two  animals,  of 
which  one  is  predatory  and  has  no  foes  but  parasites  while 
the  other  is  much  pursued,  cannot  properly  be  contrasted 
with  a  view  to  determining  the  influence  of  size  or  com- 
plexity. 

Without  multiplying  instances,  it  will  be  clear  enough 
then  that  the  aggregate  of  destructive  actions,  positive  and 
negative,  which  each  species  has  to  contend  with,  is  so 
undennable  in  the  amounts  and  kinds  of  its  components, 
that  nothing  beyond  a  vague  idea  of  its  relative  total  can  be 
formed. 

§  331.  Besides   these   immense   variations   in   the   outer 
actions  to  be  counter-balanced,  there  are  immense  variations 
74 


434:  LAWS  OP  MULTIPLICATION. 

in  the  inner  actions  required  to  counter-balance  them.  Even 
were  species  similarly  conditioned,  self-preservation  would 
require  of  them  extremely  unlike  expenditures  of  force. 

The  cost  of  locomotion  increases  in  a  greater  ratio  than 
the  size.  In  virtue  of  the  law  that  the  weights  of  animals  in- 
crease as  the  cubes  of  their  dimensions,  while  their  powers  of 
bearing  strains  increase  only  as  the  squares  of  their  dimen- 
sions (§46),  preservation  of  its  various  attitudes  requires  a 
large  animal  to  consume  more  substance  in  proportion  to  its 
weight,  than  it  requires  a  small  animal  to  consume ;  and  there 
results,  other  things  equal,  a  difficulty  of  self-maintenance 
which  augments  in  a  more  rapid  ratio  than  the  bulk.  Nor 
must  we  overlook  the  further  complication,  that  among 
aquatic  creatures  the  variation  of  resistance  of  the  medium 
tends  to  produce  an  opposite  effect. 

Again,  the  heat-consumption  is  a  changing  element  in  the 
total  expense  of  self-preservation.  Creatures  which  have  tem- 
peratures scarcely  above  that  of  the  air  or  water,  may,  other 
things  equal,  accumulate  more  surplus  nutriment  than  crea- 
tures which  have  to  keep  their  bodies  warm  spite  of  the  con- 
tinual loss  by  radiation  and  conduction.  This  difference  of 
cost  is  modified  by  the  presence  or  absence  of  natural  cloth- 
ing; and  it  is  also  modified  by  unlikenesses  of  size.  Here 
the  bulky  animals  have  the  advantage :  small  masses  cooling 
more  rapidly  than  large  ones. 

Dissimilarities  of  attack  and  defence  are  also  causes  of 
variation  in  the  outlay  for  self-maintenance.  A  creature  that 
has  to  hunt,-  as  compared  with  another  that  gets  a  sufficiency 
of  prey  by  lying  in  wait,  or  a  creature  that  escapes  by  speed 
as  compared  with  another  that  escapes  by  concealment, 
obviously  leads  a  life  that  is  physiologically  more  costly. 
Animals  which  protect  themselves  passively,  as  the  Hedge-hog 
by  its  spines  or  as  the  Skunk  and  the  Musk-rat  by  their  in- 
tolerable odours,  are  relatively  economical;  and  have  the 
more  vital  capital  for  other  purposes. 

Amplification  is  needless.    These  instances  will  show  that 


DIFFICULTIES  OF  INDUCTIVE  VERIFICATION.      435 

anything  beyond  very  general  conceptions  of  the  individual 
expenditures  in  different  cases,  cannot  be  reached. 

§  332.  Still  more  entangled  are  we  among  qualifying  con- 
siderations when  we  contrast  species  in  their  powers  of  multi- 
plication. The  total  cost  of  Genesis  admits  of  even  less 
definite  estimation  than  does  the  total  cost  of  Individua- 
tion.  I  do  not  refer  merely  to  the  truth  that  the  degree  of 
fertility  depends  on  four  factors — the  age  of  commencing 
reproduction,  the  number  in  each  brood,  the  frequency  of  the 
broods,  and  the  time  during  which  broods  continue  to  be 
repeated.  There  are  many  further  obstacles  in  the  way  of 
comparisons. 

Were  all  multiplication  carried  on  sexually,  the  problem 
would  be  less  involved;  but  there  are  many  kinds  of  asexual 
multiplication  alternating  with  the  sexual.  This  asexual 
multiplication  is  in  some  cases  perpetual  instead  of  occa- 
sional; and  often  has  more  forms  than  one  in  the  same 
species.  The  result  is  that  we  have  to  compare  what  is  here 
a  periodic  process  with  what  is  elsewhere  a  cyclical  process 
partly  continuous  and  partly  periodic:  the  calculation  of 
fertility  in  this  last  case  being  next  to  impossible. 

We  have  to  avoid  being  misled  by  the  assumption  that  the 
cost  of  Genesis  is  measured  by  the  number  of  young  produced, 
instead  of  being  measured,  as  it  is,  by  the  weight  of  nutri- 
ment abstracted  to  form  the  young,  plus  the  weight  con- 
sumed in  caring  for  them.  This  total  weight  may  be  very 
diversely  apportioned.  In  contrast  to  the  Cod  with  its 
millions  of  small  ova  spawned  without  protection,  we  may 
put  the  Hippocampus,  or  the  Pipe-fish,  with  its  few  relatively- 
large  ova  carried  about  by  the  male  in  a  caudal  pouch,  or 
seated  in  hemispherical  pits  in  its  skin;  or  we  may  put  the 
still  more  remarkable  genus  Arius,  and  especially  Arius 
BoaJceii — a  fish  some  six  or  seven  inches  long,  which  produces 
ten  or  a  dozen  eggs  5 — 10  mm.  in  diameter,  that  are  carried 
by  the  male  in  his  mouth  till  they  are  hatched.  Here  though 


436  LAWS  OF  MULTIPLICATION. 

the  degrees  of  fertility,  if  measured  by  the  numbers  of 
fertilized  germs  deposited,  are  extremely  unlike,  they  are 
less  unlike  if  measured  by  the  numbers  of  young  which  are 
hatched  and  survive  long  enough  to  take  care  of  themselves ; 
nor  will  the  tax  on  the  parent-Cod  seem  so  immensely  dif- 
ferent from  that  on  the  parent- Arms,  if  the  masses  of  the 
ova,  instead  of  their  numbers,  are  compared.  Again, 

while  sometimes  the  parental  loss  is  little  else  but  the  matter 
deducted  to  form  eggs,  &c.,  at  other  times  it  takes  the 
shape  of  a  small  direct  deduction  joined  with  a  large  indirect 
outlay.  The  Mason-wasp  furnishes  a  typical  instance.  In 
journeyings  hither  and  thither  to  fetch  bit  by  bit  the 
materials  for  building  a  cell;  in  putting  together  these 
materials,  as  well  as  in  secreting  glutinous  matter  to  act  as 
cement;  and  then,  afterwards,  in  the  labour  of  seeking  for, 
and  carrying,  the  small  caterpillars  with  which  it  fills  up  the 
cell  to  serve  its  larva  with  food  when  it  emerges  from  the 
egg;  the  Mason-wasp  expends  more  substance  than  is  con- 
tained in  the  egg  itself.  And  this  supplementary  expenditure 
is  manifestly  so  great  that  but  few  eggs  can  be  housed  and 
provisioned. 

Estimates  of  the  cost  of  Genesis  are  further  complicated 
by  variations  in  the  ratio  borne  by  the  two  sexes.  Among 
Fishes  the  mass  of  milt  approaches  in  size  the  mass  of  spawn ; 
but  among  higher  Vertebrata  the  substance  lost  by  the  one 
sex  in  the  shape  of  sperm-cells  is  small  compared  with  that 
lost  by  the  other  sex  in  the  shape  of  albumen  stored-up  in 
the  eggs,  or  blood  supplied  to  the  foetus,  or  milk  given  to  the 
young.  Then  there  come  the  differences  of  indirect  tax 
011  males  and  females.  While,  frequently,  the  fostering  of 
the  young  devolves  entirely  on  the  female,  occasionally  the 
male  undertakes  it  wholly  or  in  part.  After  building  a 
nest,  the  male  Stickleback  guards  the  eggs  till  they  are 
hatched;  as  does  also  the  great  Silurus  glanis  for  some  forty 
days,  during  which  he  takes  no  food.  And  then,  among  most 
birds,  we  have  the  male  occupied  in  feeding  the  female  during 


DIFFICULTIES  OF  INDUCTIVE  VERIFICATION.      437 

incubation,  and  the  young  afterwards.  Evidently  all  these 
differences  affect  the  proportion  between  the  total  cost  of  re- 
production and  the  total  cost  of  individuation. 

Whether  the  species  is  monogamous  or  polygamous,  and 
whether  there  are  marked  differences  of  size  or  of  structure 
between  males  and  females,  are  also  questions  not  to  be  over- 
looked. If  there  are  many  females  to  one  male,  the  total 
quantity  of  assimilated  matter  devoted  by  each  generation  to 
the  production  of  a  new  generation,  is  greater  than  if  there 
is  a  male  to  each  female.  Similarly,  where  the  requirements 
are  such  that  small  males  will  suffice,  the  larger  quantity  of 
food  left  for  the  females  makes  possible  a  greater  surplus 
available  for  reproduction.  Another  cause  has  a  like  effect. 
Where  the  habits  of  the  race  render  it  needless  that  both 
sexes  should  have  developed  powers  of  locomotion — where, 
as  in  the  Glow-worm  and  sundry  Lepidoptera,  the  female  is 
wingless  while  the  male  has  wings — the  cost  of  Individuation 
not  being  so  great  for  the  species  as  a  whole,  there  arises  a 
greater  reserve  for  Genesis:  the  matter  which  would  other- 
wise have  gone  to  the  production  of  wings  and  the  using  of 
them,  may  go  to  the  production  of  ova. 

Other  complications,  as  those  which  we  see  in  Bees  and 
Ants,  might  be  dwelt  on;  but  the  foregoing  will  amply  serve 
the  intended  purpose. 

§  333.  To  ascertain  by  comparison  of  cases  whether  Indi- 
viduation and  Genesis  vary  inversely,  is  thus  an  undertaking 
so  beset  with  difficulties,  that  we  might  despair  of  any  satis- 
factory results,  were  not  the  relation  too  marked  a  one  to  be 
hidden  even  by  all  these  complexities.  Species  are  so  ex- 
tremely contrasted  in  their  degrees  of  evolution,  and  so 
extremely  contrasted  in  their  rates  of  multiplication,  that  the 
law  of  relation  between  these  traits  becomes  unmistakable 
when  the  evidence  is  looked  at  in  its  ensemble.  This  we  shall 
soon  find  on  ranging  in  order  a  number  of  typical  cases. 

In  doing  this  it  will  be  convenient  to  neglect,  temporarily, 


438  LAWS  OF  MULTIPLICATION. 

all  unlikenesses  among  the  circumstances  in  which  organ- 
isms are  placed.  At  the  outset,  we  will  turn  our  attention 
wholly  to  the  antagonism  displayed  between  the  integrative 
process  which  results  in  individual  evolution  and  the  disinte- 
grative  process  which  results  in  multiplication  of  individuals ; 
and  this  we  will  consider  first  as  we  see  it  under  the  several 
forms  of  agamogenesis,  and  then  as  we  see  it  under  the 
several  forms  of  gamogenesis.  We  will  next  look  at  the 
antagonism  between  propagation  and  that  evolution  which  is 
shown  by  increased  complexity.  And  then  we  will  consider 
the  remaining  phase  of  the  antagonism,  as  it  exists  between 
the  degree  of  fertility  and  the  degree  of  evolution  expressed 
by  activity. 

Afterwards,  passing  to  the  varying  relations  between 
organisms  and  their  environments,  we  will  note  how  relative 
increase  in  the  supply  of  food,  or  relative  decrease  in  the 
quantity  of  force  expended  by  the  individual,  entails  relative 
increase  in  the  quantity  of  force  devoted  to  multiplication, 
and  vice  versa. 

Certain  minor  qualifications,  together  with  sundry  impor- 
tant corollaries,  may  then  be  entered  upon. 


CHAPTEE  V. 

ANTAGONISM  BETWEEN  GROWTH  AND  ASEXUAL  GENESIS. 

§  334.  WHEN  illustrating,  in  Part  IV,  the  morphological 
composition  of  plants  and  animals,  there  were  set  down  in 
groups,  numerous  facts  which  we  have  here  to  look  at  from 
another  point  of  view.  Then  we  saw  how,  hy  union  of  small 
simple  aggregates,  there  are  produced  large  compound  aggre- 
gates. Now  we  have  to  observe  the  reactive  effect  of  this 
process  on  the  relative  numbers  of  the  aggregates.  Our  pre- 
sent subject  is  the  antagonism  of  Individuation  and  Genesis 
as  seen  under  its  simplest  form,  in  the  self-evident  truth  that 
the  same  quantity  of  matter  may  be  divided  into  many  small 
wholes  or  few  large  wholes ;  but  that  number  negatives  large- 
ness and  largeness  negatives  number. 

In  setting  down  some  examples  we  may  conveniently 
adopt  the  same  arrangement  as  before.  We  will  look  at  the 
facts  as  they  are  presented  by  vegetal  aggregates  of  the  first 
order,  of  the  second  order,  and  of  the  third  order;  and  then 
as  they  are  presented  by  animal  aggregates  of  the  same  three 
orders. 

§  335.  The  ordinary  unicellular  plants  are  at  once  micro- 
scopic and  enormously  prolific.  The  often  cited  Sphcerella 
nivalis,  which  shows  its  immense  powers  of  multiplication  by 
reddening  wide  tracts  of  snow  in  a  single  night,  does  this  by 
developing  in  its  cavity  a  brood  of  young  cells,  which,  being 

439 


440  LAWS  OF  MULTIPLICATION. 

presently  set  free  by  the  bursting  of  the  parent-cell,  severally 
grow  and  quickly  repeat  the  process.  The  like  occurs  among 
sundry  of  those  kindred  forms  of  minute  A Igce  which,  by 
their  enormous  numbers,  sometimes  suddenly  change  pools  to 
an  opaque  green.  So,  too,  the  Desmidiacece  often  multiply  so 
greatly  as  to  colour  the  water;  and  among  the  Diatomacece 
the  rate  of  genesis  by  self-division,  "  is  something  really  ex- 
traordinary. So  soon  as  a  frustule  is  divided  into  two,  each 
of  the  latter  at  once  proceeds  with  the  act  of  self-division ;  so 
that,  to  use  Professor  Smith's  approximative  calculation  of 
the  possible  rapidity  of  multiplication,  supposing  the  process 
to  occupy,  in  any  single  instance,  twenty-four  hours,  *  we 
should  have,  as  the  progeny  of  a  single  frustule,  the  amazing 
number  of  one  thousand  millions  in  a  single  month.' "  In 
these  cases  the  multiplication  is  so  carried  on  that  the  parent 
is  lost  in  the  offspring — the  old  individuality  disappears  either 
in  the  swarms  of  zoospores  it  dissolves  into,  or  in  the  two 
or  four  new  individualities  simultaneously  produced  by  fis- 
sion. Vegetal  aggregates  of  the  first  order,  have,  how- 
ever, a  form  of  agamogenesis  in  which  the  parent  individual- 
ity is  not  lost :  the  young  cells  arise  from  the  old  cells  by  ex- 
ternal gemmation.  This  process,  too,  repeated  as  it  is  at 
short  intervals,  results  in  immense  fertility.  The  Yeast- 
fungus,  which  in  a  few  hours  thus  propagates  itself  through- 
out a  large  vat  of  wort,  offers  a  familiar  example. 

In  certain  compound  forms  that  must  be  classed  as  plants 
of  the  second  order  of  aggregation,  though  very  minute  ones, 
self-division  similarly  increases  the  numbers  at  high  rates. 
The  Sarcina  ventriculi,  a  parasitic  plant  which  infests  the 
stomach  and  swarms  afresh  as  fast  as  previous  swarms  are 
vomited,  shows  us  a  spontaneous  fission  of  clusters  of  cells. 
An  allied  mode  of  increase  occurs  in  Gonium  pectorale:  each 
cell  of  the  cluster  resolving  itself  into  a  secondary  cluster, 
and  the  secondary  clusters  then  separating.  "  Supposing, 
which  is  very  probable,  that  a  young  Gonium  after  twenty- 
four  hours  is  capable  of  development  by  fission,  it  follows 


GROWTH  AND  ASEXUAL  GENESIS  .441 

that  under  favourable  conditions  a  single  colony  may  on  the 
second  day  develop  16,  on  the  third  256,  on  the  fourth  4,096, 
and  at  the  end  of  a  week  268,435,456  other  organisms  like 
itself/'  In  the  Volvoclnece  this  continual  dissolution  of  a 
primary  compound  individual  into  secondary  compound  indi- 
viduals, is  carried  on  endogenously,  and  on  a  modified  system : 
some  only  of  the  component  cells  giving  origin  to  young 
colonies,  and  the  parent  bursting  to  liberate  them.  The 
numbers  arising  by  this  method  also,  are  sometimes  so  great 
as  to  tint  large  bodies  of  water.  More  fully  estab- 

lished and  organized  aggregates  of  the  second  order,  such  as 
the  higher  Thallophytes  and  the  lower  Archegoniates,  do  not 
sacrifice  their  individualities  by  fission;  but  nevertheless,  by 
the  kindred  process  of  gemmation,  are  continually  hindered 
in  the  increase  of  their  individualities.  The  gemmae  called 
tetraspores  are  cast  off  in  great  numbers  by  the  marine  Algae. 
Among  those  simple  Jungermanniacece  which  consist  of  single 
fronds,  the  young  ones  that  bud  out  grow  for  a  time  in  con- 
nexion with  their  parents,  send  rootlets  from  their  under 
sides  into  the  soil,  and  presently  separate  themselves  —  a 
habit  which  augments  the  number  of  individuals  in  propor- 
tion as  it  checks  their  growths. 

Plants  of  the  third  order  of  composition,  arising  by  arrest 
of  this  separation,  exhibit  a  further  corresponding  decrease 
in  the  abundance  of  the  aggregates  formed.  Archegoniates  of 
inferior  types,  in  which  the  axes  produced  by  integration  of 
fronds  are  but  small  and  feeble,  are  characterized  by  the 
habit  of  throwing  off  bulbils — bud-shaped  axes  which,  falling 
and  taking  root,  add  to  the  number  of  distinct  individuals. 
This  agamic  multiplication,  very^  general  among  the  Mosses 
and  their  kindred,  and  not  uncommon  under  a  modified  form 
in  such  higher  types  as  the  Ferns,  many  of  which  produce 
young  ones  from  the  surfaces  of  their  fronds,  becomes  very 
unusual  among  Phasnogams.  The  detachment  of  bulbils, 
though  not  unknown  among  them,  is  exceptional.  And  while 
it  is  true  that  some  flowering  plants,  as  the  Strawberry, 


442  LAWS  OP  MULTIPLICATION. 

multiply  by  a  process  allied  to  gemmation,  yet  this  is  not 
characteristic  of  the  class.  A  leading  trait  of  these  highest 
groups,  to  which  the  largest  members  of  the  vegetal  king- 
dom belong,  is  that  agamogenesis  has  so  far  ceased  that  it 
does  not  usually  originate  independent  plants.  Though  the 
axes  which,  budding  one  out  of  another,  compose  a  tree, 
are  the  equivalents  of  asexually-produced  individuals;  yet 
the  asexual  production  of  them  stops  short  of  separation. 
These  vast  integrations  arise  where  spontaneous  disintegra- 
tion, and  the  multiplication  effected  by  it,  have  come  to  an 
end. 

Thus,  not  forgetting  that  certain  Phasnogams,  as  Begonia 
pliyllomaniaca,  revert  to  quite  primitive  modes  of  increase, 
we  may  hold  it  as  beyond  question  that  while  among  the 
most  minute  plants  asexual  multiplication  is  universal,  and 
produces  enormous  numbers  in  short  periods,  it  becomes  step 
by  step  more  restricted  in  range  and  frequency  as  we  ad- 
vance to  large  and  compound  plants;  and  disappears  so 
generally  from  the  highest  and  largest,  that  its  occurrence  is 
regarded  as  anomalous. 

§  336.  Parallel  examples  furnished  by  animals  make  clear 
the  purely  quantitative  nature  of  this  relation  under  its  origi- 
nal form.  Among  the  Protozoa,  as  among  the  Protophyta, 
there  occurs  that  process  by  which  the  individuality  of  the 
parent  is  wholly  lost  in  producing  offspring — the  breaking 
up  of  the  parental  mass  into  a  number  of  germs.  Some  of 
the  Infusoria,  as  for  instance  those  of  the  genus  Kolpoda 
and  several  allied  genera,  become  encysted  and  subsequently 
break  up  into  young  ones.  The  more  familiar  mode 

of  increase  among  these  animal-aggregates  of  the  first  order, 
by  fission,  though  it  sacrifices  the  parent  individuality  by 
merging  it  in  the  individualities  of  the  two  produced,  sacri- 
fices it  less  completely  than  does  the  dissolution  into  a  great 
number  of  germs.  Occurring,  however,  as  this  fission  does, 
very  frequently,  and  being  completed,  in  some  cases  that 


GROWTH  AND  ASEXUAL  GENESIS.  443 

have  been  observed,  in  the  course  of  half-an-hour,  it  results 
in  immensely- rapid  multiplication.  If  all  its  offspring  sur- 
vive, and  continue  dividing  themselves,  a  single  Paramcecium 
is  said  to  be  capable  of  thus  originating  268  millions  in  the 
course  of  a  month.*  Nor  is  this  the  greatest  known  rate 
of  increase.  Another  animalcule,  visible  only  under  a  high 
magnifying  power,  "  is  calculated  to  generate  170  billions 
in  four  days."  f  And  these  enormous  powers  of  propagation 
are  accompanied  by  a  minuteness  so  extreme,  that  of  some 
species  one  drop  of  water  would  contain  as  many  individuals 
as  there  are  human  beings  on  the  Earth !  Even  if  we  allow 
a  large  margin  for  exaggeration  in  these  estimates,  it  is 
beyond  question  that  among  these  smallest  of  animals  the 
rate  of  asexual  multiplication  is  immensely  the  greatest; 
and  this  suffices  for  the  purposes  of  argument. 

Of  animal  aggregates  belonging  to  the  second  order,  that 
multiply  asexually  with  rapidity,  the  familiar  Polypes 
furnish  conspicuous  examples.  By  gemmation  in  most  cases, 
in  other  cases  by  fission,  and  in  some  »ases  by  both,  the 
agamogenesis  is  carried  on  among  these  tribes.  As  shown  in 
Fig.  148,  the  budding  of  young  ones  from  the  parent  Hydra 
is  carried  on  so  actively,  that  before  the  oldest  of  them  is 
cast  off  half-a-dozen  or  more  others  have  reached  various 
stages  of  growth;  and  even  while  still  attached,  the  first- 
formed  of  the  group  have  commenced  budding  out  from  their 
sides  a  second  generation  of  young  ones.  In  the  Hydra  tuba 

*  To  meet  a  possible  criticism  it  should  bo  remarked  that  this  calcula- 
tion assumes  that  the  power  of  asexual  reproduction  is  not  exhausted  by 
the  end  of  the  month.  It  has  been  found  that  "  the  successive  fissions  of 
Paramcecium  cannot  continue  indefinitely.  After  some  hundreds  of  genera- 
tions the  products  of  fission  are  small,  have  no  mouth,  and  die  unless  before 
this  they  have  been  allowed  to  conjugate  with  individuals  of  another  brood." 
It  may,  however,  be  fairly  taken  for  granted  that  "  some  hundreds  of  genera- 
tions "  would  take  longer  than  a  month. 

f  Even  this  number  is  far  exceeded.  Dr.  Edward  Klein,  in  a  lecture  he 
gave  at  the  Royal  Institution  on  June  2,  1898,  asserted  that  246  bacteria  in 
a  cubic  centimetre  of  nutritive  liquid  would  multiply  'to  20,000,000  in  the 
course  of  twenty-four  hours  :  a  rate  which,  at  the  end  of  the  third  day,  would 
give,  as  the  offspring  of  one  individual,  537,367, 797,000,000. 


444  LAWS  OF  MULTIPLICATION. 

this  gemmiparous  multiplication  is  from  time  to  time  inter- 
rupted by  a  transverse  splitting-up  of  the  body  into  segments, 
which  successively  separate  and  swim  away:  the  result  of 
the  two  processes  being  that,  in  the  course  of  a  season,  there 
are  produced  from  a  single  germ  great  numbers  of  young 
Medusce,  which  are  the  adult  or  sexual  forms  of  the  species. 
Eespecting  ccelenterate  animals  of  this  degree  of  composi- 
tion, it  may  be  added  that  when  we  ascend  to  the  larger 
kinds  we  find  asexual  genesis  far  less  active.  Though  com- 
parisons are  interfered  with  by  differences  of  structure  and 
mode  of  life,  yet  the  contrasts  are  too  striking  to  have  their 
meanings  much  obscured.  If,  for  instance,  we  take  a  solitary 
Actinozoon  and  a  solitary  Hydrozoon,  we  see  that  the  rela- 
tively-great bulk  of  the  first,  goes  along  with  a  relatively- 
slow  agamogenesis.  The  common  Sea-anemones  are  but 
occasionally  observed  to  undergo  self-division :  multiplication 
by  budding  being  in  some  cases  largely  followed,  but  their 
numbers  are  not  rapidly  increased  by  either  process.  A 
higher  class  of  secondary  aggregates  exemplifies  the  same 
general  truth  with  a  difference.  In  the  smaller  members  the 
agamogenesis  is  incomplete,  and  in  the  larger  it  disappears. 
The  gemmation  of  the  minute  Polyzoa,  though  it  does  not 
end  in  the  separation  of  the  young  individuals,  habitually 
goes  to  the  extent  of  producing  families  of  partially-inde- 
pendent individuals ;  but  their  near  ally,  the  Phoronis,  which 
immensely  exceeds  them  in  size,  is  solitary  and  not  gemmi- 
parous. So,  too,  is  it  with  the  Ascidioida.  And  then  among 
the  true  Mollusca,  which  are  relatively  large,  no  such  thing 
is  known  as  fission  or  gemmation. 

Take  next  the  Annulosa,  including  under  this  title  the 
Annelida  and  Arthropoda.  When  treating  of  morphological 
composition,  reasons  were  given  for  the  belief  that  the  annu- 
lose  animal  is  an  aggregate  of  the  third  order,  the  segments 
of  which,  produced  one  from  another  by  gemmation,  ori- 
ginally became  separate;  but  by  progressive  integration,  or 
arrested  disintegration,  there  resulted  a  type  in  which  many 


GROWTH  AND  ASEXUAL  GENESIS.  445 

such  segments  were  permanently  united  (§§205-7  and  note 
to  §  207).  Part  of  the  evidence  there  assigned,  is  evidence  to 
be  here  repeated  in  illustration  of  the  direct  antagonism  of 
Growth  and  Asexual-Genesis.  We  saw  how,  among  the 
loM'er  Annelids,  the  string  of  segments  produced  by  gemma- 
tion presently  divides  transversely  into  two  strings;  and 
how,  in  some  cases,  this  resolution  of  the  elongating  string 
of  segments  into  groups  that  are  to  form  separate  individuals, 
goes  on  so  actively  that  as  many  as  six  groups  are  found  in 
different  stages  of  progress  to  ultimate  independence — a  fact 
implying  a  high  rate  of  fissiparous  multiplication.*  Then  we 
saw  that,  in  the  superior  annulose  types,  distinguished  in  the 
mass  by  including  the  larger  species,  fission  does  not  occur. 
The  higher  Annelids  do  not  propagate  in  this  way;  there  is 
no  known  case  of  new  individuals  being  so  formed  among 
the  Myriapoda;  nor  do  the  Crustaceans  afford  us  a  single 
instance  of  this  primordial  mode  of  increase.  It  is, 

indeed,  true  that  while  articulate  animals  never  multiply 
asexually  after  this  simplest  method,  and  while  they  are 
characterized  in  the  mass  by  the  cessation  of  agamogenesis  of 
every  kind,  there  nevertheless  occur  in  a  few  of  their  small 
species,  those  higher  forms  of  agamogenesis  known  as  parthe- 
nogenesis and  pseudo-parthenogenesis;  and  that  by  these 
some  of  them  multiply  very  rapidly.  Hereafter  we  shall 
find,  in  the  interpretation  of  these  anomalies,  further  support 
for  the  general  doctrine. 

To  the  above  evidence  has  to  be  added  that  which  the 
Vertebrata  present.  This  may  be  very  briefly  summed  up. 
On  the  one  hand  this  class,  whether  looked  at  in  the  aggre- 
gate or  in  its  particular  species,  immensely  exceeds  all  other 
classes  in  the  sizes  of  its  individuals ;  and  on  the  other  hand, 
agamogenesis  under  any  form  is  absolutely  unknown  in  it. 
If  it  be  said  that  budding  occurs  among  the  Tunicata  which, 

*  It  has  since  been  shown  that  in  Hfyrianida  fasciata  as  many  as  29  at- 
tached groups  exist.  See  Cambridge  Natural  History,  Vol.  II,  Worms,  Roti- 
fers and  Polyzoa,  p.  280. 


446  -:     LAWS  OF  MULTIPLICATION. 

under  the  common  title  of  Chordata,  are  included  in  the  same 
phylum  with  the  Vertebrata,  then  it  may  firstly  be  replied 
that  those  types  which  have  no  vertebra  cannot  properly  be 
called  Vertebrata,  and  secondly  that  if,  as  being  Chordata, 
they  must  be  recognized,  then  the  exception  which  they  pre- 
sent further  illustrates  the  truth  that  agamogenetic  multi- 
plication occurs  only  in  creatures  small  in  size,  or  low  in 
structure,  or  both. 

§  337.  Such  are  a  few  leading  facts  serving  to  show  how 
deduction  is  inductively  verified,  in  so  far  as  the  antagonism 
between  Growth  and  Asexual  Genesis  is  concerned.  In 
whatever  way  we  explain  this  opposition  of  the  integrative 
and  disintegrative  processes,  the  facts  and  their  implications 
remain  the  same.  Indeed  we  need  not  commit  ourselves  to 
any  hypothesis  respecting  the  physical  causation.  It  suffices 
to  recognize  the  results  under  their  most  general  aspects. 
"We  cannot  help  admitting  there  are  at  work  these  two  anta- 
gonist tendencies  to  aggregation  and  separation;  and  we 
cannot  help  admitting  that  the  proportion  between  the  aggre- 
gative and  separative  tendencies,  must  in  each  case  determine 
the  relation  between  increase  in  bulk  of  the  individual  and 
increase  of  the  race  in  number. 

The  antithesis  is  as  manifest  a  posteriori  as  it  is  necessary 
d  priori.  While  the  minutest  organisms  multiply  asexually 
in  their  billions;  while  the  Infusoria  thus  multiply  in  their 
millions;  while  the  small  compound  types  next  above  them 
thus  multiply  in  their  thousands;  while  larger  and  more 
compound  types  thus  multiply  in  their  hundreds  and  their 
tens;  the  largest  types  do  not  thus  multiply  at  all.  Con- 
versely, those  which  do  not  multiply  asexually  at  all,  are  a 
billion  or  a  million  times  the  size  of  those  which  thus  mul- 
tiply with  greatest  rapidity;  and  are  a  thousand  times,  or  a 
hundred  times,  or  ten  times  the  size  of  those  which  thus 
multiply  with  less  and  less  rapidity.  Without  saying  that 
this  inverse  proportion  is  regular,  which,  as  we  shall  here- 


GROWTH  AND  ASEXUAL  GENESIS.  447 

after  see,  it  cannot  be,  we  may  unhesitatingly  assert  its 
average  truth.  That  the  smallest  organisms  habitually  re- 
produce asexually  with  immense  rapidity;  that  the  largest 
organisms  never  reproduce  at  all  in  this  manner;  and  that 
between  these  extremes  there  is  a  general  decrease  of  asexual 
reproduction  along  with  an  increase  of  bulk;  are  proposi- 
tions which  admit  of  no  dispute. 


CHAPTER  VI. 

ANTAGONISM  BETWEEN  GROWTH  AND  SEXUAL  GENESIS. 

§  338.  IN  so  far  as  it  is  a  process  of  separation,  sexual 
genesis  is  like  asexual  genesis ;  and  is  therefore,  equally  with 
asexual  genesis,  opposed  to  that  aggregation  which  results  in 
growth.  Whether  deduction  is  made  from  one  parent  or 
from  two,  whether  it  is  made  from  any  part  of  the  body 
indifferently  or  from  a  specialized  part,  or  whether  it  is  made 
directly  or  indirectly,  it  remains  in  any  case  a  deduction; 
and  in  proportion  as  it  is  great,  or  frequent,  or  both,  it  must 
restrain  the  increase  of  the  individual. 

Here  we  have  to  group  together  the  leading  illustrations 
of  this  truth.  We  will  take  them  in  the  same  order  as 
before. 

§  3*39.  The  lowest  vegetal  forms,  or  rather,  we  may  say, 
those  forms  which  we  cannot  class  as  either  distinctly  vegetal 
or  distinctly  animal,  show  us  a  process  of  sexual  multiplica- 
tion that  differs  much  less  from  the  asexual  process  than  in 
the  higher  forms.  The  common  character  which  distinguishes 
sexual  from  asexual  genesis,  is  that  the  mass  of  protoplasm 
whence  a  new  generation  is  to  arise,  has  been  produced  by  the 
union  of  two  portions  of  matter  which  were  before  more  wide- 
ly separated.  I  use  this  general  expression  because,  among 
the  simplest  Algce,  this  is  not  invariably  matter  supplied 
by  different  individuals :  certain  Diatomacece  exhibit  within 
a  single  cell,  the  formation  of  a  sporangium  by  a  drawing 
448 


GROWTH  AND  SEXUAL  GENESIS.  449 

together  of  the  opposite  halves  of  the  endochrome  into  a 
ball.  Mostly,  however,  sporangia  are  products  of  conjuga- 
tion. The  protoplasmic  contents  of  two  cells  unite  to  form 
the  germ-mass  or  zygote;  and  these  conjugating  cells  may  be 
either  entirely  independent,  as  in  many  Desmidiacece  and  in 
the  gametes  of  many  Confervoidece;  or  they  may  be  two  of 
the  adjacent  cells  forming  a  thread,  as  in  some  Conjugates 
and  the  gametes  of  Confervoidece;  or  they  may  be  cells 
belonging  to  adjacent  threads,  as  in  other  Conjugates.  But 
whether  it  is  originated  by  a  single  parent-cell,  or  by  two 
parent-cells,  the  zygote,  after  remaining  quiescent  until  there 
recur  the  fit  conditions  for  growth,  either  breaks  up  into  a 
multitude  of  spores,  each  of  which  produces  an  individual 
that  usually  multiplies  asexually,  or  germinates  directly  to 
produce  one  new  individual;  and  the  fact  here  to  be  noted 
is,  that  as  the  entire  contents  of  the  parent-cells  unite  to 
form  the  zygote,  their  individualities  are  lost  in  the  germs  of 
a  new  generation.  In  these  minute  simple  types,  sexual 
propagation  just  as  completely  sacrifices  the  life  of  the 
parent  or  parents,  as  does  that  form  of  asexual  propagation 
in  which  the  protoplasm  resolves  itself  directly  into  zoo- 
spores.  And  in  the  one  case  as  in  the  other,  this  sacrifice 
is  the  concomitant  of  a  prodigious  fertility.  Slightly 

in  advance  of  this,  but  still  showing  us  an  almost  equal  loss 
of  parental  life  in  the  lives  of  offspring,  is  the  process  seen  in 
such  unicellular  Algce  as  Botrydium,  and  in  minute  Fungi  of 
the  same  degree  of  composition.  These  exhibit  a  relatively- 
enormous  development  of  the  spore-producing  part,  and  an 
almost  entire  absorption  of  the  parental  substance  into  it. 
As  evidence  of  the  resulting  powers  of  multiplication,  we 
have  but  to  remember  that  the  spread  of  mould  over  stale 
food,  the  rapid  destruction  of  crops  by  mildew,  and  other 
kindred  occurrences,  are  made  possible  by  the  incalculably 
numerous  spores  thus  generated  and  universally  dispersed. 

Plants  a  degree  higher  in  composition  supply  a  parallel 
series  of  illustrations.    We  have  among  the  larger  Fungi,  in 
75 


450  LAWS  OF  MULTIPLICATION. 

which  the  reproductive  apparatus  is  relatively  so  enormous  as 
to  constitute  the  ostensible  plant,  a  similar  subordination  of 
the  individual  to  the  race,  and  a  similarly-immense  fertility. 
Thus,  as  quoted  by  Dr.  Carpenter,  Fries  says — "  in  a  single  in- 
dividual of  Reticularia  maxima,  I  have  counted  (calculated?) 
10,000,000  sporules."  It  needs  but  to  note  the  clouds  of 
particles,  so  minute  as  to  look  like  smoke,  which  ripe  puff- 
balls  give  off  when  they  are  burst,  and  then  to  remember 
that  each  particle  is  a  potential  fungus,  to  be  impressed  with 
the  almost  inconceivable  powers  of  propagation  which  these 
plants  possess.  The  Lichens,  too,  furnish  examples. 

Though  they  are  nothing  like  so  prolific  as  the  Fungi  (the 
difference  yielding,  as  we  shall  hereafter  see,  further  support 
to  the  general  argument),  yet  there  is  a  great  production  of 
germs,  and  a  proportionate  sacrifice  of  the  parental  indi- 
viduality. Considerable  areas  of  the  thallus  develop  into  the 
fruit-bodies  characteristic  of  the  various  fungi  which,  com- 
bined with  algae,  form  the  different  lichens  (various  members 
of  the  Ascomycetes  and  the  Basidiomycetes) .  From  these  are 
produced  great  numbers  of  ascospores  or  basidiospores,  as  the 
case  may  be.  Very  many  lichens  also  reproduce  themselves 
by  means  of  Soredia,  i.e.,  little  masses  of  algal  cells  closely 
wrapped  in  a  weft  of  fungal  hyphaB.  Some  con- 

trasts presented  by  the  higher  Algce  may  also  be  named  as 
exemplifying  the  inverse  proportion  between  the  size  of  the 
individual  and  the  extent  of  the  generative  structures.  While 
in  the  smaller  kinds  relatively  large  portions  of  the  fronds  are 
transformed  into  reproductive  elements,  in  the  larger  kinds 
these  portions  are  relatively  small :  instance  the  Macrocystis 
pyrifera,  a  gigantic  sea-weed  which  sometimes  attains  a 
length  of  1,500  feet,  of  which  Dr.  Carpenter  remarks — 
"This  development  of  the  nutritive  surface  takes  place  at 
the  expense  of  the  fructifying  apparatus,  which  is  here  quite 
subordinate." 

When  we  turn  to  vegetal  aggregates  of  the  third  order  of 
composition,  facts  having  the  same  meaning  are  conspicuous. 


GROWTH  AND  SEXUAL  GENESIS.  451 

On  the  average  these  higher  plants  are  far  larger  than 
plants  of  a  lower  degree  of  composition;  and  on  the  average 
their  rates  of  sexual  reproduction  are  far  less.  Similarly  if, 
among  Archegoniates  and  Phsenogams,  we  compare  the 
smaller  types  with  the  larger,  we  find  them  proportionately 
more  prolific.  This  is  not  manifest  if  we  simply  calculate 
the  number  of  seeds  ripened  by  an  individual  in  a  single 
season;  but  it  becomes  manifest  if  we  take  into  account  the 
further  factor  which  here  complicates  the  result — the  age  at 
which  sexual  genesis  commences.  The  smaller  Phaenogams 
are  mostly  either  annuals,  or  perennials  that  die  down 
annually;  and  seeding  as  they  do  annually  before  their 
deaths,  or  the  deaths  of  their  reproductive  parts,  it  results 
that  in  the  course  of  a  year  each  gives  origin  to  a  multitude 
of  potential  plants,  of  which  every  one  may  the  next  year,  if 
preserved,  give  origin  to  an  equal  multitude.  Supposing  but 
a  hundred  offspring  to  be  produced  the  first  year,  ten 
thousand  may  be  produced  in  the  second  year,  a  million  in 
the  third,  a  hundred  millions  in  the  fourth.  Meanwhile, 
what  has  been  the  possible  multiplication  of  a  large  Phaeno- 
gam  ?  While  its  small  congener  has  been  seeding  and  dying, 
and  leaving  multitudinous  progeny  to  seed  and  die,  it  has 
simply  been  growing;  and  may  so  continue  to  grow  for  ten 
or  a  dozen  years  without  bearing  fruit.  Before  a  Cocoa-nut 
tree  has  ripened  its  first  cluster  of  nuts,  the  descendants  of 
a  wheat  plant,  supposing  them  all  to  survive  and  multiply, 
will  have  become  numerous  enough  to  occupy  the  whole 
surface  of  the  Earth.  So  that  though,  when  it  begins  to 
bear,  a  tree  may  annually  shed  as  many  seeds  as  a  herb,  yet 
in  consequence  of  this  delay  in  bearing,  its  fertility  is  incom- 
parably less;  and  its  relatively-small  fertility  becomes  still 
further  reduced  where,  as  in  Lodoicea  callipyge,  the  seeds 
take  two  years  from  the  date  of  fertilization  to  the  date  of 
germination. 

§  340.  Some  observers  state  that  in  certain  Protozoa  there 


452  LAWS  OF  MULTIPLICATION. 

occurs  a  process  of  conjugation  akin  to  that  which  the 
Protophyta  exhibit — a  coalescence  of  the  substance  of  two 
individuals  to  form  a  germ-mass.  This  has  been  alleged 
more  especially  of  Actinophrys.  If  this  statement  should 
be  proved  true,*  then  of  the  minute  forms  that  appear  to  be 
more  animal  than  vegetal  in  their  characters,  some  have  a 
mode  of  sexual  multiplication  by  which  the  parents  are 
sacrificed  bodily  in  the  production  of  a  new  generation. 

Among  small  animal  aggregates  of  the  second  order,  the 
first  to  be  considered  are  of  course  the  Ccelenterata.  A  Hydra 
occasionally  devotes  a  large  part  of  its  substance  to  sexual 
genesis.  In  the  walls  of  its  body  groups  of  ova,  or  sperma- 
tozoa, or  both,  take  their  rise;  and  develop  into  masses 
greatly  distorting  the  creature's  form,  and  leaving  it  much 
diminished  when  they  escape.  Here,  however,  gamogenesis  is 
obviously  supplementary  to  agamogenesis — the  immensely 
rapid  multiplication  by  budding  continues  as  long  as  food  is 
abundant  and  warmth  sufficient,  and  is  replaced  by  gamo- 
genesis only  at  the  close  of  the  season.  A  better  example 

*  To  this  passage  Prof.  MacBride  appends  the  remark :— "  This  is  quite 
proven  now,  and  the  statement  as  it  stands  is  quite  correct ;  but  far  better 
and  more  minutely  worked  out  cases  are  to  be  found  amongst  the  Infusoria. 
In  ParamcRcium  for  example,  there  are  normally  present  a  large  macro- 
nucleus  and  a  small  micronucleus  lying  alongside  of  it.  When  two  indi- 
viduals adhere  preparatory  to  conjugation,  the  macronucleus  breaks  up  into 
fragments  which  are  absorbed :  the  micronucleus — which  has  some  time  pre- 
viously divided  into  two — begins  to  break  up  further  and  eventually  forms 
eight  bodies ;  all  of  these  except  one  disappear ;  this  last  piece  then  divides 
into  two ;  of  these  two  one  represents  a  male  genital  cell,  for  it  passes  over 
into  the  body  of  the  other  Paramoecium  and  fuses  with  one  of  the  two  corre- 
sponding nuclei  there ;  thus  each  of  the  two  individuals  which  adhere 
fertilizes  the  other.  The  two  individuals  then  separate  and  the  nucleus 
(result  of  fusion  of  male  and  female  nuclei)  in  each  divides  into  four.  Of 
these,  two  move  to  one  end  of  the  animal  and  two  to  the  other.  The 
animal  then  divides  into  two  transversely— each  of  the  products  thus  hav- 
ing two  nuclei  which  form  the  micro-  and  macro-nucleus  of  it.  Thus  it 
appears  that  the  function  of  sexual  union  is  simply  to  give  increased  vigour 
to  all  the  vital  processes  including  fadon.  Since  as  mentioned  above  (p. 
443)  if  it  is  prevented,  the  products  of  fission  are  eventually  unable  to  feed 
themselves." 


GROWTH  AND  SEXUAL  GENESIS.  453 

of  the  relation  between  small  size  and  active  gamogenesis 
among  low  types  of  the  Metazoa  is  supplied  by  the  Rotifera. 
Microscopic  as  these  are,  they  have  a  great  rate  of  sexual 
increase.  According  to  Ehrenberg,  Hydatina  senta  "  is 
capable  of  a  four-fold  propagation  every  twenty-four  or 
thirty  hours,  bringing  forth  in  this  time  four  ova,  which  grow 
from  the  embryo  to  maturity,  and  exclude  their  fertile  ova 
in  the  same  period.  The  same  individual,  producing  in  ten 
days  forty  eggs,  developed  with  the  rapidity  above  cited,  this 
rate,  raised  to  the  tenth  power,  gives  one  million  of  indi- 
viduals from  one  parent,  on  the  eleventh  day  four  millions, 
and  on  the  twelfth  day  sixteen  millions,  and  so  on." 
Ehrenberg,  however,  characterized  by  Huxley  as  "  the 
greatest  looker  and  the  worst  observer,"  is  not  a  safe  autho- 
rity, and  it  is  better  to  state  the  estimate  of  Ludwig  Plate, 
who  says  that  Hydatina  lays  fifty  eggs  in  two  to  three  weeks 
— a  number  which,  multiplying  in  the  manner  described, 
will  yield  in  the  time  named  a  much  smaller  total  though  still 
an  enormous  total. 

The  Annul osa,  including  among  them  the  inferior  types, 
have  habits  and  conditions  of  life  so  various  that  only  the 
broadest  contrasts  can  be  instanced  in  support  of  the  pro- 
position before  us.  The  differences  of  organization  and 
activity  greatly  complicate  the  inverse  variation  of  fertility 
and  bulk.  Bearing  in  mind,  however,  that  the  rate  of  multi- 
plication depends  much  less  on  the  number  of  each  brood 
than  on  the  quickness  with  which  maturity  is  reached  and  a 
new  generation  commenced,  it  will  be  obvious  that  though 
Annelids,  relatively  enormous  in  size,  produce  great  numbers 
of  ova,  yet  as  they  do  this  at  comparatively  long  inter- 
vals, their  rates  of  increase  fall  immensely  below  that  just 
instanced  in  the  Kotifers.  And  when  at  the  other  extreme 
we  come  to  the  large  articulate  animals,  such  as  the  Crab 
and  the  Lobster,  the  further  diminution  of  fertility  is  seen  in 
the  still  longer  delay  which  occurs  before  each  new  generation 
begins  to  reproduce. 


454  LAWS  OF  MULTIPLICATION. 

Perhaps  the  best  examples  are  supplied  by  vertebrate 
animals,  and  especially  those  that  are  most  familiar  to  us. 
Comparisons  between  Fishes  are  unsatisfactory,  because  of 
our  ignorance  of  their  histories.  In  some  cases  Fishes  equal 
in  bulk  produce  widely  different  numbers  of  eggs;  as  the 
Cod  which  spawns  millions  at  once,  and  the  Salmon  by 
which  nothing  like  so  great  a  number  is  spawned.  But  then 
the  eggs  are  very  unlike  in  size ;  and  if  the  ovaria  of  the  two 
fishes  be  compared,  the  difference  between  their  masses  is 
comparatively  moderate.  There  are,  indeed,  contrasts  which 
seem  at  variance  with  the  alleged  relation;  as  that  between 
the  Cod  and  the  Stickleback  which,  though  so  much  smaller, 
produces  fewer  ova.  The  Stickleback's  ova,  however,  are 
relatively  large ;  and  their  total  bulk  bears  as  great  a  ratio  to 
the  bulk  of  the  Stickleback  as  does  the  bulk  of  the  Cod's  ova 
to  that  of  the  Cod.  Moreover  if,  as  is  not  improbable,  the 
reproductive  age  is  arrived  at  earlier  by  the  Stickleback  than 
by  the  Cod,  the  fertility  of  the  species  may  be  greater  not- 
withstanding the  smaller  number  produced  by  each  indi- 
vidual. Evidence  which  admits  of  being  tolerably 
well  disentangled  is  furnished  by  Birds.  They  differ  but 
little  in  their  grades  of  organization;  and  the  habits  of  life 
throughout  extensive  groups  of  them  are  so  similar,  that 
comparisons  may  be  fairly  made.  It  is  true  that,  as  here- 
after to  be  shown,  the  differences  of  expenditure  which 
differences  of  bulk  entail,  have  doubtless  much  to  do  with 
the  differences  of  fertility.  But  we  may  set  down  under  the 
present  head  some  of  those  cases  in  which  the  activity,  being 
relatively  slight,  does  not  greatly  interfere  with  the  relation 
we  are  considering;  and  may  note  that  among  such  birds 
having  similarly  slight  activities,  the  small  produce  more  eggs 
than  the  large,  and  eggs  that  bear  in  their  total  mass  a 
greater  ratio  to  the  mass  of  the  parent.  Consider,  for  ex- 
ample, the  gallinaceous  birds;  which  are  like  one  another 
and  unlike  birds  of  most  other  groups  in  flying  comparatively 
little.  Taking  first  the  wild  members  of  this  order,  which 


GROWTH  AND  SEXUAL  GENESIS.  455 

rarely  breed  more  than  once  in  a  season,  we  find  that  the 
Pheasant  has  from  10  to  14  eggs,  the  Black-cock  from  6  to 
10,  the  Grouse  8  to  14,  the  Partridge  12  to  20,  the  Quail 
still  more,  sometimes  reaching  two  broods  of  7  to  12  in  each. 
Here  the  only  exception  to  the  relation  between  decreasing 
bulk  and  increasing  number  of  eggs,  occurs  in  the  cases  of 
the  Pheasant  and  the  Black-cock;  and  it  is  to  be  remem- 
bered, in  explanation,  that  the  Pheasant  is  constitutionally 
adapted  to  a  warmer  region,  is  better  fed — often  artificially — 
and  leads  a  less  active  life.  If  we  pass  to  domesticated 
genera  of  the  same  order,  we  meet  with  parallel  differences. 
From  the  numbers  of  eggs  laid,  little  can  be  inferred;  for 
under  the  favourable  conditions  artificially  maintained,  the 
laying  is  carried  on  indefinitely.  But  though  in  the  sizes  of 
their  broods  the  Turkey  and  the  Fowl  do  not  greatly  differ, 
the  Fowl  begins  breeding  at  a  much  earlier  age  than  the 
Turkey,  and  produces  broods  more  frequently :  a  consider- 
ably higher  rate  of  multiplication  being  the  result.  Now 
these  contrasts  among  domestic  creatures  which  are  similarly 
conditioned,  and  closely-allied  by  constitution,  may  be 
held  to  show,  more  clearly  than  most  other  contrasts,  the 
inverse  variation  between  bulk  and  sexual  genesis;  since 
here  the  cost  of  activity  is  diminished  to  a  comparatively 
small  amount.  There  is  little  expenditure  in  flight — some- 
times almost  none;  and  the  expenditure  in  walking  about 
is  not  great:  there  is  more  of  standing  than  of  actual 
movement.  It  is  true  that  young  Turkeys  commence 
their  existence  as  larger  masses  than  chickens;  but  it  is 
tolerably  manifest  that  the  total  weight  of  the  eggs"  laid 
by  a  Turkey  during  each  season,  bears  a  less  ratio  to  the 
Turkey's  weight,  than  the  total  weight  of  the  eggs  which  a 
Hen  lays  during  each  season,  bears  to  the  Hen's  weight; 
and  this  is  the  fairest  way  of  making  the  comparison.  The 
comparison  so  made  shows  a  greater  difference  than  appears 
likely  to  be  due  to  the  different  costs  of  locomotion;  con- 
sidering the  inertness  of  the  creatures.  Kemembering  that 


456  LAWS  OF  MULTIPLICATION. 

the  assimilating  surface  increases  only  as  the  squares  of  the 
dimensions,  while  the  mass  of  the  fabric  to  be  built  up  by  the 
absorbed  nutriment  increases  as  the  cubes  of  the  dimensions, 
it  will  be  seen  that  the  expense  of  growth  becomes  relatively 
greater  with  each  increment  of  size;  and  that  hence,  of  two 
similar  creatures  commencing  life  with  different  sizes,  the 
larger  one  in  reaching  its  superior  adult  bulk,  will  do  this  at 
a  more  than  proportionate  expense;  and  so  will  either  be 
delayed  in  commencing  its  reproduction,  or  will  have  a 
diminished  reserve  for  reproduction,  or  both.  Other  orders 
of  Birds,  active  in  their  habits,  show  more  markedly  the  con- 
nexion between  augmenting  mass  and  declining  fertility. 
But  in  them  the  increasing  cost  of  locomotion  becomes  an 
important,  and  probably  the  most  important,  factor.  The 
evidence  they  furnish  will  therefore  come  better  under 
another  head.  Contrasts  among  Mammals,  like 

those  which  Birds  present,  have  their  meanings  obscured  by 
inequalities  of  the  expenditures  for  motion.  The  smaller 
fertility  which  habitually  accompanies  greater  bulk,  must 
in  all  cases  be  partly  ascribed  to  this.  Still,  it  may  be 
well  if  we  briefly  note,  for  as  much  as  they  are  worth, 
the  broader  contrasts.  While  a  large  Mammal  bears  but 
a  single  young  one  at  a  time,  is  several  years  before  it  com- 
mences doing  this,  and  then  repeats  the  reproduction  at  long 
intervals;  we  find,  as  we  descend  to  the  smaller  members  of 
the  class,  a  very  early  commencement  of  breeding,  an  increas- 
ing number  at  a  birth,  reaching  in  small  Eodents  to  10  or 
even  more,  and  a  much  more  frequent  recurrence  of  broods: 
the  combined  result  being  a  relatively  prodigious  fertility. 
If  a  specific  comparison  be  desired  between  Mammals  that 
are  similar  in  constitution,  in  food,  in  conditions  of  life,  and 
all  other  things  but  size,  the  Deer-tribe  supplies  it.  "While 
the  large  Ked-deer  has  but  one  at  a  birth,  the  small  Eoe-deer 
has  frequently  two  at  a  birth.* 

§  341.  The  antagonism  between  growth  and  sexual  gene- 
*  A  passage  translated  for  me  from  the  German  may  be  here  given  in 


GROWTH  AND  SEXUAL  GENESIS.  457 

sis,  visible  in  these  general  contrasts,  may  also  be  traced  in  the 
history  of  each  plant  and  animal.  So  familiar  is  the  fact 
that  sexual  genesis  does  not  occur  early  in  life,  and  in  all 
organisms  which  expend  much  begins  only  when  the  limit  of 
size  is  nearly  reached,  that  we  do  not  sufficiently  note  its 
significance.  It  is  a  general  physiological  truth,  however, 
that  while  the  building-up  of  the  individual  is  going  on 
rapidly,  the  reproductive  organs  remain  imperfectly  deve- 
loped and  inactive;  and  that  the  commencement  of  repro- 
duction at  once  indicates  a  declining  rate  of  growth,  and 
becomes  a  cause  of  arresting  growth.  As  was  shown  in  §  78, 
the  exceptions  to  this  rule  are  found  where  the  limit  of 
growth  is  indefinite;  either  because  the  organism  expends 
little  or  nothing  in  action,  or  expends  in  action  so  moderate 
an  amount  that  the  supply  of  nutriment  is  never  equilibrated 
by  its  expenditure. 

We  will  pass  over  the  inferior  plants  and,  limiting 
ourselves  to  Phgenogams,  will  not  dwell  on  the  less  con- 
spicuous evidence  with  the  smaller  types  present.  A  few 
cases  such  as  gardens  supply  will  serve.  All  know  that 
a  Pear-tree  increases  in  size  for  years  before  it  begins  to 
bear;  and  that,  producing  but  few  pears  at  first,  it  is  long 
before  it  fruits  abundantly.  A  young  Mulberry-tree,  branch- 
ing out  luxuriantly  season  after  season,  but  covered  with 
nothing  but  leaves,  at  length  blossoms  sparingly  and  sets 
some  small  and  imperfect  berries,  which  it  drops  while  they 
are  green;  and  it  makes  these  futile  attempts  time  after 

verification.  Dr.  Dionys  Hellin  in  an  essay  on  the  origin  of  Multiparity 
and  Twin-births,  refers  to  the  thesis  above  set  forth,  and  says  that  "  the 
fact  that  it  is  generally  women  of  small  growth  who  bear  twins  is  in  com- 
plete agreement  with  it."  He  adds  that  "  Puech  is  right  in  his  opinion  that 
twin  pregnancies  are  a  direct  result  of  relatively  large  ovaries  (i.e.,  in  com- 
parison with  the  whole  body).  He  has  observed  that  for  the  same  size  of 
body  the  ovarium  of  a  pluriparous  animal  is  always  of  greater  volume  than 
that  of  a  uniparous  animal  ....  a  sow  has  ovaries  as  large  as  a  cow's ; 
bnt  while  the  latter  bears  only  one  calf  [at  a  timel,  the  sow  brings  forth 
6 — 15  at  each  litter.  Even  in  animals  of  the  same  species  but  belonging  to 
different  races  these  relations  may  be  verified,"  e.g.,  Barbary  sheep  and  ordi- 
nary sheep. 


458  LAWS  OF  MULTIPLICATION. 

time  before  it  succeeds  in  ripening  any  seeds.  But  these 
multi-axial  plants,  or  aggregates  of  individuals  some  of 
which  continue  to  grow  while  others  become  arrested  and 
transformed  into  seed-bearers,  show  us  the  relation  less  de- 
finitely than  certain  plants  that  are  substantially,  if  not 
literally,  uni-axial.  Of  these  the  Cocoa-nut  may  be  in- 
stanced. For  some  years  it  goes  on  shooting  up  without 
making  any  sign  of  becoming  fertile.  About  the  sixth  year 
it  flowers;  but  the  flowers  wither  without  result.  In  the 
seventh  year  it  flowers  and  produces  a  few  nuts;  but  these 
prove  abortive  and  drop.  In  the  eighth  year  it  ripens  a 
moderate  number  of  nuts;  and  afterwards  increases  the 
number  until,  in  the  tenth  year,  it  comes  into  full  bearing. 
Meanwhile,  from  the  time  of  its  first  flowering  its  growth 
begins  to  diminish,  and  goes  on  diminishing  till  the  tenth 
year,  when  it  ceases.  Here  we  see  the  antagonism  between 
growth  and  sexual  genesis  under  both  its  aspects — see  a 
struggle  between  self-evolution  and  race-evolution,  in  which 
the  first  for  a  time  overcomes  the  last,  and  the  last  ultimately 
overcomes  the  first.  The  continued  aggrandizement  of  the 
parent-individual  makes  abortive  for  two  seasons  the  tendency 
to  produce  new  individuals;  and  the  tendency  to  produce 
new  individuals,  becoming  more  decided,  stops  any  further 
aggrandizement  of  the  parent  individual. 

Parallel  illustrations  occur  in  the  animal  kingdom.  The 
eggs  laid  by  a  pullet  are  relatively  small  and  few.  Similarly, 
it  is  alleged  that,  as  a  general  rule,  "a  bitch  has  fewer 
puppies  at  first,  than  afterwards."  According  to  Burdach, 
as  quoted  by  Dr.  Duncan,  "the  elk,  the  bear,  &c.,  have  at 
first  only  a  single  young  one,  then  they  come  to  have  most 
frequently  two,  and  at  last  again  only  one.  The  young 
hamster  produces  only  from  three  to  six  young  ones,  while 
that  of  a  more  advanced  age  produces  from  eight  to  sixteen. 
The  same  is  true  of  the  pig."  It  is  remarked  by  Buff  on  that 
when  a  sow  of  less  than  a  year  old  has  young,  the  number  of 
the  litter  is  small,  and  its  members  are  feeble  and  even  im- 


GROWTH  AND  SEXUAL  GENESIS.  459 

perfect.  Here  we  have  evidence  that  in  animals  growth 
checks  sexual  genesis.  And  then,  on  the  other  hand,  we 
have  evidence  that  sexual  genesis  checks  growth.  It  is  well 
known  to  breeders  that  if  a  filly  is  allowed  to  bear  a  foal, 
she  is  thereby  prevented  from  reaching  her  proper  size.  And 
a  like  loss  of  perfection  as  an  individual,  is  suffered  by  a 
cow  which  breeds  too  early.  It  may  be  added,  as  a  converse 
fact,  that  castrated  animals,  as  capons  and  notably  cats, 
often  become  larger  than  their  unmutilated  associates. 

§  342.  Notwithstanding  the  way  in  which  the  inverse 
variation  of  growth  and  sexual  genesis  is  complicated  with 
other  relations,  its  existence  is,  I  think,  sufficiently  mani- 
fest. Individually,  many  of  the  foregoing  instances  are  open 
to  criticism,  and  have  to  be  taken  with  qualifications;  but 
when  looked  at  in  the  mass  their  meaning  is  beyond  doubt. 
Comparisons  between  the  largest  with  the  smallest  types, 
whether  vegetal  or  animal,  yield  results  which  are  unmis- 
takable. On  the  one  hand,  remembering  the  fact  that  dur- 
ing its  centuries  of  life  an  Oak  does  not  produce  as  many 
acorns  as  a  Fungus  does  spores  in  a  single  night,  we  see  that 
the  Fungus  has  a  fertility  exceeding  that  of  the  Oak  in  a 
degree  literally  beyond  our  powers  of  calculation  or  imagina- 
tion. On  the  other  hand  when,  taking  a  microscopic  pro- 
tophyte  which  has  billions  of  descendants  in  a  few  days, 
we  ask  how  many  such  would  be  required  to  build  up  the 
forest  tree  which  is  years  before  it  drops  a  seed,  we  are  met  by 
a  parallel  difficulty  in  conceiving  the  number,  if  not  in  setting 
it  down.  Similarly,  if  from  the  minute  and  prodigiously- 
fertile  Eotifer  we  turn  to  the  Elephant,  which  approaches 
thirty  years  before  it  bears  a  solitary  young  one,  we  find  the 
connexions  between  small  size  and  great  fertility  and  between 
great  size  and  small  fertility,  too  intensely  marked  to  be 
much  disguised  by  the  perturbing  relations  that  have  been 
indicated.  Finally,  as  this  induction,  reached  by  a  survey  of 
organisms  in  general,  is  verified  by  observations  on  the  rela- 


460  LAWS  OF  MULTIPLICATION. 

tion  between  decreasing  growth  and  commencing  reproduc- 
tion in  individual  organisms,  we  may,  I  think,  consider  the 
alleged  antagonism  as  proved.* 

*  When,  after  having  held  for  some  years  the  general  doctrine  elaborated 
in  these  chapters,  I  agreed,  early  in  1852,  to  prepare  an  outline  of  it  for  the 
Westminster  Review,  I  consulted,  among  other  works,  the  just-issued  third 
edition  of  Dr.  Carpenter's  Principles  of  Physiology,  General  and  Compara- 
tive— seeking  in  it  for  facts  illustrating  the  different  degrees  of  fertility  of 
different  organisms,  I  met  with  a  passage,  quoted  above  in  §  339,  which 
seemed  tacitly  to  assert  that  individual  aggrandizement  is  at  variance  with 
the  propagation  of  the  race  ;  but  nowhere  found  a  distinct  enunciation  of 
this  truth.  I  did  not  then  read  the  Chapter  entitled  "  General  View  of  the 
Functions,"  which  held  out  no  promise  of  such  evidence  as  I  was  looking  for. 
But  on  since  referring  to  this  chapter,  I  discovered  in  it  the  definite  state- 
ment that — "  there  is  a  certain  degree  of  antagonism  between  the  Nutritive 
and  Reproductive  functions,  the  one  being  executed  at  the  expense  of  the 
other.  The  reproductive  apparatus  derives  the  materials  of  its  operations 
through  the  nutritive  system,  and  is  entirely  dependent  upon  it  for  the  con- 
tinuance of  its  function.  If,  therefore,  it  be  in  a  state  of  excessive  activity, 
it  will  necessarily  draw  off  from  the  individual  fabric  some  portion  of  the 
aliment  destined  for  its  maintenance.  It  may  be  universally  observed  that, 
when  the  nutritive  functions  are  particularly  active  in  supporting  the  indi- 
vidual, the  reproductive  system  is  in  a  corresponding  degree  undeveloped, — 
and  vice  versd."  P.  692. 


CHAPTER  VII. 

THE  ANTAGONISM  BETWEEN  DEVELOPMENT  AND  GENESIS, 
ASEXUAL   AND   SEXUAL. 

§  343.  BY  Development,  as  here  to  be  dealt  with  apart 
from  Growth,  is  meant  increase  of  structure  as  distinguished 
from  increase  of  mass.  As  was  pointed  out  in  §  50,  this  is 
the  biological  definition  of  the  word.  In  the  following  sec- 
tions, then,  we  have  to  note  how  complexity  of  organization 
is  hindered  by  reproductive  activity,  and  conversely. 

This  relation  partially  coincides  with  that  which  we  have 
just  contemplated ;  for,  as  was  shown  in  §  44,  degree  of 
growth  is  to  a  considerable  extent  dependent  on  degree  of 
organization.  But  while  the  antagonism  to  be  illustrated  in 
this  chapter  is  much  entangled  with  that  illustrated  in  the 
last  chapter,  it  may  be  so  far  separated  as  to  be  identified  as 
an  additional  antagonism. 

Besides  the  direct  opposition  between  that  continual  dis- 
integration which  rapid  genesis  implies,  and  the  fulfilment  of 
that  pre-requisite  to  extensive  organization — the  formation  of 
an  extensive  aggregate,  there  is  an  indirect  opposition  which 
we  may  recognize  under  several  aspects.  The  change 

from  homogeneity  to  heterogeneity  takes  time;  and  time 
taken  in  transforming  a  relatively-structureless  mass  into  a 
developed  individual,  delays  the  period  of  reproduction.  Usu- 
ally this  time  is  merged  in  that  taken  for  growth ;  but  certain 
cases  of  metamorphosis  show  us  the  one  separate  from  the 
other.  An  insect,  passing  from  its  lowly-organized  cater- 

461 


462  LAWS  OF  MULTIPLICATION. 

pillar-stage  into  that  of  chrysalis,  is  afterwards  a  week,  a 
fortnight,  or  a  longer  period  in  completing  its  structure :  the 
re-commencement  of  genesis  being  by  so  much  postponed, 
and  the  rate  of  multiplication  therefore  diminished.  Further, 
that  re-arrangement  of  substance  which  development  implies, 
entails  expenditure.  The  chrysalis  loses  weight  in  the  course 
of  its  transformation;  and  that  its  loss  is  not  loss  of  water 
only,  may  be  inferred  from  the  fact  that  it  respires,  and  that 
respiration  indicates  consumption.  Clearly  the  matter  con- 
sumed is,  other  things  equal,  a  deduction  from  the  surplus 
which  may  go  to  reproduction.  Yet  again,  the  more 

widely  and  completely  an  organic  mass  becomes  differentiated, 
the  smaller  is  the  portion  of  it  which  retains  the  relatively- 
undifferentiated  state  that  admits  of  being  moulded  into  new 
individuals,  or  the  germs  of  them.  Protoplasm  which  has 
become  specialized  tissue  cannot  be  generalized  afresh,  and 
afterwards  transformed  into  something  else;  and  hence  the 
progress  of  structure  in  an  organism,  by  diminishing  the 
unstructured  part,  diminishes  the  amount  available  for  mak- 
ing offspring. 

It  is  true  that  higher  structure,  like  greater  growth,  may 
insure  to  a  species  advantages  which  eventually  further  its 
multiplication — may  give  it  access  to  larger  supplies  of  food, 
or  enable  it  to  obtain  food  more  economically;  and  we  shall 
hereafter  see  how  the  inverse  variation  we  are  considering  is 
thus  qualified.  But  here  we  are  concerned  only  with  the 
necessary  and  direct  effects;  not  with  those  that  are  con- 
tingent and  remote.  These  necessary  and  direct  effects  we 
will  now  look  at  as  exemplified. 

§  344.  Speaking  generally,  the  simpler  plants  propagate 
both  sexually  and  asexually;  and,  speaking  comparatively, 
the  complex  plants  propagate  only  sexually:  their  asexual 
propagation  is  usually  incomplete — produces  a  united  aggre- 
gate of  individuals  instead  of  numerous  distinct  individuals. 
The  Protophytes  that  perpetually  subdivide,  the  merely- 


DEVELOPMENT  AND  GENESIS.  463 

cellular  Algce  that  shed  their  tetraspores,  the  Archegoniates 
that  spontaneously  separate  their  fronds  or  drop  their  gem- 
mae, show  us  an  extra  mode  of  multiplication  which,  among 
flowering  plants,  is  exceptional.  This  extra  mode  of  multipli- 
cation among  these  simpler  plants,  is  made  easy  by  their  low 
development.  Tetraspores  arise  only  where  the  frond  con- 
sists of  untransformed  cells;  gemmae  bud  out  and  drop  off 
only  where  the  tissue  is  comparatively  homogeneous. 

Should  it  be  said  that  this  is  but  another  aspect  of  the 
antagonism  already  set  forth,  since  these  undeveloped  forms 
are  also  the  smaller  forms;  the  reply  is  that  though  in  part 
true  this  is  not  wholly  true.  Various  marine  Algce  which 
propagate  asexually,  are  larger  than  some  Phaenogams  which 
do  not  thus  propagate.  The  objection  that  difference  of 
medium  vitiates  this  comparison,  is  met  by  the  fact  that  it  is 
the  same  among  land-plants  themselves.  Sundry  of  the  low- 
ly-organized Liverworts  which  are  habitually  gemmiparous, 
exceed  in  size  many  flowering  plants.  And  the  Ferns  show 
us  agamic  multiplication  occurring  in  plants  which,  while 
they  are  inferior  in  complexity  of  structure,  are  superior  in 
bulk  to  numbers  of  annual  Monocotyledons  and  Dicotyledons. 

§  345.  In  the  ability  of  the  lowly-organized  substance  of  a 
Sponge  to  transform  itself  into  multitudes  of  gemmules,  we 
have  an  instance  of  this  same  direct  relation  in  the  animal 
kingdom.  Moreover,  the  instance  yields  very  distinct  proof 
of  an  antagonism  between  development  and  genesis,  inde- 
pendent of  the  antagonism  between  growth  and  genesis; 
for  the  Sponge  which  thus  multiplies  itself  asexually,  as  well 
as  sexually,  is  far  larger  than  hosts  of  more  complex  animals 
which  do  not  multiply  asexually. 

Once  again  may  be  cited  the  creature  so  often  brought  in 
evidence,  the  Hydra,  as  showing  us  how  rapidity  of  agamic 
propagation  is  associated  with  inferiority  of  structure.  Its 
power  to  produce  young  ones  from  nearly  all  parts  of  its 
body,  is  due  to  the  comparative  homogeneity  of  its  body.  In 


464  LAWS  OF  MULTIPLICATION. 

kindred  but  more-organized  types,  the  gemmiparity  is 
greatly  restricted,  or  disappears.  Among  the  free-swimming 
Hydrozoa,  multiplication  by  budding,  when  it  occurs  at  all, 
occurs  only  at  special  places.  That  increase  of  structure 
apart  from  increase  of  size,  is  here  a  cause  of  declining  agamo- 
genesis,  we  may  see  in  the  contrast  between  the  simple  Hydra 
and  the  compound  Hydroids.  These  last,  along  with  more- 
differentiated  tissues,  show  us  a  gemmation  which  does  not 
go  on  all  over  the  body  of  each  polype,  and  much  of  it  does 
not  end  in  separation. 

It  is,  however,  among  the  Annulosa  that  progressing 
organization  is  most  conspicuously  operative  in  diminishing 
agamogenesis.  The  segments  or  "  somites  "  composing  an 
animal  belonging  to  this  class,  are  primordially  alike;  and, 
as  before  argued  (§§  205-7),  are  probably  the  homologues  of 
what  were  originally  independent  individuals.  The  progress 
from  the  lower  to  the  higher  types  of  the  class,  is  at  once  a 
progress  towards  types  in  which  the  strings  of  segments  cease 
to  undergo  subdivision,  and  towards  types  in  which  the  seg- 
ments, no  longer  alike  in  their  structures  and  functions,  have 
become  physiologically  integrated  or  mutually  dependent. 
Already  this  group  of  cases  has  been  named  as  illustrating 
the  antagonism  between  growth  and  asexual  genesis;  but  it 
is  proper  also  to  name  it  here,  since,  on  the  one  hand,  the 
greater  size  due  to  the  ceasing  of  fission,  is  made  possible 
only  by  the  specialization  of  parts  and  the  development  of  a 
co-ordinating  apparatus  to  combine  their  actions,  and  since, 
on  the  other  hand,  specialization  and  co-ordination  can  ad- 
vance only  in  proportion  as  fission  ceases. 

§  346.  The  inverse  variation  of  development  and  sexual 
genesis  is  by  no  means  easy  to  follow.  One  or  two  facts  indi- 
cative of  it  may,  however,  be  named. 

Phsenogams  that  have  but  little  supporting  tissue  may 
fairly  be  classed  as  structurally  inferior  to  those  having  stems 
with  a  bulky  and  complex  woody  system;  for  these  imply 


DEVELOPMENT  AND  GENESIS.  465 

additional  differentiations,  and  constitute  wider  departures 
from  the  primitive  type  of  vegetal  tissue.  That  the  con- 
comitant of  this  higher  organization  is  a  slower  gamogenesis, 
scarcely  needs  pointing  out.  While  the  herbaceous  annual 
is  blossoming  and  ripening  seed,  the  young  tree  is  transform- 
ing its  originally-succulent  axis  into  dense  fibrous  substance; 
and  year  by  year  the  young  tree  expends  in  doing  the  like, 
nutriment  which  successive  generations  of  the  annual  expend 
in  fruit.  Here  the  inverse  relation  is  between  sexual  repro- 
duction and  complexity,  and  not  between  sexual  reproduction 
and  bulk,  seeing  that  besides  seeding,  the  annual  often  grows 
to  a  size  greater  than  that  reached  by  the  young  infertile  tree 
in  several  years. 

Proof  of  the  antagonism  between  complexity  and  gamo- 
genesis in  animals,  is  still  more  difficult  to  disentangle. 
Perhaps  the  evidence  most  to  the  point  is  furnished  by  the 
contrast  between  Man  and  certain  other  Mammals  approach- 
ing him  in  mass.  To  compare  him  with  the  domestic 
Sheep  which,  though  not  very  unlike  in  size,  is  relatively 
prolific,  is  objectionable  because  of  the  relative  inactivity  of 
Sheep ;  and  this,  too,  may  be  alleged  as  a  reason  why  the  Ox, 
though  far  more  bulky,  is  also  far  more  fertile,  than  Man. 
Further,  against  a  comparison  with  the  Horse  which,  while 
both  larger  and  more  prolific,  is  tolerably  active,  it  may  be 
urged  that  in  his  case,  and  the  cases  of  herbivorous  creatures 
generally,  the  small  exertion  required  to  procure  food,  joined 
with  the  great  ratio  borne  by  the  alimentary  organs  to  the 
organs  they  have  to  build  up  and  repair,  vitiates  the  result. 
We  may,  however,  fairly  draw  a  parallel  between  Man  and  a 
large  carnivore.  The  Lion,  superior  in  size,  and  perhaps 
equal  in  activity,  has  a  digestive  system  not  proportionately 
greater;  and  yet  has  a  higher  rate  of  multiplication  than 
Man.  Here  the  only  decided  want  of  parity,  besides  that  of 
organization,  is  that  of  food.  Possibly  a  carnivore  gains  an 
advantage  in  having  a  surplus  nutriment  consisting  almost 
wholly  of  those  nitrogenous  materials  from  which  the  bodies 
76 


4G6  LAWS  OF  MULTIPLICATION. 

of  young  ones  are  mainly  formed.  But,  allowing  for  all  other 
differences,  it  appears  not  improbable  that  the  smallness  of 
human  fertility  compared  with  the  fertility  of  large  feline 
animals,  is  due  to  the  greater  complexity  of  the  human 
organization — more  especially  the  organization  of  the  nervous 
system.  Taking  degree  of  nervous  organization  as  the  chief 
correlative  of  mental  capacity ;  and  remembering  the  physio- 
logical cost  of  that  slow  evolution  whereby  high  mental 
capacity  is  reached;  we  may  suspect  that  nervous  organiza- 
tion is  very  expensive:  the  inference  being  that  bringing  it 
up  to  the  level  it  reaches  in  Man,  whose  digestive  system,  by 
.no  means  large,  has  at  the  same  time  to  supply  materials  for 
general  growth  and  daily  waste,  involves  a  great  retardation 
of  maturity  and  sexual  genesis. 


CHAPTER  VIII. 

ANTAGONISM   BETWEEN   EXPENDITURE  AND  GENESIS. 

§  347.  UNDER  this  head  we  have  to  set  down  no  evidence 
derived  from  the  vegetal  kingdom.  Plants  are  not  expenders 
of  force  in  such  degrees  as  to  affect  the  general  relations  with 
which  we  are  dealing.  They  have  not  to  maintain  a  heat 
above  that  of  their  environment,  nor  have  they  to  generate 
motion;  and  hence  consumption  for  these  two  purposes  does 
not  diminish  the  stock  of  material  which  serves  on  the  one 
hand  for  growth  and  on  the  other  hand  for  propagation. 

It  will  be  well,  too,  if  we  pass  over  the  lower  animals: 
especially  those  aquatic  ones  which,  being  nearly  of  the 
same  temperature  as  the  water,  and  nearly  of  the  same 
specific  gravity,  lose  but  little  in  evolving  motion,  sensible 
and  insensible.  A  further  reason  for  excluding  from  con- 
sideration these  inferior  types,  is  that  we  do  not  know  enough 
of  their  rates  of  genesis  to  permit  of  our  making,  with  any 
satisfaction,  those  involved  comparisons  here  to  be  entered 
upon. 

The  facts  on  which  we  must  mainly  depend  are  those  to  be 
gathered  from  terrestrial  animals,  and  chiefly  from  those 
higher  classes  of  them  which  are  at  the  same  time  great 
expenders  and  have  rates  of  multiplication  about  which  our 
knowledge  is  tolerably  definite.  We  will  restrict  ourselves, 
then,  to  the  evidence  which  Birds  and  Mammals  supply. 

§  348.  Satisfactory  proof  that  loss  of  substance  in  the 

467 


468  LAWS  OF  MULTIPLICATION. 

maintenance  of  heat  diminishes  the  rapidity  of  propagation, 
is  difficult  to  obtain.  It  is,  indeed,  obvious  that  the  warm- 
blooded Vertebrata  are  less  prolific  than  the  cold-blooded; 
but  then  they  are  at  the  same  time  more  vivacious.  Simi- 
larly, between  Mammals  and  Birds  (which  are  the  warmer- 
blooded  of  the  two)  there  is,  other  things  equal,  a  parallel, 
though  much  smaller,  difference;  but  here,  too,  the  unlike- 
nesses  of  muscular  action  complicate  the  evidence.  Again, 
the  annual  return  of  generative  activity  has  an  average  cor- 
respondence with  the  annual  return  of  a  warmer  season, 
which,  did  it  stand  alone,  might  be  taken  as  evidence  that  a 
diminished  cost  of  heat-maintenance  leads  to  such  a  surplus 
as  makes  reproduction  possible.  But  then,  this  periodic  rise 
of  temperature  is  habitually  accompanied  by  an  increase  in 
the  quantity  of  food — a  factor  of  equal  or  greater  importance. 
We  must  be  content,  therefore,  with  such  few  special  facts  as 
admit  of  being  disentangled. 

Certain  of  these  we  are  introduced  to  by  the  general  rela- 
tion last  named — the  habitual  recurrence  of  genesis  with  the 
recurrence  of  spring.  For  in  some  cases  a  domesticated  crea- 
ture has  its  supplies  of  food  almost  equalized ;  and  hence  the 
effect  of  varying  nutrition  may  be  in  great  part  eliminated 
from  the  comparison.  The  common  Fowl  yields  an  illustra- 
tion. It  is  fed  through  the  cold  months,  but  nevertheless,  in 
mid-winter,  it  either  wholly  leaves  off  laying  or  lays  very 
sparingly.  And  then  we  have  the  further  evidence  that  if  it 
lays  sparingly,  it  does  so  only  on  condition  that  the  heat,  as 
well  as  the  food,  is  artificially  maintained.  Hens  lay  in  cold 
weather  only  when  they  are  kept  warm.  To  which  fact  may 
be  added  the  kindred  one  that  "when  pigeons  receive  arti- 
ficial heat,  they  not  only  continue  to  hatch  longer  in  autumn, 
but  will  recommence  in  spring  sooner  than  they  would  other- 
wise do."  An  analogous  piece  of  evidence  is  that,  in 
winter,  inadequately  sheltered  Cows  either  cease  to  give  milk 
or  give  it  in  diminished  quantity.  For  though  giving  milk 
is  not  the  same  thing  as  bearing  a  young  one,  yet,  as  milk 


EXPENDITURE  AND  GENESIS.  469 

is  part  of  the  material  from  which  a  young  one  is  built  up, 
it  is  part  of  the  outlay  for  reproductive  purposes,  and  diminu- 
tion of  it  is  a  loss  of  reproductive  power.  Indeed  the  case 
aptly  illustrates,  under  another  aspect,  the  struggle  between 
self-preservation  and  race-preservation.  Maintenance  of  the 
cow's  life  depends  on  maintenance  of  its  heat;  and  main- 
tenance of  its  heat  may  entail  such  reduction  in  the  supply 
of  milk  as  to  cause  the  death  of  the  calf. 

Evidence  derived  from  the  habits  of  the  same  or  allied 
genera  in  different  climates,  may  naturally  be  looked  for ;  but 
it  is  difficult  to  get,  and  it  can  scarcely  be  expected  that  the 
remaining  conditions  of  existence  will  be  so  far  similar  as  to 
allow  of  a  fair  comparison  being  made.  The  only  illustrative 
facts  I  have  met  with  which  seem  noteworthy,  are  some  named 
by  Mr.  Gould  in  his  work  on  The  Birds  of  Australia.  He 
says : — "  I  must  not  omit  to  mention,  too,  the  extraordinary 
fecundity  which  prevails  in  Australia,  many  of  its  smaller 
birds  breeding  three  or  four  times  in  a  season;  but  laying 
fewer  eggs  in  the  early  spring  when  insect  life  is  less  de- 
veloped, and  a  greater  number  later  in  the  season,  when  the 
supply  of  insect  food  has  become  more  abundant.  I  have 
also  some  reason  to  believe  that  the  young  of  many  species 
breed  during  the  first  season,  for  among  others,  I  frequently 
found  one  section  of  the  Honey-eaters  (the  Melithrepti)  sit- 
ting upon  eggs  while  still  clothed  in  the  brown  dress  of  imma- 
turity; and  we  know  that  such  is  the  case  with  the  intro- 
duced Gallinacece  (or  poultry)  three  or  four  generations  of 
which  have  been  often  produced  in  the  course  of  a  year." 
Though  here  Mr.  Gould  refers  only  to  variation  in  the 
quantity  of  food  as  a  cause  of  variation  in  the  rate  of  multi- 
plication, may  we  not  suspect  that  warmth  is  a  part-cause 
of  the  high  rate  which  he  describes  as  general? 

§  349.  Of  the  inverse  variation  between  activity  and 
genesis,  we  get  clear  proof.  Let  us  begin  with  that  which 
Birds  furnish. 


470  LAWS  OP  MULTIPLICATION. 

First  we  have  the  average  contrast,  already  hinted,  between 
the  fertility  of  Birds  and  the  fertility  of  Mammals.  Compar- 
ing the  large  with  the  large  and  the  small  with  the  small,  we 
see  that  creatures  which  continually  go  through  the  muscular 
exertion  of  sustaining  themselves  in  the  air  and  propelling 
themselves  rapidly  through  it,  are  less  prolific  than  creatures 
of  equal  weights  which  go  through  the  smaller  exertion  of 
moving  about  over  solid  surfaces.  Predatory  Birds  have 
fewer  young  ones  than  predatory  Mammals  of  approximately 
the  same  sizes.  If  we  compare  Eooks  with  Rats,  or  Finches 
with  Mice,  we  find  like  differences.  And  these  differences  are 
greater  than  at  first  appears.  For  whereas  among  Mammals 
a  mother  is  able,  unaided,  to  bear  and  suckle  and  rear  half- 
way to  maturity,  a  brood  that  probably  weighs  more  in  pro- 
portion than  does  the  brood  of  a  Bird;  a  Bird,  or  at  least  a 
Bird  that  flies  much,  is  unable  to  do  this.  Both  parents  have 
to  help;  and  this  indicates  that  the  margin  for  reproduction 
in  each  adult  individual  is  smaller. 

Among  Birds  themselves  occur  contrasts  which  may  be 
next  considered.  In  the  Raptorial  class,  various  species  of 
which,  differing  in  their  sizes,  are  similarly  active  in  their 
habits,  we  see  that  the  small  are  more  prolific  than  the  large. 
The  Golden  Eagle  has  usually  2  eggs:  sometimes  3,  some- 
times only  1.  As  we  descend  to  the  Kites  and  Falcons,  the 
number  is  2  or  3,  and  3  or  4.  And  when  we  come  to  the 
Sparrow-Hawk,  3  to  5  is  the  specified  number.  Similarly 
among  the  Owls :  while  the  Great  Eagle-Owl  has  2  or  3 
eggs,  the  comparatively  small  Common  Owl  has  4  or  5.  As 
before  hinted,  it  is  impossible  to  say  what  proportions  of  these 
differences  are  due  to  unlikenesses  of  bulk  merely,  and  what 
proportions  are  due  to  unlikenesses  in  the  costs  of  locomo- 
tion. But  we  may  fairly  assume  that  the  unlikenesses  in  the 
costs  of  locomotion  are  here  the  more  important  factors. 
Weights  varying  as  the  cubes  of  the  dimensions,  while  the 
surfaces  of  digestive  systems  vary  as  the  squares,  the  expense 
of  flight  increases  more  rapidly  than  does  the  ability  to  take 


EXPENDITURE  AND  GENESIS.  471 

in  nutriment;  and  as  motion  through  the  air  requires  more 
effort  than  motion  on  the  ground,  this  geometrical  progression 
tells  more  rapidly  on  Birds  than  on  Mammals.  Be  this  as  it 
may,  however,  these  contrasts  support  the  argument;  as  do 
various  others  which  may  be  set  down.  The  Finch-family,  for 
example,  have  broods  averaging  about  5  in  number,  and  have 
commonly  2  broods  in  the  season;  while  in  the  Crow- family 
the  number  of  the  brood  is  on  the  average  less,  and  there  is 
but  one  brood  in  the  season.  And  then  on  descending  to  such 
small  birds  as  the  Wrens  and  the  Tits,  we  have  8,  10,  12  to 
15  eggs,  and  sometimes  two  broods  in  the  year.  One  of  the 
best  illustrations  is  furnished  by  the  Swallow-tribe,  through- 
out which  there  is  little  or  no  difference  in  mode  of  life  or  in 
food.  The  Sand-Martin,  much  the  least  of  them,  has  4  to  6 
eggs  and  two  broods ;  the  Swallow,  somewhat  larger,  has  4  or 
5;  and  the  Swift  (similar  in  habits  though  unrelated),  larger 
still,  has  but  2.  Here  we  see  a  lower  fertility  associated 
in  part  with  greater  size,  but  associated  still  more  con- 
spicuously with  greater  expenditure.  For  the  difference  of 
fertility  is  more  than  proportionate  to  the  difference  of  bulk, 
as  shown  in  other  cases ;  and  for  this  greater  difference  there 
is  the  reason,  that  the  Swift  has  to  support  not  only  the  cost 
of  propelling  its  larger  mass  through  the  air,  but  also  the 
cost  of  propelling  it  at  a  higher  velocity. 

Omitting  much  evidence  of  like  nature,  let  us  note  that 
disclosed  by  comparisons  of  certain  groups  of  birds  with 
other  groups.  "  Skulkers  "  is  the  descriptive  title  applied  to 
the  Water-Kail,  the  Corn-Crake,  and  their  allies,  which  evade 
enemies  by  concealment — consequently  expending  but  little 
in  locomotion.  These  birds  have  relatively  large  broods — 6 
to  11,  8  to  12,  &c.  Not  less  instructive  are  the  contrasts  be- 
tween the  Gallinaceous  Birds  and  other  Birds  of  like  sizes  but 
more  active  habits.  The  Partridge  and  the  Wood-Pigeon  are 
about  equal  in  bulk  and  have  much  the  same  food.  Yet  while 
the  one  has  from  12  to  20  young  ones,  the  other  has  but  2 
young  ones  twice  a-year :  its  annual  reproduction  is  less  than 


472  LAWS  OF  MULTIPLICATION. 

one-third.  It  may  be  said  that  the  ability  of  the  Partridge 
to  bring  up  so  large  a  brood,  is  due  to  that  habit  of  its  tribe 
which  one  of  its  names,  "  Scrapers,"  describes ;  and  to  the 
accompanying  habit  of  the  young,  which  begin  to  get  their 
own  living  as  soon  as  they  are  hatched :  so  saving  the  parents' 
labour.  Conversely,  it  may  be  said  that  the  inability  of 
Pigeons  to  rear  more  than  2  at  a  time,  is  caused  by  the  neces- 
sity of  fetching  everything  they  eat.  But  the  alleged  relation 
holds  nevertheless.  On  the  one  hand,  a  great  part  of  the  food 
which  the  Partridge  chicks  pick  up,  is  food  which,  in  their 
absence,  the  mother  would  have  picked  up.  Though  each  chick 
costs  her  far  less  than  a  young  Pigeon  costs  its  parents,  yet 
the  whole  of  her  chicks  cost  her  a  great  deal  in  the  shape  of 
abstinence — an  abstinence  she  can  bear  because  she  has  to  fly 
but  little.  On  the  other  hand,  the  Pigeon's  habit  of  laying 
and  hatching  but  two  eggs,  must  not  be  referred  to  any  fore- 
seen necessity  of  going  through  so  much  labour  in  supporting 
the  young,  but  to  a  constitutional  tendency  established  by 
such  labour.  This  is  proved  by  the  curious  fact  that  when 
domesticated,  and  saved  from  such  labour  by  artificial  feeding, 
Pigeons,  says  Macgillivray  (quoting  Aitkin),  "  arc  frequently 
seen  sitting  on  eggs  long  before  the  former  brood  is  able  to 
leave  the  nest,  so  that  the  parent  bird  has  at  the  same  time 
young  birds  and  eggs  to  take  care  of." 

§  350.  Made  to  illustrate  the  effect  of  activity  on  fertility, 
most  comparisons  among  Mammals  are  objectionable:  other 
circumstances  are  not  equal.  A  few,  however,  escape  this 
criticism. 

One  is  that  between  the  Hare  and  the  Eabbit.  These  are 
closely-allied  species  of  the  same  genus,  similar  in  their  diet 
but  unlike  in  their  expenditures  for  locomotion.  The  rela- 
tively-inert Eabbit  has  6  young  ones  in  a  litter,  and  four 
litters  a-year;  while  the  relatively-active  Hare  has  but  2  to 
5  in  a  litter.  This  is  not  all.  The  Eabbit  begins  to  breed 
at  six  months  old ;  but  a  year  elapses  before  the  Hare  begins 


EXPENDITURE  AND  GENESIS.  473 

to  breed.  These  two  factors  compounded,  result  in  a  dif- 
ference of  fertility  far  greater  than  can  be  ascribed  to  un- 
likeness  of  the  two  creatures  in  size. 

Perhaps  the  most  striking  piece  of  evidence  which  Mam- 
mals furnish,  is  the  extreme  infertility  of  our  common  Bat. 
The  Cheiroptera  and  the  Rodentia  are  not  very  dissimilar  in 
their  internal  structures.  Diversity  of  constitution,  therefore, 
cannot  vitiate  the  comparison  between  Bats  and  Mice,  which 
are  about  the  same  in  size.  Though  their  diets  differ,  the 
difference  is  in  favour  of  the  Bat:  its  food  being  exclusively 
animal  while  that  of  the  Mouse  is  mainly  vegetal.  What 
now  are  their  respective  rates  of  genesis?  The  Mouse  has 
several  litters  in  a  year  of  5  to  7  in  each ;  while  the  Bat  pro- 
duces only  one  at  a  time.  Whether  the  Bat  repeats  its  one 
more  frequently  than  the  Mouse  repeats  its  7  is  not  stated; 
but  it  is  quite  certain  that  even  if  it  does  so  (an  absurd 
supposition),  the  more  frequent  repetition  cannot  be  such 
as  to  raise  its  fertility  to  anything  like  that  of  the  Mouse. 
And  this  relatively-low  rate  of  multiplication  we  may  fairly 
ascribe  to  its  relatively-high  rate  of  expenditure. 

Here  let  us  note,  in  passing,  an  interesting  example  of  the 
way  in  which  a  species  which  has  no  specially-great  power  of 
self-preservation,  while  its  power  of  multiplication  is  ex- 
tremely small,  nevertheless  avoids  extinction  because  it  has 
to  meet  an  unusually-small  total  of  race-destroying  forces. 
Leaving  out  parasites,  the  only  enemy  of  the  Bat  is  the  Owl ; 
and  the  Owl  is  sparingly  distributed. 

§  351.  These  general  evidences  may  be  enforced  by  some 
special  evidences.  We  have  few  opportunities  of  observing 
how,  within  the  same  species,  variations  of  expenditure  are 
related  to  variations  of  fertility.  But  a  fact  or  two  showing 
the  connexion  may  be  named. 

Doctor  Duncan  quotes  a  statement  to  the  point  respecting 
the  breeding  of  dogs.  Already  in  §  341 1  have  extracted  a  part 
of  this  statement,  to  the  effect  that  before  her  growth  is  com- 


474  LAWS  OP  MULTIPLICATION. 

plete,  a  bitch  bears  at  a  birth  fewer  puppies  than  when  she 
becomes  full-grown.  An  accompanying  allegation  is,  that 
her  declining  vigour  is  shown  by  a  decrease  in  the  number  of 
puppies  contained  in  a  litter,  "  ending  in  one  or  two."  And 
then  it  is  further  alleged  that,  "  as  regards  the  amount  of 
work  a  dog  has  to  perform,  so  will  the  decline  be  rapid  or 
gradual ;  and  hence,  if  a  bitch  is  worked  hard  year  after  year, 
she  will  fail  rapidly,  and  the  diminution  of  her  puppies  will 
be  accordingly;  but  if  worked  moderately  and  well  kept,  she 
will  fail  gradually,  and  the  diminution  will  be  less  rapid." 

In  this  place,  more  fitly  than  elsewhere,  may  be  added  a 
fact  of  like  implication,  though  of  a  different  order.  Of 
course  whether  excessive  expenditure  be  in  the  continual  re- 
pairs of  nervo-muscular  tissues  or  in  replacing  other  tissues, 
the  reactive  effects,  if  not  quite  the  same,  will  be  similar — 
there  will  be  a  decrease  of  the  surplus  available  for  genesis. 
If,  then,  in  any  animals  there  from  time  to  time  occur  unusual 
outlays  for  self-maintenance,  we  may  expect  the  periods  of 
such  outlays  to  be  periods  of  diminished  or  arrested  repro- 
duction. That  they  are  so  the  moulting  of  birds  shows  us. 
When  hens  begin  to  moult  they  cease  to  lay.  While  they 
are  expending  so  much  in  producing  new  clothing,  they  have 
nothing  to  expend  for  producing  eggs. 


CHAPTER  IX. 

COINCIDENCE   BETWEEN   HIGH   NUTRITION   AND   GENESIS. 

§  352.  UNDER  this  head  may  be  grouped  various  facts 
which,  in  another  way,  tell  the  same  tale  as  those  contained 
in  the  last  chapter.  The  evidence  there  put  together  went 
to  show  that  increased  cost  of  self-maintenance  entailed  de- 
creased power  of  propagation.  The  evidence  to  be  set  down 
here,  will  go  to  show  that  power  of  propagation  is  augmented 
by  making  self-maintenance  unusually  easy.  For  into  this 
may  be  translated  the  effect  of  abundant  food. 

To  put  the  proposition  more  specifically — we  have  seen 
that  after  individual  growth,  development,  and  daily  con- 
sumption, have  been  provided  for,  the  surplus  nutriment 
measures  the  rate  of  multiplication.  This  surplus  may  be 
raised  in  amount  by  such  changes  in  the  environment  as 
bring  a  larger  supply  of  the  materials  or  forces  on  which 
both  parental  life  and  the  lives  of  offspring  depend.  Be 
there,  or  be  there  not,  any  expenditure,  a  higher  nutrition 
will  make  possible  a  greater  propagation.  We  may  expect 
this  to  hold  both  of  agamogenesis  and  of  gamogenesis;  and 
we  shall  find  that  it  does  so. 

§  353.  On  multi-axial  plants,  the  primary  effect  of  surplus 
nutriment  is  a  production  of  large  and  numerous  leaf-shoots. 
How  this  asexual  multiplication  results  from  excessive  nutri- 
tion, is  well  shown  when  the  leading  axis,  or  a  chief  branch, 
is  broken  off  towards  its  extremity.  The  axillary  buds  below 

475 


476  LAWS  OP  MULTIPLICATION. 

the  breakage  quickly  swell  and  burst  into  lateral  shoots, 
which  often  put  forth  secondary  shoots :  two  generations  of 
agamic  individuals  arise  where  there  probably  would  have 
been  none  but  for  the  local  abundance  of  sap,  no  longer 
drawn  off.  In  like  manner  the  abnormal  agamogenesis  which 
we  have  in  proliferous  flowers,  is  habitually  accompanied  by 
a  general  luxuriance,  implying  an  unusual  plethora. 

No  less  conclusive  is  the  evidence  furnished  by  agamo- 
genesis in  animals.  Sir  John  Dalyell,  speaking  of  Hydra 
tuba,  and  of  the  period  before  strobilization  commences,  says 
— "  It  is  singular  how  much  propagation  is  promoted  by 
abundant  sustenance."  This  Polype  goes  on  budding-out 
young  polypes  from  its  sides,  with  a  rapidity  proportionate 
to  the  supply  of  materials.  So,  too,  is  it  with  the 

agamic  reproduction  of  the  Aphis.  As  cited  by  Professor 
Huxley,  Kyber  "  states  that  he  raised  viviparous  broods  of 
both  this  species  (Aphis  Dianthi)  and  A.  Rosce  for  four  con- 
secutive years,  without  any  intervention  of  males  or  ovi- 
parous females,  and  that  the  energy  of  the  power  of  agamic 
reproduction  was  at  the  end  of  that  period  undiminished. 
The  rapidity  of  the  agamic  proliferation  throughout  the 
whole  period  was  directly  proportional  to  the  amount  of 
warmth  and  food  supplied." 

In  these  cases  the  relation  is  not  appreciably  complicated 
by  expenditure.  The  parent  having  reached  its  limit  of 
growth,  the  absorbed  food  goes  to  asexual  multiplication: 
scarcely  any  being  deducted  for  the  maintenance  of  parental 
life. 

§  354.  The  sexual  multiplication  of  organisms  under 
changed  conditions,  undergoes  variations  conforming  to  a 
parallel  law.  Cultivated  plants  and  domesticated  animals 
yield  us  proof  of  this. 

Facts  showing  that  in  cultivated  plants  sexual  genesis  in- 
creases with  nutrition,  are  obscured  by  facts  showing  that  a 
less  rapid  asexual  genesis,  and  an  incipient  sexual  genesis, 


NUTRITION  AND  GENESIS.  477 

accompany  the  fall  from  a  high  to  a  moderate  nutrition.  The 
confounding  of  these  two  relations  has  led  to  mistaken  infer- 
ences. When  treating  of  Genesis  inductively,  we  reached  the 
generalization  that  "  the  products  of  a  fertilized  germ  go 
on  accumulating  by  simple  growth,  so  long  as  the  forces 
whence  growth  results  are  greatly  in  excess  of  the  antagonist 
forces ;  but  that  when  diminution  of  the  one  set  of  forces,  or 
increase  of  the  other,  causes  a  considerable  decline  in  this  ex- 
cess, and  an  approach  towards  equilibrium,  fertilized  germs 
are  again  produced.''  (§  78.)  It  was  pointed  out  that  this 
holds  of  organisms  which  multiply  by  heterogenesis,  as  well 
as  those  which  multiply  by  homogenesis.  And  plants  were 
referred  to  as  illustrating,  both  generally  and  locally,  the 
decline  of  agamic  multiplication  and  commencement  of  gamic 
multiplication,  along  with  a  lessening  rate  of  nutrition.  Now 
the  many  cases  which  are  given  of  fruitfulness  caused  in  trees 
by  depletion,  are  really  cases  of  this  change  from  agamo- 
genesis  to  gamogenesis;  and  simply  go  to  prove  that  what 
would  naturally  arise  when  decreased  peripheral  growth  had 
followed  increased  size,  may  be  brought  about  artificially  by 
diminishing  the  supply  of  materials  for  growth.  Cramping 
its  roots  in  a  pot,  or  cutting  them,  or  ringing  its  branches, 
will  make  a  tree  bear  very  early:  bringing  about  a  prema- 
ture establishment  of  that  relative  innutrition  which  would 
have  spontaneously  arisen  in  course  of  time.  Such  facts 
by  no  means  show  that  in  plants  sexual  genesis  increases  as 
nutrition  diminishes.  When  it  has  once  set  in,  sexual 
genesis  is  scanty  or  imperfect  unless  nutrition  is  good. 
Though  the  starved  plant  may  blossom,  yet  many  of  its 
blossoms  will  fail;  and  such  seeds  as  it  produces  will  be  ill- 
furnished  with  those  enveloping  structures  and  that  store  of 
albumen,  &c.,  needed  to  give  good  chances  of  successful  ger- 
mination— the  number  of  surviving  offspring  will  be  dimin- 
ished. Were  it  otherwise,  the  manuring  of  fields  which  are  to 
bear  seed-crops,  would  be  not  simply  useless  but  injurious. 
Were  it  otherwise,  dunging  the  roots  of  a  fruit-tree  would  in 


478  LAWS  OF  MULTIPLICATION. 

all  cases  be  impolitic;  instead  of  being  impolitic  only  where 
the  growth  of  sexless  axes  is  still  luxuriant.  Were  it  other- 
wise, a  tree  which  has  borne  a  heavy  crop  should,  by  the 
consequent  depletion,  be  led  to  bear  a  still  heavier  crop  next 
year;  whereas  it  is  apt  to  be  wholly  or  partially  barren  next 
year — has  to  recover  a  state  of  tolerably-high  nutrition  before 
its  sexual  genesis  again  becomes  large. 

But  the  best  illustrations  are  yielded  by  animals — those 
animals  at  least  in  which  we  have,  besides  an  increased  sup- 
ply of  nutriment,  a  diminished  expenditure.  Two  classes  of 
comparisons,  alike  in  their  implications,  may  be  made — 
comparisons  between  tame  and  wild  animals  of  the  same 
species  or  genus,  and  comparisons  between  tame  animals  of 
the  same  species  differently  treated. 

To  begin  with  Birds,  let  us  first  contrast  the  farm-yard 
Gallinacece  with  their  kindred  of  the  fields  and  woods.  Not- 
withstanding their  greater  size,  which,  other  things  equal, 
should  be  accompanied  by  smaller  fertility,  the  domesticated 
kinds  have  more  numerous  offspring  than  the  wild  kinds.  A 
Turkey  has  a  dozen  in  a  brood,  while  a  Pheasant  has  from  6 
to  10.  Twice  or  thrice  in  a  season,  a  Hen  rears  as  many 
chickens  as  a  Partridge  rears  once  in  a  season.  Anserine 
birds  show  us  parallel  differences.  The  Tame  Goose  sits  on 
13  to  18  eggs  and  often  sits  a  second  time;  but  the  Wild 
Goose  sits  on  5,  6,  or  7,  and  these  are  noted  as  considerably 
smaller.  It  is  the  same  with  Ducks.  The  domesticated 
variety  lays  and  hatches  twice  as  many  eggs  as  the  wild 
variety.  And  the  like  holds  of  Pigeons.  After  remarking 
of  the  Columba  lima  that  "  in  spring  when  they  have  plenty 
of  corn  to  pick  from  the  newly-sown  fields,  they  begin  to 
get  fat  and  pair;  and  again  in  harvest,  when  the  corn  is  cut 
down,"  Macgillivray  goes  on  to  say  that  "  the  same  pair  when 
tamed  generally  breed  four  times  "  in  the  year.  That 

between  different  poultry-yards  inequalities  of  fertility  are 
caused  by  inequalities  in  the  supplies  of  food,  is  a  familiar 
truth.  High  feeding  shows  its  effects  not  only  in  the  con- 


NUTRITION  AND  GENESIS.  479 

tinuous  laying,  but  also  in  the  sizes  of  the  eggs.  Among 
directions  given  for  obtaining  eggs  from  pullets  late  in  the 
year,  it  is  especially  insisted  on  that  they  shall  have  a 
generous  diet.  Eespecting  Pigepns  Macgillivray  writes : — 
"  that  their  breeding  depends  much  on  their  having  plenty  of 
food  to  fatten  them,  seems,  I  think,  evident  from  the  circum- 
stance that,  when  tamed,  which  they  easily  are,  they  are 
observed  to  breed  in  every  month  of  the  year.  I  do  not  mean 
that  the  same  pair  will  breed  every  month;  but  some  in  the 
flock,  if  well  fed,  will  breed  at  any  season."  There  may 

be  added  a  fact  of  like  meaning  which  partially-domesticated 
birds  yield.  The  Sparrow  is  one  of  the  Finch  tribe  that  has 
taken  to  the  neighbourhood  of  houses;  and  by  its  boldness 
secures  food  not  available  to  its  congeners.  The  result  is 
that  it  has  several  broods  in  a  season,  while  its  field-haunting 
kindred  have  none  of  them  more  than  two  broods,  and  some 
have  only  one. 

Equally  clear  proof  that  abundant  nutriment  raises  the 
rate  of  multiplication,  occurs  among  Mammals.  Compare  the 
litters  of  the  Dog  with  the  litters  of  the  Wolf  and  the  Fox. 
Whereas  those  of  the  one  range  in  number  from  6  to  14,  those 
of  the  others  contain  respectively  5  or  6  or  occasionally  7,  and 
4  or  5  or  rarely  6.  Again,  the  Wild  Cat  has  4  or  5  kittens; 
but  the  tame  Cat  has  5  or  6  kittens  2  or  3  times  a-year. 
So,  too,  is  it  with  the  Weasel  tribe.  The  Stoat  has  5  young 
ones  once  a-year.  The  Ferret  has  2  litters  yearly,  each  con- 
taining from  6  to  9 ;  and  this  notwithstanding  that  it  is  the 
larger  of  the  two.  Perhaps  the  most  striking  contrast  is  that 
between  the  wild  and  tame  varieties  of  the  Pig.  While  the 
one  produces,  according  to  its  age,  from  4  to  8  or  10  young 
ones  once  a  year,  the  other  produces  sometimes  as  many  as  17 
in  a  litter ;  or,  in  other  cases,  will  bring  up  5  litters  of  10  each 
in  two  years — a  rate  of  reproduction  which  is  unparalleled 
in  animals  of  as  large  a  size.*  And  let  us  not  omit  to  note 
that  this  excessive  fertility  occurs  where  there  is  the 
*  The  climate,  the  locality,  and  the  kind  of  food,  are  of  course  all  factors ; 


480  LAWS  OF  MULTIPLICATION. 

greatest  inactivity — where  there  is  plenty  to  eat  and  nothing 
to  do.  There  is  no  less  distinct  evidence  that  among 

domesticated  Mammals  themselves,  the  well-fed  individuals 
are  more  prolific  than  the»ill-fed  individuals.  On  the  high 
and  comparatively-infertile  Cotswolds,  it  is  unusual  for 
ewes  to  have  twins;  but  they  very  commonly  have  twins 
in  the  adjacent  rich  valley  of  the  Severn.  Similarly,  among 
the  barren  hills  of  the  west  of  Scotland,  two  lambs  will  be 
borne  by  about  one  ewe  in  twenty;  whereas  in  England, 
something  like  one  ewe  in  three  will  bear  two  lambs.  Nay, 
in  rich  pastures,  twins  are  more  frequent  than  single  births; 
and  it  occasionally  happens  that,  after  a  genial  autumn  and 
consequent  good  grazing,  a  flock  of  ewes  will  next  spring 
yield  double  their  number  of  lambs — the  triplets  balancing 
the  uniparas.  So  direct  is  this  relation,  that  I  have  heard  a 
farmer  assert  his  ability  to  foretell,  from  the  high,  medium, 
or  low,  condition  of  an  ewe  in  the  autumn,  whether  she  will 
next  spring  bear  two,  or  one,  or  none. 

§  355.  An  objection  must  here  be  met.  Many  facts  may 
be  brought  to  prove  that  fatness  is  not  accompanied  by  ferti- 
lity but  by  barrenness ;  and  the  inference  drawn  is  that  high 
feeding  is  unfavourable  to  genesis.  The  premiss  may  be 
admitted  while  the  conclusion  is  denied. 

There  is  a  distinction  between  what  may  be  called  normal 
plethora,  and  an  abnormal  plethora,  liable  to  be  confounded 
with  it.  The  one  is  a  mark  of  constitutional  wealth ;  but  the 
other  is  a  mark  of  constitutional  poverty.  Normal  plethora 
is  a  superfluity  of  materials  both  for  the  building  up  of 

and  hence,  probably,  the  differences  between  the  statements  of  different  au- 
thorities concerning  these  several  cases.  Prof.  MacBride  writes  : — 

"According  to  Flower  (Mammals,  Living  and  Extinct)  the  Ferret  is  a 
domesticated  variety  of  the  common  polecat,  which  has  3  to  8  young.  Dar- 
win (Animals  and  Plants)  says  that  the  wild  sow  often  breeds  twice  a  year 
and  produces  a  litter  of  4  to  8,  and  sometimes  even  12.  The  domestic  sow 
breeds  twice  and  would  breed  oftener  if  permitted,  and  if  any  good  at  all  pro- 
duces 8  in  litter." 


NUTRITION  AND  GENESIS.  481 

tissue  and  the  evolution  of  force;  and  this  is  the  plethora 
which  we  have  found  to  be  associated  with  unusual  fecundity. 
Abnormal  plethora  which,  as  truly  alleged,  is  accompanied 
by  infecundity,  is  a  superfluity  of  force-evolving  materials 
joined  with  either  a  positive  or  a  relative  deficiency  of  tissue- 
forming  materials :  the  increased  bulk  indicating  this  state, 
being  really  the  bulk  of  so  much  inert  or  dead  matter.  Note, 
first,  a  few  of  the  facts  which  show  us  that  obesity  implies 
physiological  impoverishment. 

Neither  in  brutes  nor  men  does  it  ordinarily  occur  either 
in  youth  or  in  that  early  maturity  during  which  the  vigour 
is  the  greatest  and  the  digestion  the  best:  it  does  not 
habitually  accompany  the  highest  power  of  taking  up  nutri- 
tive materials.  When  fatness  arises  in  the  prime  of  life, 
whether  from  peculiarity  of  food  or  other  circumstance,  it  is 
not  the  sign  of  an  increased  total  vitality.  On  the  contrary, 
if  great  muscular  action  has  to  be  gone  through,  the  fat 
must  be  got  rid  of;  either,  as  in  a  man,  by  training,  or  as  in 
a  horse  that  has  grown  bulky  while  out  at  grass,  by  putting 
him  on  such  more  nutritive  diet  as  oats.  The  frequency 

of  senile  fatness,  both  in  domesticated  creatures  and  in  our- 
selves, has  a  similar  implication.  Whether  we  consider  the 
smaller  ability  of  those  who  display  it  to  withstand  large 
demands  on  their  powers,  or  whether  we  consider  the  com- 
paratively-inferior digestion  common  among  them,  we  see 
that  the  increased  size  indicates,  not  an  abundance  of  mate- 
rials which  the  organism  requires,  but  an  abundance  of 
materials  which  it  does  not  require.  Of  like  meaning 

is  the  fact  that  women  who  have  had  several  children,  and 
animals  after  they  have  gone  on  bearing  young  for  some 
time,  frequently  become  fat;  and  lose  their  fecundity  as 
they  do  this.  In  such  cases  the  fatness  is  not  to  be  taken 
as  the  cause  of  the  infecundity;  but  the  constitutional 
exhaustion  which  the  previous  production  of  offspring  has 
left,  shows  itself  at  once  in  the  failing  fecundity  and  the 
commencing  fatness.  There  is  yet  another  kind  of  evi- 

77 


482  LAWS  OF  MULTIPLICATION. 

dence.  Obesity  not  uncommonly  sets  in  after  the  system  has 
been  subject  to  debilitating  influences.  Often  a  serious  illness 
is  followed  by  a  corpulence  to  which  there  was  previously 
no  tendency.  And  the  prolonged  administration  of  mercury, 
constitutionally  injurious  as  it  is,  sometimes  produces  a  like 
effect. 

Closer  inquiry  verifies  the  conclusion  to  which  these  facts 
point.  The  microscope  shows  that  along  with  the  increase  of 
bulk  common  in  advanced  life,  there  goes  on  what  is  called 
"  fatty  degeneration :  "  oil-globules  are  deposited  where  there 
should  be  particles  of  flesh — or  rather,  we  may  say,  the 
hydrocarbonaceous  molecules  locally  produced  by  decom- 
position of  the  nitrogenous  molecules,  have  not  been  replaced 
by  other  nitrogenous  molecules,  as  they  should  have  been. 
This  fatty  degeneration  is,  indeed,  a  kind  of  local  death. 
For  so  regarding  it  we  have  not  simply  the  reason  that  an 
active  substance  has  its  place  occupied  by  an  inert  substance ; 
but  we  have  the  further  reason  that  the  flesh  of  dead  bodies, 
under  certain  conditions,  is  transformed  into  a  fatty  matter 
called  adipocere. 

The  infertility  that  accompanies  fatness  in  domestic 
animals  has,  however,  other  causes  than  that  declining  con- 
stitutional vigour  which  the  fatness  commonly  indicates. 
Being  artificially  fed,  these  animals  cannot  always  obtain 
what  their  systems  need.  That  which  is  given  to  them  is 
given  expressly  because  of  its  fattening  quality.  And  since 
the  capacity  of  the  digestive  apparatus  remains  the  same, 
the  absorption  of  fat-producing  materials  in  excess,  implies 
defect  in  the  absorption  of  materials  from  which  the 
tissues  are  formed,  and  out  of  which  young  ones  are  built 
up.  Moreover,  this  special  feeding  with  a  view 

to  rapid  and  early  fattening,  continued  as  it  is  through 
generations,  and  accompanied  as  it  is  by  a  selection  of  indi- 
viduals and  varieties  which  fatten  most  readily,  tends  to 
establish  a  modified  constitution,  more  fitted  for  producing 
fat  and  correspondingly-less  fitted  for  producing  flesh — a 


NUTRITION  AND  GENESIS.  483 

constitution  which,  from  this  relatively-deficient  absorption 
of  nitrogenous  matters,  is  likely  to  become  infertile;  as,  in- 
deed, these  varieties  often  do  become.  Hence,  no  con- 
clusions respecting  the  effects  of  high  nutrition,  properly  so 
called,  can  be  drawn  from  cases  of  this  kind.  The  cases  are, 
in  truth,  of  a  kind  which  could  not  exist  but  for  human 
agency.  Under  natural  conditions  no  animal  would  diet 
itself  in  the  way  required  to  produce  such  results.  And  if  it 
did  its  race  would  quickly  disappear.* 

There  is  yet  another  mode  in  which  accumulation  of  fat 
diminishes  fertility.  Even  supposing  it  unaccompanied  by 
a  smaller  absorption  of  nitrogenous  materials,  it  is  still  a 
cause  of  lessening  the  surplus  of  nitrogenous  materials.  For 
the  repair  of  the  motor  tissues  becomes  more  costly.  Fat 
stored-up  is  weight  to  be  carried.  A  creature  loaded  with 
inert  matter  must,  other  things  equal,  consume  a  greater 
amount  of  tissue-forming  substances  for  keeping  its  loco- 
motive apparatus  in  order ;  and  thus  expending  more  for  self- 
maintenance  can  expend  less  for  race-maintenance.  Abnormal 
plethora  is  thus  antagonistic  to  reproduction  in  a  double  way. 
It  ordinarily  implies  a  smaller  absorption  of  tissue-forming 
matters,  and  an  increased  demand  on  the  diminished  supply. 
Hence  fertility  decreases  in  a  geometrical  progression. 

The  counter-conclusion  drawn  from  facts  of  this  class  is, 
then,  due  to  a  misconception  of  their  nature — a  misconception 

*  It  is  worth  while  inquiring  whether  unfitness  of  the  food  given  to  them, 
is  not  the  chief  cause  of  that  sterility  which,  as  Mr.  Darwin  says,  "  is  the 
great  bar  to  the  domestication  of  animals."  He  remarks  that  "  when  animals 
and  plants  are  removed  from  their  natural  conditions,  they  are  extremely 
liable  to  have  their  reproductive  systems  seriously  affected."  Possibly  the 
relative  or  absolute  arrest  of  genesis,  is  less  due  to  a  direct  effect  on  the 
reproductive  system,  than  to  a  changed  nutrition  of  which  the  reproductive 
system  most  clearly  shows  the  results.  The  matters  required  for  forming  an 
embryo  are  in  a  greater  proportion  nitrogenous  than  are  the  matters  required 
for  maintaining  an  adult.  Hence,  an  animal  forced  to  live  on  insufficientlv- 
nitrogenized  food,  may  have  its  surplus  for  reproduction  cut  off,  but  still  have 
a  sufficiency  to  keep  its  own  tissues  in  repair,  and  appear  to  be  in  good  henlth 
— meanwhile  increasing  in  bulk  from  excess  of  the  non-nitrogenous  matters 
it  eats. 


484:  LAWS  OP  MULTIPLICATION. 

arising  partly  from  the  circumstance  that  the  increase  of  bulk 
produced  by  fat  is  somewhat  like  the  increase  of  bulk  which 
growth  of  tissues  causes,  and  partly  from  the  circumstance 
that  abundance  of  good  food  normally  produces  a  certain 
quantity  of  fat,  which,  within  narrow  limits,  is  a  valuable 
store  of  force-evolving  material.  When,  however,  we  limit 
the  phrase  high  nutrition  to  its  proper  meaning — an  abun- 
dance of,  and  due  proportion  among,  all  the  substances  which 
the  organism  needs — we  find  that,  other  things  equal,  fertility 
always  increases  as  nutrition  increases.  And  we  see  that  these 
apparently-exceptional  cases,  are  cases  which  really  show  us 
the  same  thing ;  since  they  are  cases  of  relative  innutrition. 

[NOTE. — By  a  strange  oversight  when  writing  this  chapter 
in  the  first  edition — an  oversight  I  was  on  the  eve  of  repeat- 
ing in  this  present  edition — I  omitted  to  bring  forward  the 
familiar  and  all-important  evidence  furnished  by  the  varia- 
tions of  genesis  which  ordinarily  accompany  the  alternations 
of  the  seasons.  These  variations,  in  multitudinous  creatures 
of  all  types,  show  unmistakably  that  reproduction  begins  at 
those  times  of  the  year  when  greater  warmth  and  larger 
supplies  of  food  render  maintenance  of  individual  life 
relatively  easy,  and  when  there  is  therefore  a  surplus  avail- 
able for  producing  new  individuals.  Conversely,  along 
with  the  decrease  of  heat  and  the  relative  deficiency  of  food 
which  make  it  comparatively  difficult  in  winter  to  maintain 
individual  life,  there  ceases  to  be  the  power  of  producing 
other  lives :  the  reproductive  organs  become  quiescent  and 
often  dwindle.  With  this  general  fact  is  associated  a  special 
fact.  Though  among  wild  animals — birds,  mammals,  and 
others — breeding  ceases  when  Nature  no  longer  supplies 
abundant  food  and  warmth;  in  domesticated  mammals  and 
birds,  artificially  supplied  with  food  and  warmth,  the  breed- 
ing season  is  greatly  extended  and  often  made  continuous, 
as,  under  the  same  conditions,  it  is  in  Man  himself. 

Evidence  yielded  by  the  vegetal  world  is  less  conspicuous, 


NUTRITION  AND  GENESIS.  485 

for  the  reason  that  the  cold  which  arrests  reproductive 
activity  also  arrests  individual  activity:  growth  of  the  indi- 
vidual and  multiplication  of  the  race  vary  simultaneously 
with  variations  in  the  seasons.  Still  there  are  some  familiar 
facts  showing  that  the  external  conditions  which  favour 
nutrition  also  bring  about  reproduction.  Early  in  the  year 
we  are  supplied  with  flowers  from  regions  warmer  than  our 
own,  and  by  and  by  there  come  to  our  markets  fruits  and 
vegetables  from  the  south  of  France,  the  Channel  Islands, 
and  even  from  the  Scilly  Isles,  which  are  much  in  advance 
of  those  furnished  by  the  gardens  of  our  own  colder  regions : 
reproduction  commences  earlier  where  the  light  and  heat 
furthering  nutrition  are  greater.  And  then  there  is  a 
kindred  meaning  in  the  not  unfrequent  occurrence  of  a  sec- 
ond flowering  and  even  of  a  second  fruiting  in  warm,  bright 
and  prolonged  autumns.  Here  the  abnormal  recommence- 
ment of  reproduction  is  determined  by  an  abnormal  increase 
of  nutrition.] 


CHAPTER  X. 

SPECIALITIES   OF   THESE   RELATIONS. 

§  356.  TESTS  of  the  general  doctrines  set  forth  in  preced- 
ing chapters,  are  afforded  by  organisms  having  modes  of  life 
which  diverge  widely  from  ordinary  modes.  Here,  as  else- 
where, aberrant  cases  yield  crucial  proofs. 

If  certain  organisms  are  so  circumstanced  that  highly- 
nutritive  matter  is  supplied  to  them  without  stint,  and  they 
have  nothing  to  do  but  absorb  it,  we  may  infer  that  their 
powers  of  propagation  will  be  enormous. 

If  there  are  classes  of  creatures  which  expend  very  little 
for  self-support  in  comparison  with  allied  creatures,  a  rela- 
tively-extreme prolificness  may  be  expected  of  them. 

Or  if,  again,  we  find  species  presenting  the  peculiarity 
that  while  some  of  their  individuals  have  much  to  do  and 
little  to  eat,  others  of  their  individuals  have  much  to  eat  and 
little  to  do,  we  may  look  for  great  fertility  in  these  last  and 
comparative  infertility  or  barrenness  in  the  first. 

These  several  anticipations  we  shall  find  completely 
verified. 

§  357.  Plants  which,  like  the  Rafflesiacece,  carry  their  para- 
sitism to  the  extent  of  living  on  the  juices  they  absorb  from 
other  plants,  exhibit  one  of  these  relations  in  the  vegetal 
kingdom.  In  them  the  organs  for  self-support  being  need- 
less, are  rudimentary;  and  the  parts  directly  or  indirectly 
concerned  in  the  production  and  distribution  of  germs,  con- 
stitute the  mass  of  the  organism.  That  small  ratio  which 
486 


SPECIALITIES  OF  THESE  RELATIONS.  487 

the  race-preserving  structures  bear  to  the  self-preserving 
structures  in  ordinary  Phaenogams,  is,  in  these  Phaenogams, 
inverted.  A  like  relation  occurs  in  the  common  Dodder. 

There  may  be  added  a  kindred  piece  of  evidence  which  the 
Fungi  present.  Those  of  them  which  grow  on  living  plants, 
repeat  the  above  connection  completely;  and  those  of  them 
which,  though  not  parasitic,  nevertheless  subsist  on  organized 
materials  previously  elaborated  by  other  plants,  substantially 
repeat  it.  The  spore-producing  part  is  relatively  enormous; 
and  the  fertility  is  far  greater  than  that  of  Cryptogams  of 
like  sizes,  which  have  to  form  for  themselves  the  organic 
compounds  of  which  they  and  their  germs  consist. 

§  358.  The  same  lesson  is  taught  us  by  animal-parasites. 
Along  with  the  decreased  cost  of  Individuation,  they  similarly 
show  us  an  increased  expenditure  for  Genesis ;  and  they  show 
us  this  in  the  most  striking  manner  where  the  deviation  from 
ordinary  conditions  of  life  is  the  greatest. 

Take,  among  the  Epizoa,  such  an  instance  as  Chondracan- 
tlius  gibbosus.  Belonging  to  the  Entomostraca,  both  males 
and  females  of  this  species  are,  in  their  early  days,  similar  to 
their  allies ;  and  the  males,  practically  parasitic,  though  they 
become  greatly  degraded,  continue  throughout  life  to  show 
by  their  segmentation  and  other  external  traits  their  original 
nature.  The  female,  however,  having  fixed  herself  where 
she  can  suck  the  juices  of  her  host,  the  Lophius,  grows  to 
twelve  times  the  length  of  the  male  and  probably  a  thousand 
times  its  bulk,  and  becomes  utterly  transformed  by  loss  of 
the  organs  of  animal  life  and  enormous  development  of  the 
organs  of  reproduction.  "  N~o  heart  is  discoverable,  and  the 
nervous  system  and  organs  of  sense  (if  any)  are  equally  un- 
distinguishable.  The  interspace  between  the  alimentary 
canal  and  the  walls  of  the  body  is  almost  wholly  occupied 
by  the  ovarium."  *  And  then  beyond  this  there  are  ap- 
pended ovi-sacs  twice  the  length  of  the  body.  So  that  the 
*  Huxley,  Anatomy  of  Invcrtebrated  Animals,  p.  274. 


488  LAWS  OP  MULTIPLICATION. 

germ-producing  organs  and  their  contents,  eventually 
acquire  a  total  bulk  many  times  that  of  all  the  other  organs 
put  together.  Numerous  species  of  this  type  and  habit, 
repeat  this  relation  between  a  life  of  inaction  with  high 
feeding,  and  an  enormous  rate  of  genesis.  Parasites  belong- 
ing to  another  great  division  of  the  animal  kingdom,  the 
Platyhelminthes,  supply  an  example  of  an  epizoon  in  which 
the  rate  of  multiplication  is  made  great  not  so  much  by 
immense  development  of  the  egg-producing  organs  as  by 
the  rapidity  with  which  generations  succeed  one  another — a 
rapidity  such  that  each  generation  partially  develops  the 
next  before  it  is  itself  anything  like  ready  for  independent 
life.  This  is  the  Gyrodactylus  elegans,  of  which  it  is  said 
that  "  its  most  remarkable  feature  is  that  it  is  viviparous, 
and  its  embryos  before  they  leave  the  body  of  their  mother 
have  already  developed  their  embryos  inside  them;  and  the 
latter  may  contain  their  embryos,  so  that  four  generations 
may  be  included  under  the  cuticle  of  the  sexually  mature 
animal."  * 

Entozoa  yield  us  many  examples  of  this  causal  relation, 
raised  to  a  still  higher  degree.  The  Gordius,  or  Hair-worm, 
is  a  creature  which,  finding  its  way  when  young  into  the 
body  of  an  insect  which  is  afterwards  swallowed  by  a  fish, 
there  grows  rapidly,  and  then  emerging  to  breed,  lays  as 
many  as  8,000,000  eggs  in  less  than  a  day.  Similarly  with 
those  larger  types  infesting  the  higher  animals.  It  has  been 
calculated  by  Dr.  Eschricht,  as  quoted  by  Professor  Owen, 
that  there  are  "64,000,000  of  ova  in  the  mature  female 
Ascaris  lumbricoides."  Very  many  of  the  Entozoa  belong  to 
the  Platyhelminthes,  and  among  them  occur  examples  of  fer- 
tility caused  not  only  by  great  numbers  of  ova,  but  by  rapid 
succession  of  partially-developed  individuals  and  also  exam- 
ples of  fertility  caused  by  production  of  ova  almost  exceeding 
numeration.  Among  the  first  the  Liver-fluke  may  be  named. 
Of  the  half-million  eggs  it  produces  each  yields  a  free-swim- 
*  Shipley,  Zoology  of  Irivertebrata,  p.  112. 


SPECIALITIES  OF  THESE  RELATIONS.  489 

ming  ciliated  embryo,  and  any  one  of  these,  which  finds  its 
way  into  a  water-snail,  becomes  a  sporocyst — a  bag,  presently 
occupied  exclusively  by  masses  of  cells :  each  mass  by  and 
by  becoming  a  Redia,  which  makes  its  way  out.  Like  all 
its  fellows  which  develop  in  succession,  this,  with  the  excep- 
tion of  a  small  space  occupied  by  the  stomach,  devotes  the. 
whole  of  its  interior  partly  to  the  formation  of  other  RedicB 
(which  presently  escape  and  become  similarly  transformed), 
and  partly  to  the  development  of  Cercarice,  into  which  the 
internal  substance  of  all  the  Redice  is  eventually  transformed : 
Cercarice  which,  escaping  from  the  host,  become  agents  for 
infecting  other  creatures.  So  that  each  ovum  thus  gives 
rise  to  a  number  of  forms  which  severally  subserve  multi- 
plication in  different  ways.  Of  the  other  division  of  Platy- 
helminthes  referred  to  as  carrying  on  its  multiplication  by 
production  of  ova  only,  the  commonest  of  the  Cestoidea 
furnishes  the  best  example.  Immersed  as  a  Tape-worm  is  in 
nutritive  liquid,  which  it  absorbs  through  its  integument,  it 
requires  no  digestive  apparatus.  The  room  which  one  would 
occupy,  and  the  materials  it  would  use  up,  are  therefore  avail- 
able for  germ-producing  organs,  which  nearly  fill  each  seg- 
ment :  each  segment,  sexually  complete  in  itself,  is  little  else 
than  an  enormous  reproductive  system,  with  just  enough  of 
other  structures  to  bind  it  together.  Remembering  that  the 
Tape-worm,  retaining  its  hold,  continues  to  bud-out  such 
segments  as  fast  as  the  fully-developed  ones  are  cast  off,  and 
goes  on  doing  this  as  long  as  the  infested  individual  lives; 
we  see  that  here,  where  there  is  no  expenditure,  where  the 
cost  of  individuation  is  reduced  to  the  greatest  extent  while 
the  nutrition  is  the  highest  possible,  the  degree  of  fertility 
reaches  its  extreme.  These  Entozoa  yield  us  further 

interesting  evidence.  Of  their  various  species,  most  if  not 
all  undergo  passive  migration  from  animal  to  animal  before 
they  become  mature.  Usually,  the  form  assumed  in  the  body 
of  the  first  host  is  devoid  of  all  that  part  in  which  the  repro- 
ductive structures  take  their  rise;  and  this  part  grows  and 


490  LAWS  OF  MULTIPLICATION. 

develops  reproductive  structures,  only  in  some  predatory 
animal  to  which  its  first  host  falls  a  sacrifice.  Occasionally, 
however,  the  egg  gives  origin  to  the  sexual  form  in  the 
animal  that  originally  swallowed  it,  but  the  development 
remains  incomplete — there  is  no  sexual  genesis,  no  formation 
of  eggs  in  the  rudimentary  segments.  That  these  may 
become  fertile  it  is  needful,  as  before,  for  the  containing 
animal  to  be  devoured;  so  that  the  imperfect  Tape- worm 
may  find  its  way  into  the  intestine  of  a  higher  animal. 
Thus  the  Bothriocephalus  solidus,  found  in  the  abdominal 
cavity  of  the  Stickleback,  is  barren  while  it  remains  there; 
but  if  the  Stickleback  be  eaten  by  a  Water-fowl,  the  re- 
productive system  of  the  transferred  Bothriocephalus  (then 
known  as  B.  nodosus)  becomes  developed  and  active.  So,  too, 
a  kind  of  Tape-worm  which  remains  infertile  while  in  the 
intestine  of  a  Mouse,  becomes  fertile  in  the  intestine  of  a  Cat 
that  devours  the  mouse.  May  we  not  regard  these  facts  as 
again  showing  the  dependence  of  fertility  on  nutrition? 
Barrenness  here  accompanies  conditions  unfavourable  to  the 
absorption  of  nutriment ;  and  it  gives  way  to  fecundity  where 
nutriment  is  large  in  quantity  and  superior  in  quality. 

§  359.  Extremely  significant  are  those  cases  of  partial 
reversion  to  primitive  forms  of  genesis,  which  occur  under 
special  conditions  in  some  of  the  higher  Annulosa.  I  refer 
to  the  pseudo-parthenogenesis  and  metagenesis  in  Insects. 

Under  what  conditions  do  the  Aphides  exhibit  this  strange 
deviation  from  the  habits  of  their  order?  Why  among  them 
should  imperfect  females  produce,  agamically,  others  like 
themselves,  generation  after  generation,  with  great  rapidity? 
There  is  the  obvious  explanation  that  they  get  plenty  of 
easily-assimilated  food  without  exertion.  Piercing  the  tender 
coats  of  young  shoots,  they  sit  and  suck — appropriating  the 
nitrogenous  elements  of  the  sap  and  ejecting  its  saccha- 
rine matter  as  "  honey  dew."  Along  with  a  sluggishness 
strongly  contrasted  with  the  activity  of  most  insects — along 


SPECIALITIES  OP  THESE  RELATIONS.  491 

with  a  very  low  rate  of  consumption  and  a  correlative  degra- 
dation of  structure;  we  have  here  a  retrogression  to  asexual 
genesis,  and  a  greatly-increased  rate  of  multiplication. 

The  recently  discovered  instance  of  internal  metagenesis 
in  the  maggots  of  certain  Flies  has  a  like  meaning.  In- 
credible as  it  at  first  seemed  to  naturalists,  it  is  now  proved 
that  the  Cecydomia-laTva,  develops  in  its  interior  a  brood  of 
larvae  of  like  structure  with  itself.  In  this  case,  as  in  the 
last,  abundant  food  is  combined  with  low  expenditure.  These 
larvae  are  found  in  such  b.abitats  as  the  refuse  of  beet-root- 
sugar  factories — masses  of  nitrogenous  debris  remaining  after 
the  extraction  of  the  saccharine  matter.  Each  larva  has  a 
practically-unlimited  supply  of  sustenance  imbedding  it  on 
all  sides.* 

It  is  true  that  some  other  maggots,  as  those  of  the  Flesh- 
fly,  are  similarly,  or  still  better,  circumstanced;  and,  it  may 
be  said,  ought  therefore  to  have  the  same  habit.  But  this 
does  not  necessarily  follow.  Survival  of  the  fittest  will 
determine  whether  such  specially-favourable  conditions  result 
in  aggrandizement  of  the  individual  or  in  multiplication 
of  the  race.  And  in  the  case  of  the  Flesh-fly  there  is  a 
reason  why  greater  individuation  rather  than  more  rapid 
genesis  will  occur.  For  a  decomposing  animal  body  lasts  so 
short  a  time,  that  were  Flesh-fly  larvae  to  multiply  agamically, 
the  second  generation  would  die  from  the  disappearance  of 
their  food.  Hence  individuals  in  which  the  excessive 
nutrition  led  to  internal  metagenesis,  would  leave  no  pos- 
terity, and  natural  selection  would  establish  the  variety  in 

*  I  am  told  that  "  Wagner,  who  described  the  larva,  found  that  it  bored 
into  the  bark  of  trees.  It  attacks  also  the  wheat  plant,  and  is  a  most 
destructive  parasite."  Apparently  this  statement  is  at  variance  with  the 
foregoing  inference.  It  is  clear,  however,  that  since  these  heaps  of  nitro- 
genous refuse  in  which  it  has  been  found  are  artificial  and  recent,  they  can- 
not be  its  natural  habitats ;  and  it  seems  not  improbable  that  these  larvae, 
suddenly  supplied  with  a  more  nutritive  food  in  unlimited  amount,  may  have 
as  a  consequence  acquired  this  habit  of  agamotrenetic  multiplication  which 
did  not  characterize  the  species  under  its  natural  conditions  and  relatively  low 
nutrition. 


492  LAWS  OF  MULTIPLICATION. 

which  greater  growth  resulted.  All  which  the  argument 
requires  is  that  when  such  reversion  to  agamogenesis  does 
take  place,  it  shall  be  where  the  food  is  unusually  abundant 
and  the  expenditure  unusually  small;  and  this  the  cases 
instanced  go  to  show. 

§  360.  The  physiological  lesson  taught  us  by  Bees  and 
Ants,  not  quite  harmonizing  with  the  moral  lesson  they  are 
supposed  to  teach,  is  that  highly-fed  idleness  is  favourable  to 
fertility,  and  that  excessive  industry  has  barrenness  for  its 
concomitant. 

The  egg  of  a  Bee  develops  into  a  small  barren  female  or 
into  a  large  fertile  female,  according  to  the  supply  of  food 
given  to  the  larva  hatched  from  it.  We  here  see  that  the 
germ-producing  action  is  an  overflow  of  the  surplus  remain- 
ing after  completion  of  the  individual;  and  that  the  lower 
feeding  which  the  larva  of  a  working  Bee  has,  results  in  a 
dwarfing  of  the  adult  and  an  arrested  development  of  the 
generative  organs.  Further,  we  have  the  fact  that  the  con- 
dition under  which  the  perfect  female,  or  mother-Bee,  goes 
on,  unlike  insects  in  general,  laying  eggs  continuously,  is 
that  she  has  plenty  of  food  brought  to  her,  is  kept  warm,  and 
goes  through  no  considerable  exertion.  While,  contrariwise, 
it  is  to  be  noted  that  the  infertility  of  the  workers  is  asso- 
ciated with  the  ceaseless  labour  of  bringing  materials  for  the 
combs  and  building  them,  as  well  as  the  labour  of  feeding 
the  queen,  the  larvae,  and  themselves. 

Ants  also  show  us  these  relations,  and  they  are  shown  in  a 
greatly  exaggerated  form  by  what  are  called  white  ants — 
insects  belonging  to  a  quite  different  order.  The  contrast 
in  bulk  between  the  fecund  and  infecund  females  is  here 
immensely  greater.  The  mother- Ant  has  the  reproductive 
system  so  enormously  developed,  that  the  remainder  of  her 
body  is  relatively  insignificant.  Entirely  incapable  of  loco- 
motion, she  is  unable  to  deposit  her  eggs  in  the  places  where 
they  are  to  be  hatched ;  so  that  they  have  to  be  carried  away 


SPECIALITIES  OF  THESE  RELATIONS.  493 

by  the  workers  as  fast  as  they  are  extruded.  Her  life  is  thus 
reduced  substantially  to  that  of  a  parasite — an  absorption  of 
abundant  food  supplied  gratis,  a  total  absence  of  expendi- 
ture, and  a  consequent  excessive  rate  of  genesis.  "The 
queen-ant  of  the  African  Termites  lays  80,000  eggs  in  twenty- 
four  hours." 

§  361.  It  may  be  needful  to  say  that  these  exceptional 
relations  cannot  be  ascribed  to  the  assigned  causes  acting 
alone.  The  extreme  fertility  which,  among  parasites  and 
social  insects,  accompanies  extremely  high  feeding  and  an 
expenditure  reduced  nearly  to  zero,  presupposes  typical  struc- 
tures and  tendencies  of  suitable  kinds;  and  these  are  not 
directly  accounted  for.  On  creatures  otherwise  organized, 
unlimited  supplies  of  food  and  total  inactivity  are  not  fol- 
lowed by  such  results.  There  of  course  requires  a  consti- 
tution fitted  to  the  special  conditions,  and  the  evolution  of 
this  cannot  be  due  simply  to  plethora  joined  with  rest.  These 
cases  are  given  as  illustrating  the  conditions  under  which 
extreme  exaltations  of  fertility  become  possible.  Their  mean- 
ings, thus  limited,  are  clear,  and  completely  to  the  point.  We 
see  in  them  that  the  devotion  of  nutriment  to  race-preserva- 
tion, is  carried  furthest  where  the  cost  of  self-preservation 
is  reduced  to  a  minimum;  and,  conversely,  that  nothing  is 
devoted  directly  to  race-preservation  by  individuals  on  which 
falls  an  excessive  expenditure  for  self-preservation  and  pre- 
servation of  other's  offspring. 


[NOTE. — Among  specialities  of  these  relations  may  be  fitly 
added  here  a  very  strange  one,  for  a  description  of  which  I 
am  indebted  to  M.  Charles  Julin.  Professor  of  Comparative 
Anatomy  in  the  University  of  Liege.  In  the  Revue  Generate 
des  Sciences  for  30th  August,  1894,  in  an  account  of  certain 
investigations  of  M.  Giard,  he  describes  what  he  calls  "la 
castration  parasitaire  " — a  castration  not  of  a  literal  kind 


494  LAWS  OF  MULTIPLICATION. 

but  one  effected  by  the  arrest  of  development  which  follows 
from  the  depletion  caused  by  a  parasite.  The  Sacculina  is 
an  amazingly  transformed  type  belonging  to  the  Cirrhipedia 
— a  type  without  segments  or  appendages  and  without  mouth 
and  alimentary  canal.  Fixing  itself,  during  its  early  loco- 
motive stage,  under  the  abdomen  of  a  decapodous  crustacean, 
and  leaving  behind  its  exo-skeleton,  it  makes  its  way  into 
the  interior,  and  there  becoming  a  mere  bag  containing  the 
reproductive  organs,  obtains  the  needful  nutriment  by  de- 
veloping what  are  practically  roots  and  rootlets  which  run 
everywhere  among  the  viscera  and  absorb  nutriment  from 
the  surrounding  tissues.  Here  we  are  concerned  merely 
with  the  effect  produced  upon  the  host  by  this  physiological 
robbery.  This  effect  is  to  arrest  the  development  not  only 
of  the  primary  sexual  organs  devoted  to  the  production  of 
germs,  but  also  of  those  secondary  sexual  organs  which  char- 
acterize the  male.  M.  Julin  writes : — 

"II  convient  ce pendant  de  dire,  pour  etre  plus  exact,  que,  dana 
les  cas  des  Crabes  infested  par  des  Sacculines,  il  n'y  a  pas,  en  realit6, 
apparition  de  caracteres  femelles  chez  le  sexe  male,  mais  plut&t 
absence  de  developpement  des  caracteres  males.  En  fait,  1'animal 
reste  a  un  stade  jeune,  non  diffe>enci6  sexuellement,  tout  en  prenant 
une  taille  plus  considerable.  Cela  nous  porte  a  attribuer  les  modi- 
fications dont  nous  avons  par!6  a  un  simple  arret  de  developpement, 
qui  est  plus  sensible  chez  le  male,  parce  que  chez  lui  les  caracteres 
sexuels  secondaires  sont  a  l'6tat  normal  plus  developp6s  que  chez  la 
femelle. 

D'une  maniere  g6n6rale,  nous  croyons,  avec  M.  Giard,  qu'il  faut 
assimiler  les  modifications  dues  a  la  castration  parasitaire  a  celles  qui 
sont  le  r£sultat  de  la  progenese  ou  qui  engendrent  le  dimorphisms 
saisonnier. 

II  y  a  progenhe  lorsque,  chez  un  animal,  la  reproduction  sexude 
s'opere  d'une  fa^on  plus  ou  moins  pr6coce,  c'est-a-dire  lorsque  les 
produits  sexuels  (oeufs  ou  spermatozoldes)  se  forment  et  murissent 
avant  que  1'etre  n'ait  atteint  son  complet  d6veloppement.  On  peut 
citer  comme  exemples  les  Axolotls  et  les  larves  de  Tritons  qui,  les  uns 
normalement,  les  autres  accidentellement,  pendent  en  ayant  encore 
leurs  branchies. 

Tres  souvent  la  progendse  n'affecte  qu'un  seul  sexe.     Tantdt,  c'est 


SPECIALITIES  OF  THESE  RELATIONS.  495 

le  sexe  femelle  qui  murit  a  l'6tat  larvaire  comme  chez  les  pucerons, 
les  Stylops,  etc...Tantot  c'est  le  sexe  male,  comme  chez  la  Bonellie,  les 
males  complementaires  de  Cirripedes,  les  males  pygmees  des  Rotiferes, 
le  male  de  1'Anguille,  etc.  D'autres  fois,  enfin,  1'animal  presente 
successivement  les  deux  sexes  avec  progenese  pour  1'un  d'entre  eux. 
C'est  ainsi  qu'il  y  a  progenese  protandrique  chez  les  Crustaces  cymo- 
thoadiens,  et,  parmi  les  Vertebres,  chez  les  Myxines,  qui,  males  dans 
le  jeune  age,  deviennent  femelles  en  vieillissant  et  en  achevant  de 
prendre  leur  developpement.  Le  cas  des  vieilles  femelles  de  Galli- 
naces  a  plumage  et  a  instincts  masculins  semble  etre,  au  contraire,  un 
exemple  imparfait  de  progentse  protogynique,  puisque  ces  femelles  ont 
pondu  lorsqu'elles  avaient  encore  la  livree  des  jeunes  et  qu'elles  ont 
continue  plus  tard  leur  developpement,  et  presentent  le  caractere  des 
males  sans  que,  cependant,  1'on  ait  constate  la  production  de  sper- 
matozoides. 

Dans  les  cas  extremes  de  progenese  femelle,  la  reproduction  se  fait 
meme  sans  le  concours  de  1'element  male,  revenant  ainsi  a  la  forme 
agamique  primordiale.  Ces  cas  sont  connus  depuis  longtemps  sous  le 
nom  de  pedogenese.  On  les  a  observe  chez  les  larves  de  Miastor,  de 
Chirwiomus  et  chez  certains  pucerons. 

Chaque  fois  qu'il  y  a  progenese  dans  un  type  determine,  on  cons- 
tate soit  momentanement,  soit  d'une  facon  definitive,  un  arret  de 
croissance  et  de  developpement:  1'animal  progen6tique  a,  par  suite, 
Taspect  d'une  larve  sexude,  lorsqu'on  le  compare  soit  a  1'autre  sexe, 
soit  aux  formes  voisines,  qui  ne  pr6sentent  pas  le  ph6nomene  de  la 


Cela  est  en  parfaite  harmonic  avec  le  principe,  si  Men  mis  en 
lumiere  par  Herbert  Spencer,  de  Vantagonixme  entre  la  genese  et  la 
croissance  et  entre  la  genese  et  le  developpement.  Get  antagonisme 
s'explique  facilement  si  1'on  songe  que  les  mat^riaux  employes  pour 
la  reproduction  ne  peuvent  servir  a  I'accroissement  de  1'individu. 
S'il  est  avantageux  pour  un  organisme  de  se  reproduire  sans  acqueVir 
des  organes  inutiles,  la  selection  naturelle  d6terminera  bient6t  une 
progenese  de  plus  en  plus  complete.  Les  animaux  parasites,  outre 
qu'ils  tirent  de  leur  h6te  une  nourriture  abondante,  n'ont  guere  besoin 
d'une  foule  d'organes  qui  servent  a  leurs  cong6n6res  libres  dans  la 
vie  de  relation.  Aussi  voyons-nous  qu'un  tres  grand  nombre  d'ani- 
maux  parasites  sont  progenetiques.  Les  males  prog6n6tiques  de  la 
Bonellie  et  des  Cirripedes  vivent  en  parasites  dans  leurs  femelles. 
Chez  certains  types,  les  pucerons,  la  progendse  cesse  des  que,  la 
nourriture  devenant  moins  abondante,  un  d6placement  pourra  etre 
necessaire. 


406  LAWS  OP  MULTIPLICATION. 

En  resumd,  1'arret  de  d6veloppement  du  a  la  progenese  resulte 
d'une  derivation  des  principes  nourriciers  au  detriment  de  1'animal 
progenetique.  Dans  les  exemples  de  castration  parasitaire  que  nous 
avons  examines,  Ic  parasite  joue,  par  rapport  a  sou  h6te,  absolument 
le  meme  r61e  que  la  glande  genitale  d'uii  type  progenetique.  11 
detourne,  pour  sa  propre  subsistance,  une  partie  des  principes  qui 
auraient  servi  au  developpement  de  Fanimal.  Aussi  les  effets  produits 
sont-ils  tout  a  fait  de  iiiuiue  ordre." 

A  phenomenon  so  anomalous  as  this,  explicable  upon  the 
hypothesis  set  forth  but  not  otherwise  explicable,  furnishes 
striking  verification.] 


CHAPTER  XI. 

INTERPRETATION   AND   QUALIFICATION. 

§  362.  CONSIDERING  the  difficulties  of  inductive  verifica- 
tion, we  have,  I  think,  as  clear  a  correspondence  between  the 
a  priori  and  a  posteriori  conclusions,  as  can  be  expected.  The 
many  factors  co-operating  to  bring  about  the  result  in  every 
case,  are  so  variable  in  their  absolute  and  relative  amounts, 
that  we  can  rarely  disentangle  the  effect  of  each  one,  and 
have  usually  to  be  content  with  qualified  inferences.  Though 
in  the  mass  organisms  show  us  an  unmistakable  relation 
between  great  size  and  small  fertility,  yet  special  compari- 
sons among  them  are  nearly  always  partially  vitiated  by 
differences  of  structure,  differences  of  nutrition,  differences  of 
expenditure.  Though  it  is  beyond  question  that  the  more 
complex  organisms  are  the  less  prolific,  yet  as  complexity 
has  a  certain  general  connexion  with  bulk,  and  in  animals 
with  expenditure,  we  cannot  often  identify  its  results  as  inde- 
pendent of  these.  And,  similarly,  though  the  creatures  which 
waste  much  matter  in  producing  motion,  sensible  and  insen- 
sible, have  lower  rates  of  multiplication  than  those  which 
waste  less,  yet,  as  the  creatures  which  waste  much  are 
generally  larger  and  more  complex,  we  are  again  met  by  an 
obstacle  which  limits  our  comparisons,  and  compels  us  to 
accept  conclusions  less  definite  than  are  desirable. 

Such  difficulties  arise,  however,  only  when  we  endeavour, 
as  in  foregoing  chapters,  to  prove  the  inverse  variation 
78  497 


498  LAWS  OF  MULTIPLICATION. 

between  Genesis  and  each  separate  element  of  Individuation 
— growth,  development,  activity.  We  are  scarcely  at  all 
hampered  by  qualifications  when,  from  contemplating  these 
special  relations,  we  return  to  the  general  relation.  The 
antagonism  between  Individuation  and  Genesis  is  shown  by 
all  the  facts  which  have  been  grouped  under  each  head.  We 
have  seen  that  in  ascending  from  the  lowest  to  the  highest 
types,  there  is  a  decrease  of  fertility  so  great  as  to  be  abso- 
lutely inconceivable,  and  even  inexpressible  by  figures;  and 
whether  the  superiority  of  type  consists  in  relative  largeness, 
in  greater  complexity,  in  higher  activity,  or  in  some  or  all  of 
these  combined,  matters  not  to  the  ultimate  inference.  The 
broad  fact,  enough  for  us  here,  is  that  organisms  in  which 
the  integration  and  differentiation  of  matter  and  motion  have 
been  carried  furthest,  are  those  in  which  the  rate  of  multipli- 
cation has  fallen  lowest.  How  much  of  the  decline  of  repro- 
ductive power  is  due  to  the  greater  integration  of  matter, 
how  much  to  its  greater  differentiation,  how  much  to  the 
larger  amounts  of  integrated  and  differentiated  motions  gene- 
rated, it  may  be  impossible  to  say;  and  it  is  not  needful  to 
say.  These  are  all  elements  of  a  higher  degree  of  life,  an 
augmented  ability  to  maintain  the  organic  equilibrium  amid 
environing  actions,  an  increased  power  of  self-preservation; 
and  we  find  their  invariable  accompaniment  to  be,  a  dimi- 
nished expenditure  of  matter,  or  motion,  or  both,  in  race- 
preservation. 

In  brief,  then,  examination  of  the  evidence  shows  that 
there  does  exist  that  relation  which  we  inferred  must  exist. 
Arguing  from  general  data,  we  saw  that  for  the  maintenance 
of  a  species,  the  ability  to  produce  offspring  must  be  great, 
in  proportion  as  the  ability  of  the  individuals  to  contend  with 
destroying  forces  is  small;  and  conversely.  Arguing  from 
other  general  data,  we  saw  that,  derived  a's  the  self-sustain- 
ing and  race-sustaining  forces  are  from  a  common  stock  of 
force,  it  necessarily  happens  that,  other  things  equal,  increase 
of  one  involves  decrease  of  the  other.  And  then,  turning 


INTERPRETATION  AND  QUALIFICATION.  499 

to  special  facts,  we  have  found  that  this  inverse  variation  is 
clearly  traceable  throughout  both  the  animal  and  vegetal 
kingdoms.  We  may  therefore  set  it  down  as  a  law,  that 
every  higher  degree  of  organic  evolution,  has  for  its  concomi- 
tant a  lower  degree  of  that  peculiar  organic  dissolution  which 
is  seen  in  the  production  of  new  organisms. 

§  363.  Something  remains  to  be  said  in  reply  to  the  in- 
quiry— how  is  the  ratio  between  Individuation  and  Genesis 
established  in  each  case  ?  This  inquiry  has  been  but  partially 
answered  in  the  course  of  the  foregoing  argument. 

Many  specialities  of  the  reproductive  process  are  mani- 
festly due  to  the  natural  selection  of  favourable  variations. 
Whether  a  creature  lays  a  few  large  eggs  or  many  small  ones 
equal  in  weight  to  the  few  large,  is  not  determined  by  any 
physiological  necessity:  here  the  only  assignable  cause  is  the 
survival  of  varieties  in  which  the  matter  devoted  to  repro- 
duction happens  to  be  divided  into  portions  of  such  size  and 
number  as  most  to  favour  multiplication.  Whether  in  any 
case  there  are  frequent  small  broods  or  larger  broods  at 
longer  intervals,  depends  wholly  on  the  constitutional  pecu- 
liarity that  has  arisen  from  the  dying  out  of  families  in 
which  the  sizes  and  intervals  of  the  broods  were  least  suited 
to  the  conditions  of  life.  Whether  a  species  of  animal  pro- 
duces many  offspring  of  which  it  takes  no  care  or  a  few  of 
which  it  takes  much  care — that  is,  whether  its  reproductive 
surplus  is  laid  out  wholly  in  germs  or  partly  in  germs  and 
partly  in  labour  on  their  behalf — must  have  been  decided  by 
that  moulding  of  constitution  to  conditions  slowly  effected 
through  the  more  frequent  preservation  of  descendants  from 
those  whose  reproductive  habits  were  best  adapted  to  the 
circumstances  of  the  species.  Given  a  certain  surplus  avail- 
able for  race-preservation,  and  it  is  clear  that  by  indirect 
equilibration  only,  can  there  be  established  the  more  or 
less  peculiar  distribution  of  this  surplus  which  we  see  in 
each  case.  Obviously,  too,  survival  of  the  fittest 


500  LAWS  OF  MULTIPLICATION. 

has  a  share  in  determining  the  proportion  between  the 
amount  of  matter  that  goes  to  Individuation  and  the  amount 
that  goes  to  Genesis.  Whether  the  interests  of  the  species 
are  most  subserved  by  a  higher  evolution  of  the  individual 
joined  with  a  diminished  fertility,  or  by  a  lower  evolution  of 
the  individual  joined  with  an  increased  fertility,  are  ques- 
tions ever  being  experimentally  answered.  If  the  more- 
developed  and  less-prolific  variety  has  a  greater  number  of 
survivors,  it  becomes  established  and  predominant.  If,  con- 
trariwise, the  conditions  of  life  being  simple,  the  larger  or 
more-organized  individuals  gain  nothing  by  their  greater  size 
or  better  organization;  then  the  greater  fertility  of  the  less 
evolved  ones,  will  insure  to  their  descendants  an  increasing 
predominance. 

But  direct  equilibration  all  along  maintains  the  limits 
within  which  indirect  equilibration  thus  works.  The 
necessary  antagonism  we  have  traced,  rigidly  restricts  the 
changes  that  natural  selection  can  produce,  under  given  con- 
ditions, in  either  direction.  A  greater  demand  for  Individua- 
tion, be  it  a  demand  caused  by  some  spontaneous  variation  or 
by  an  adaptive  increase  of  structure  and  function,  inevitably 
diminishes  the  supply  for  Genesis;  and  natural  selection 
cannot,  other  things  remaining  the  same,  restore  the  rate  of 
Genesis  while  the  higher  Individuation  is  maintained.  Con- 
versely, survival  of  the  fittest,  acting  on  a  species  that  has, 
by  spontaneous  variation  or  otherwise,  become  more  prolific, 
cannot  again  raise  its  lowered  Individuation,  so  long  as  every- 
thing else  continues  constant. 

§  364.  Here,  however,  a  qualification  must  be  made.  It 
was  parenthetically  remarked  in  §  327,  that  the  inverse  varia- 
tion between  Individuation  and  Genesis  is  not  exact;  and  it 
was  hinted  that  a  slight  modification  of  statement  would  be 
requisite  at  a  more  advanced  stage  of  the  argument.  We 
have  now  reached  the  proper  place  for  specifying  this  modi- 
fication. 


INTERPRETATION  AND  QUALIFICATION.  501 

Each  increment  of  evolution  entails  a  decrement  of  repro- 
duction which  is  not  accurately  proportionate,  but  somewhat 
less  than  proportionate.  The  gain  in  the  one  direction  is  not 
wholly  cancelled  by  a  loss  in  the  other  direction,  but  only 
partially  cancelled :  leaving  a  margin  of  profit  to  the  species. 
Though  augmented  power  of  self-maintenance  habitually 
necessitates  diminished  power  of  race-propagation,  yet  the 
product  of  the  two  factors  is  greater  than  before;  so  that  the 
forces  preservative  of  race  become,  thereafter,  in  excess  of  the 
forces  destructive  of  race,  and  the  race  spreads.  We  shall 
soon  see  why  this  happens. 

Every  advance  in  evolution  implies  an  economy.  That  any 
increase  in  bulk,  or  structure,  or  activity,  may  become  estab- 
lished, the  life  of  the  organism  must  be  to  some  extent 
facilitated  by  the  change — the  cost  of  self-support  must  be, 
on  the  average,  reduced.  If  the  greater  complexity,  or  the 
larger  size,  or  the  more  agile  movement,  entails  on  the  in- 
dividual an  outlay  that  is  not  repaid  in  food  more-easily 
obtained,  or  danger  more-easily  escaped;  then  the  individual 
will  be  at  a  relative  disadvantage,  and  its  diminished  posterity 
will  disappear.  If  the  extra  outlay  is  but  just  made  good 
by  the  extra  advantage,  the  modified  individual  will  not  sur- 
vive longer,  or  leave  more  descendants,  than  the  unmodified 
individuals.  Consequently,  it  is  only  when  the  expense  of 
greater  individuation  is  out-balanced  by  a  subsequent  saving, 
that  it  can  tend  to  subserve  the  preservation  of  the  indi- 
vidual, and,  by  implication,  the  preservation  of  the  race. 
The  vital  capital  invested  in  the  alteration  must  bring  a 
more  than  equivalent  return.  A  few  instances 

will  show  that,  whether  the-  change  results  from  direct 
equilibration  or  from  indirect  equilibration,  this  must  happen. 
Suppose  a  creature  takes  to  performing  some  act  in  an  un- 
usual way — leaps  where  ordinarily  its  kindred  crawl,  eludes 
pursuit  by  diving  instead  of,  like  others  of  its  kind,  by  swim- 
ming along  the  surface,  escapes  by  doubling  instead  of  by 
speed.  Clearly,  perseverance  in  the  modified  habit  will,  other 


502  LAWS  OF  MULTIPLICATION. 

things  equal,  imply  that  it  takes  less  effort.  The  creature's 
sensations  will  ever  prompt  desistance  from  the  more  labori- 
ous course;  and  hence  a  congenital  habit  is  not  likely  to  be 
diverged  from  unless  an  economy  of  force  is  achieved  by  the 
divergence.  Assuming,  then,  that  the  new  method  has  no 
advantage  over  the  old  in  directly  diminishing  the  chances 
of  death,  the  establishment  of  it,  and  of  the  structural 
complications  involved,  nevertheless  implies  a  physiological 
gain.  Suppose,  again,  that  an  animal  takes  to  some 
abundant  food  previously  refused  by  its  kind.  It  is  likely  to 
persist  only  if  the  comparative  ease  in  obtaining  this  food, 
more  than  compensates  for  any  want  of  adaptation  to  its 
digestive  organs;  so  that  superposed  modifications  of  the 
digestive  organs  are  likely  to  arise  only  when  an  average 
economy  results.  What  now  must  be  the  influence 

on  the  creature's  system  as  a  whole  ?  Diminished  expenditure 
in  any  direction,  or  increased  nutrition  however  effected, 
will  leave  a  greater  surplus  of  materials.  The  animal  will  be 
physiological  richer.  Part  of  its  augmented  wealth  will  go 
towards  its  own  greater  individuation — its  size,  or  its 
strength,  or  both,  will  increase;  while  another  part  will  go 
towards  more  active  genesis.  Just  as  a  state  of  plethora 
directly  produced  enhances  fertility;  so  will  such  a  state 
indirectly  produced. 

In  another  way,  the  same  thing  must  result  from  those 
additions  to  bulk  or  complexity  or  activity  that  are  due  to 
survival  of  the  fittest.  Any  change  which  prolongs  individual 
life  will,  other  things  remaining  the  same,  further  the  pro- 
duction of  offspring.  Even  when  it  is  not,  like  the  foregoing, 
a  means  of  economizing  the  forces  of  the  individual,  still,  if  it 
increases  the  chances  of  escaping  destruction,  it  increases  the 
chances  of  leaving  posterity.  Any  further  degree  of  evolu- 
tion, therefore,  will  be  established  only  where  the  cost  of 
it  is  more  than  repaid:  part  of  the  gain  being  shown  in  the 
lengthened  life  of  the  individual,  and  part  in  the  greater 
production  of  other  individuals. 


INTERPRETATION  AND  QUALIFICATION.  503 

We  have  here  the  solution  of  various  minor  anomalies  by 
which  the  inverse  variation  of  Individuation  and  Genesis  is 
obscured.  Take  as  an  instance  the  fertility  of  the  Blackbird 
as  compared  with  that  of  the  Linnet.  Both  birds  lay  five  eggs, 
and  both  usually  have  two  broods.  Yet  the  Blackbird  is  far 
the  larger  of  the  two,  and  ought,  according  to  the  general 
law,  to  be  much  less  prolific.  What  causes  this  noncon- 
formity? We  shall  find  an  answer  in  their  respective  foods 
and  habits.  Except  during  the  time  that  it  is  rearing  its 
young,  the  Linnet  collects  only  vegetal  food — lives  during 
the  winter  on  the  seeds  it  finds  in  the  fields,  or,  when  hard 
pressed,  picks  up  around  farms;  and  to  obtain  this  spare 
diet  is  continually  flying  about.  The  result,  if  it  survives  the 
frost  and  snow,  is  a  considerable  depletion;  and  it  recovers 
its  condition  only  after  some  length  of  spring  weather.  The 
Blackbird,  on  the  other  hand,  is  omnivorous.  While  it  eats 
grain  and  fruit  when  they  come  in  its  way,  it  depends  largely 
on  animal  food.  It  cuts  to  pieces  and  devours  the  dew-worms 
which,  morning  and  evening,  it  finds  on  the  surface  of  a  lawn, 
and,  even  discovering  where  they  are,  unearths  them;  it 
swallows  slugs,  and  breaking  snail-shells,  either  with  its  beak 
or  by  hammering  them  against  stones,  tears  out  their  tenants ; 
and  it  eats  beetles  and  larvaB.  Thus  the  strength  of  the 
Blackbird  opens  to  it  a  store  of  good  food,  much  of  which  is 
inaccessible  to  so  small  and  weak  a  bird  as  a  Linnet — a  store 
especially  helpful  to  it  during  the  cold  months,  when  the 
hybernating  snails  in  hedge-bottoms  yield  it  abundant  pro- 
vision. The  result  is  that  the  Blackbird  is  ready  to  breed 
very  early  in  spring,  and  is  able  during  the  summer  to  rear 
a  second,  and  sometimes  even  a  third,  brood.  Here,  then,  a 
higher  degree  of  Tndividuation  secures  advantages  so  great, 
as  to  much  more  than  compensate  its  cost.  It  is  not  that  the 
decline  of  Genesis  is  less  than  proportionate  to  the  increase  of 
Individuation,  but  there  is  no  decline  at  all.  Com- 

parison of  the  Eat  with  the  Mouse  yields  a  parallel  result. 
Though  they  differ  greatly  in  size,  yet  the  one  is  as  prolific 


504  LAWS  OF  MULTIPLICATION. 

as  the  other.  This  absence  of  difference  cannot  be  ascribed 
to  their  unlike  degrees  of  activity.  We  must  seek  its  cause 
in  some  facility  of  living  secured  to  the  Eat  by  its  greater 
intelligence,  greater  power  and  courage,  greater  ability  to 
utilize  what  it  finds.  The  Eat  is  notoriously  cunning ;  and  its 
cunning  gives  success  to  its  foraging  expeditions.  It  is  not, 
like  the  Mouse,  limited  mainly  to  vegetal  food;  but  while  it 
eats  grain  and  beans  like  the  Mouse,  it  also  eats  flesh  and 
carrion,  devours  young  poultry  and  eggs.  The  result  is  that, 
without  a  proportionate  increase  of  expenditure,  it  gets  a  far 
larger  supply  of  nourishment  than  the  Mouse;  and  relative 
excess  of  nourishment  makes  possible  a  larger  size  without 
a  smaller  rate  of  multiplication.  How  clearly  this  is  the 
cause,  we  see  in  the  contrast  between  the  common  Eat  and 
the  Water-Eat.  While  the  common  Eat  has  ordinarily 
several  broods  a-year  of  from  10  to  12  each,  the  Water-Eat, 
though  somewhat  smaller,  has  but  5  or  6  in  a  brood,  and  but 
one  brood,  or  sometimes  two  broods,  a-year.  But  the  Water- 
Eat  lives  on  vegetal  food,  and  it  lacks  all  that  its  bold,  saga- 
cious, omnivorous  congener  gains  from  the  warmth  as  well  as 
the  abundance  which  men's  habitations  yield. 

The  inverse  variation  of  Individuation  and  Genesis  is, 
therefore,  but  approximate.  Eecognizing  the  truth  that 
every  increment  of  evolution  which  is  appropriate  to  the 
circumstances  of  an  organism,  brings  an  advantage  somewhat 
in  excess  of  its  cost ;  we  see  the  general  law,  as  more  strictly 
stated,  to  be  that  Genesis  decreases  not  quite  so  fast  as 
Individuation  increases.  Whether  the  greater  Individuation 
takes  the  form  of  a  larger  bulk  and  accompanying  access  of 
strength;  whether  it  be  shown  in  higher  speed  or  agility; 
whether  it  consists  in  a  modification  of  structure  which  facili- 
tates some  habitual  movement,  or  in  a  visceral  change  that 
helps  to  utilize  better  the  absorbed  aliment;  the  ultimate 
effect  is  identical.  There  is  either  a  more  economical  per- 
formance of  the  same  actions,  internal  or  external,  or  there 
is  a  securing  of  greater  advantages  by  modified  actions,  which 


INTERPRETATION  AND  QUALIFICATION.  505 

cost  no  more,  or  have  an  increased  cost  less  than  the  in- 
creased gain.  In  any  case  the  result  is  a  greater  surplus  of 
vital  capital,  part  of  which  goes  to  the  aggrandizement  of 
the  individual,  and  part  to  the  formation  of  new  individuals. 
While  the  higher  tide  of  nutritive  matters,  everywhere  filling 
the  parent-organism,  adds  to  its  power  of  self-maintenance,  it 
also  causes  a  reproductive  overflow  larger  than  before. 

Hence  every  type  which  is  best  adapted  to  its  conditions, 
(and  this  on  the  average  means  every  higher  type),  has  a  rate 
of  multiplication  that  insures  a  tendency  to  predominate. 
Survival  of  the  fittest,  acting  alone,  is  ever  replacing  in- 
ferior species  by  superior  species.  But  beyond  the  longer 
survival,  and  therefore  greater  chance  of  leaving  offspring, 
which  superiority  gives,  we  see  here  another  way  in  which 
the  spread  of  the  superior  is  insured.  Though  the  more- 
evolved  organism  is  the  less  fertile  absolutely,  it  is  the  more 
fertile  relatively. 


CHAPTER  XII. 

MULTIPLICATION   OF   THE   HUMAN   RACE. 

§  365.  THE  relative  fertility  of  Man  considered  as  a 
species,  and  those  changes  in  Man's  fertility  which  occur  under 
changed  conditions,  must  conform  to  the  laws  which  we  have 
traced  thus  far.  As  a  matter  of  course,  the  inverse  variation 
between  Individuation  and  Genesis  holds  of  him  as  of  all 
other  organized  beings.  His  extremely  low  rate  of  multipli- 
cation— far  below  that  of  all  terrestrial  Mammals  except  the 
Elephant,  (which  though  otherwise  less  evolved  is,  in  extent 
of  integration,  more  evolved) — we  shall  recognize  as  the 
necessary  concomitant  of  his  much  higher  evolution.  And 
the  causes  of  increase  or  decrease  in  his  fertility,  special  or 
general,  temporary  or  permanent,  we  shall  expect  to  find  in 
those  changes  of  bulk,  of  structure,  or  of  expenditure,  which 
we  have  in  all  other  cases  seen  associated  with  such  effects. 

In  the  absence  of  detailed  proof  that  these  parallelisms 
exist,  it  might  suffice  to  contemplate  the  several  communities 
between  the  reproductive  function  in  human  beings  and  other 
beings.  I  do  not  refer  simply  to  the  fact  that  genesis  pro- 
ceeds in  a  similar  manner;  but  I  refer  to  the  similarity  of 
the  relation  between  the  generative  function  and  the  func- 
tions which  have  for  their  joint  end  the  preservation  of  the 
individual.  In  Man,  as  in  other  creatures  that  expend  much, 
genesis  commences  only  when  growth  and  development  are 
declining  in  rapidity  and  approaching  their  termination. 
Among  the  higher  organisms  in  general,  the  reproductive 
506 


MULTIPLICATION  OF  THE   HUMAN  RACE.          507 

activity,  continuing  during  the  prime  of  life,  ceases  when  the 
vigour  declines,  leaving  a  closing  period  of  infertility ;  and  in 
like  manner  among  ourselves,  barrenness  supervenes  when 
middle  age  brings  the  surplus  vitality  to  an  end.  So,  too, 
it  is  found  that  in  Man,  as  in  beings  of  lower  orders,  there  is 
a  period  at  which  fecundity  culminates.  In  §  341,  facts  were 
cited  showing  that  at  the  commencement  of  the  reproductive 
period,  animals  bear  fewer  offspring  than  afterwards;  and 
that  towards  the  close  of  the  reproductive  period,  there  is  a 
decrease  in  the  number  produced.  In  like  manner  it  is  shown 
by  the  tables  of  Dr.  Duncan's  recent  work,  that  the  fecundity 
of  women  increases  up  to  the  age  of  about  25  years,  and 
continuing  high  with  but  slight  diminution  till  after  30, 
then  gradually  wanes.  It  is  the  same  with  the  sizes  and 
weights  of  offspring.  Infants  born  of  women  from  25  to  29 
years  of  age,  are  both  longer  and  heavier  than  infants  born 
of  younger  or  older  women ;  and  this  difference  has  the  same 
implication  as  the  greater  total  weight  of  the  offspring  pro- 
duced at  a  birth,  during  the  most  fecund  age  of  a  pluriparous 
animal.  Once  more,  there  is  the  fact  that  a  too-early  bearing 
of  young  produces  on  a  woman  the  same  injurious  effects  as 
on  an  inferior  creature — an  arrest  of  growth  and  an  enfeeble- 
ment  of  constitution. 

Considering  these  general  and  special  parallelisms,  we 
might  safely  infer  that  variations  of  human  fertility  conform 
to  the  same  laws  as  do  variations  of  fertility  in  general. 
But  it  is  not  needful  to  content  ourselves  with  an  implication. 
Evidence  is  assignable  that  what  causes  increase  or  decrease 
of  genesis  in  other  creatures,  causes  increase  or  decrease  of 
genesis  in  Man.  It  is  true  that,  even  more  than  hitherto,  our 
reasonings  are  beset  by  difficulties.  So  numerous  are  the 
inequalities  in  the  conditions,  that  but  few  unobjectionable 
comparisons  can  be  made.  The  human  races  differ  consider- 
ably in  their  sizes,  and  notably  in  their  degrees  of  cerebral 
development.  The  countries  they  inhabit  entail  on  them 
widely  different  consumptions  of  matter  for  maintenance  of 


508  LAWS  OP  MULTIPLICATION. 

temperature.  Both  in  their  qualities  and  quantities  the 
foods  they  live  on  are  unlike;  and  the  supply  is  here  regular 
and  there  very  irregular.  Their  expenditures  in  bodily  action 
are  extremely  unequal;  and  even  still  more  unequal  are 
their  expenditures  in  mental  action.  Hence  the  factors, 
varying  so  much  in  their  amounts  and  combinations,  can 
scarcely  ever  have  their  respective  effects  identified.  Never- 
theless there  are  a  few  comparisons  the  results  of  which  may 
withstand  criticism. 

§  366.  The  increase  of  fertility  caused  by  a  nutrition  that 
is  greatly  in  excess  of  the  expenditure,  is  to  be  detected  by 
contrasting  populations  of  the  same  race,  or  allied  races, 
one  of  which  obtains  good  and  abundant  sustenance  much 
more  easily  than  the  other.  Three  cases  may  here  be  set 
down. 

The  traveller  Barrow,  describing  the  Cape-Boers,  says: — 
"  Unwilling  to  work  and  unable  to  think,"  ..."  indulging 
to  excess  in  the  gratification  of  every  sensual  appetite,  the 
African  peasant  grows  to  an  unwieldy  size ; "  and  respecting 
the  other  sex,  he  adds — "  the  women  of  the  African  peasantry 
lead  a  life  of  the  most  listless  inactivity."  Then,  after  illus- 
trating these  statements,  he  goes  on  to  note  "  the  prolific 
tendency  of  all  the  African  peasantry.  Six  or  seven  children 
in  a  family  are  considered  as  very  few;  from  a  dozen  to 
twenty  are  not  uncommon."  The  native  races  of 

this  region  yield  evidence  to  the  same  effect.  Speaking  of 
the  cruelly-used  Hottentots  (he  is  writing  a  century  ago), 
who,  while  they  are  poor  and  ill-fed,  have  to  do  all  the  work 
for  the  idle  Boers,  Barrow  says  that  they  "  seldom  have  more 
than  two  or  three  children;  and  many  of  the  women  are 
barren."  This  unusual  infertility  stands  in  remarkable  con- 
trast with  the  unusual  fertility  of  the  Kaffirs,  of  whom  he 
afterwards  gives  an  account.  Eich  in  cattle,  leading  easy 
lives,  and  living  almost  exclusively  on  animal  food  (chiefly 
milk  with  occasional  flesh),  these  people  were  then  reputed 


MULTIPLICATION  OF  THE  HUMAN  RACE.          509 

to  have  a  very  high  rate  of  multiplication.  Barrow  writes : — 
"They  are  said  to  be  exceedingly  prolific;  that  twins  are 
almost  as  frequent  as  single  births,  and  that  it  is  no  un- 
common thing  for  a  woman  to  have  three  at  a  time."  Pro- 
bably both  these  statements  are  in  excess  of  the  truth;  but 
there  is  room  for  large  discounts  without  destroying  the 
extreme  difference.  A  third  instance  is  that  of  the 

French  Canadians.  "Nous  sommes  terribles  pour  les  en- 
fants!  "  observed  one  of  them  to  Prof.  Johnston,  who  tells  us 
that  the  man  who  said  this  "  was  one  of  fourteen  children — 
was  himself  the  father  of  fourteen,  and  assured  me  that  from 
eight  to  sixteen  was  the  usual  number  of  the  farmers' 
families.  He  even  named  one  or  two  women  who  had 
brought  their  husbands  five-and-twenty,  and  threatened  '  le 
vingt-sixieme  pour  le  pretre.' "  From  these  large  families, 
joined  with  the  early  marriages  and  low  rate  of  mortality,  it 
results  that,  by  natural  increase,  "  there  are  added  to  the 
French-Canadian  population  of  Lower  Canada  four  persons 
for  every  one  that  is  added  to  the  population  of  England." 
Now  these  French-Canadians  are  described  by  Prof.  Johnston 
as  home-loving,  contented,  unenterprising;  and  as  living  in 
a  region  where  "land  and  subsistence  are  easily  obtained." 
Very  moderate  industry  brings  to  them  liberal  supplies  of 
necessaries ;  and  they  pass  a  considerable  portion  of  the  year 
in  idleness.  Hence  the  cost  of  Individuation  being  much 
reduced,  the  rate  of  Genesis  is  much  increased.  That  this 
uncommon  fertility  is  not  due  to  any  direct  influence  of  the 
locality,  is  implied  by  the  fact  that  along  with  the  "  restless, 
discontented,  striving,  burning  energy  of  their  Saxon  neigh- 
bours," no  such  rate  of  multiplication  is  observed;  while 
further  south,  where  the  physical  circumstances  are  more 
favourable  if  anything,  the  Anglo-Saxons,  leading  lives  of 
excessive  activity,  have  a  fertility  below  the  average.  And 
that  the  peculiarity  is  not  a  direct  effect  of  race,  is  proved  by 
the  fact  that  in  Europe,  the  rural  French  are  certainly  not 
more  prolific  than  the  rural  English. 


510  LAWS  OF  MULTIPLICATION. 

To  every  reader  there  will  probably  occur  the  seemingly- 
adverse  evidence  furnished  by  the  Irish;  who,  though  not 
well  fed,  multiply  fast.  Part  of  this  more  rapid  increase  is 
due  to  the  earlier  marriages  common  among  them,  and  con- 
sequent quicker  succession  of  generations — a  factor  which, 
as  we  have  seen,  has  a  larger  effect  than  any  other  on  the 
rate  of  multiplication.  Part  of  it  is  due  to  the  greater 
generality  of  marriage — to  the  comparative  smallness  of  the 
number  who  die  without  having  had  the  opportunity  of  pro- 
ducing offspring.  The  effects  of  these  causes  having  been 
deducted,  we  may  doubt  whether  the  Irish,  individually  con- 
sidered, would  be  found  more  prolific  than  the  English. 
Perhaps,  however,  it  will  be  said  that,  considering  their  diet, 
they  ought  to  be  less  prolific.  This  is  by  no  means  obvious. 
It  is  not  simply  a  question  of  nutriment  absorbed.  It  is  a 
question  of  how  much  remains  after  the  expenditure  in  self- 
maintenance.  Now  a  notorious  peculiarity  in  the  life  of  the 
Irish  peasant  is,  that  he  obtains  a  return  of  food  which  is 
large  in  proportion  to  his  outlay  in  labour.  The  cultivation 
of  his  potatoe-ground  occupies  each  cottager  but  a  small  part 
of  the  year ;  and  the  domestic  economy  of  his  wife  is  not  of  a 
kind  to  entail  on  her  much  daily  exertion.  Consequently  the 
crop,  tolerably  abundant  in  quantity  though  innutritive  in 
quality,  possibly  suffices  to  meet  the  comparatively-low  ex- 
penditure, and  to  leave  a  good  surplus  for  genesis — perhaps 
a  greater  surplus  than  remains  to  the  males  and  females  of 
the  English  peasantry,  who,  though  fed  on  better  food,  are 
harder  worked. 

We  conclude,  then,  that  in  the  human  race,  as  in  all  other 
races,  such  absolute  or  relative  abundance  of  nutriment  as 
leaves  a  large  excess  after  defraying  the  cost  of  carrying  on 
parental  life,  is  accompanied  by  a  high  rate  of  genesis.* 

§  367.  Evidence  of  the  converse  truth,  that  relative  in- 

*  This  is  exactly  the  reverse  of  Mr.  Doubleday's  doctrine ;  which  is  that 
throughout  both  the  animal  and  vegetable  kingdoms,  "over-feeding  checks 
increase ;  whilst,  on  the  other  hand,  a  limited  or  deficient  nutriment  stimu- 


MULTIPLICATION   OF  THE  HUMAN  EACE.          5H 

crease  of  expenditure,  leaving  a  diminished  surplus,  reduces 
the  degree  of  fertility,  is  not  wanting.  Some  of  it  has  been 
set  down  for  the  sake  of  antithesis  in  the  foregoing  section. 
Here  may  be  grouped  a  few  facts  of  a  more  special  kind 
having  the  same  implication. 

To  prove  that  much  bodily  labour  renders  women  less  pro- 
lific, requires  more  evidence  than  has  at  present  been  collected. 
Nevertheless  it  may  be  noted  that  De  Boismont  in  France  and 
Dr.  Szukits  in  Austria,  have  shown  by  extensive  statistical 
comparisons,  that  the  reproductive  age  is  reached  a  year 
later  by  women  of  the  labouring  class  than  by  middle-class 
women;  and  while  ascribing  this  delay  in  part  to  inferior 

latca  and  adds  to  it."  Or,  as  he  elsewhere  says — "  Be  the  range  of  the 
natural  power  to  increase  in  any  species  what  it  may,  the  plethoric  state  inva- 
riably checks  it,  and  the  deplethoric  state  invariably  develops  it ;  and  this  hap- 
pens in  the  exact  ratio  of  the  intensity  and  completeness  of  each  state,  until 
each  state  be  carried  so  far  as  to  bring  about  the  actual  death  of  the  animal 
or  plant  itself." 

I  have  space  here  only  to  indicate  the  misinterpretations  on  which  Mr. 
Doubleday  has  based  his  argument. 

In  the  first  place,  he  has  confounded  normal  plethora  with  what  I  have,  in 
§  355,  distinguished  as  abnormal  plethora.  The  cases  of  infertility  accom- 
panying fatness,  which  he  cites  in  proof  that  over-feeding  checks  increase,  are 
not  cases  of  high  nutrition  properly  so  called  ;  but  cases  of  such  defective  ab- 
sorption or  assimilation  as  constitutes  low  nutrition.  In  Chap.  IX,  abundant 
proof  was  given  that  a  truly  plethoric  state  is  an  unusually  fertile  state.  It 
may  be  added  that  much  of  the  evidence  by  which  Mr.  Doubleday  seeks  to 
show  that  among  men,  highly-fed  classes  are  infertile  classes,  may  be  out- 
balanced by  counter-evidence.  Many  years  ago  Mr.  G.  H.  Lewes  pointed  this 
out :  extracting  from  a  book  on  the  peerage,  the  names  of  16  peers  who  had, 
at  that  time,  186  children  ;  giving  an  average  of  1T6  in  a  family. 

Mr.  Doubleday  insists  much  on  the  support  given  to  his  theory  by  the 
barrenness  of  very  luxuriant  plant",  and  the  fruitfulness  produced  in  plants 
by  depletion.  Had  he  been  aware  that  the  change  from  barrenness  to  fruit- 
fulness  in  plants,  is  a  change  from  agamogenesis  to  gamogenesis — had  it  been 
as  well  known  at  the  time  when  he  wrote  as  it  is  now,  that  a  tree  which  goes 
on  putting  out  sexless  shoots,  is  thus  producing  new  individuals ;  and  that 
when  it  begins  to  bear  fruit,  it  simply  begins  to  produce  new  individuals  after 
another  manner— he  would  have  perceived  that  facts  of  this  class  do  not  tell 
in  his  favour. 

In  the  law  which  Mr.  Doubleday  alleges,  he  sees  a  guarantee  for  the 
maintenance  of  species.  He  argues  that  the  plethoric  state  of  the  indivi- 


512  LAWS  OF  MULTIPLICATION. 

nutrition,  we  may  suspect  that  it  is  in  part  due  to  greater 
muscular  expenditure.  A  kindred  fact,  admitting  of  a 
kindred  interpretation,  may  be  added.  Though  the  com- 
paratively-low rate  of  increase  in  France  is  attributed  to 
other  causes,  yet,  very  possibly,  one  of  its  causes  is  the 
greater  proportion  of  hard  work  entailed  on  French  women, 
by  the  excessive  abstraction  of  men  for  non-productive 
occupations,  military  and  civil.  The  higher  rate  of  multipli- 
cation in  England  than  in  continental  countries  generally,  is 
not  improbably  furthered  by  the  easier  lives  which  English 
women  lead. 

That  absolute  or  relative  infertility  is  commonly  produced 
in  women  by  mental  labour  carried  to  excess,  is  more  clearly 
shown.  Though  the  regimen  of  upper-class  girls  is  not  what 
it  should  be,  yet,  considering  that  their  feeding  is  better  than 
that  of  girls  belonging  to  the  poorer  classes,  while,  in  most 
other  respects,  their  physical  treatment  is  not  worse,  the 

duals  constituting  any  race  of  organisms,  presupposes  conditions  so  favour, 
able  to  life  that  the  race  can  be  in  no  danger ;  and  that  rapidity  of  multi- 
plication becomes  needless.  Conversely,  he  argues  that  a  deplethoric  state 
implies  unfavourable  conditions — implies,  consequently,  unusual  mortality; 
that  is — implies  a  necessity  for  increased  fertility  to  prevent  the  race  from 
dying  out.  It  may  be  readily  shown,  however,  that  such  an  arrangement 
would  be  the  reverse  of  self-adjusting.  Suppose  a  species,  too  numerous 
for  its  food,  to  be  in  the  resulting  deplethoric  state.  It  will,  according  to 
Mr.  Doubleday,  become  unusually  fertile ;  and  the  next  generation  will  be 
more  numerous  rather  than  less  numerous.  For,  by  the  hypothesis,  the  un- 
usual fertility  due  to  the  deplethoric  state,  is  the  cause  of  undue  increase  of 
population.  But  if  the  next  generation  is  more  numerous  while  the  supply 
of  food  has  not  increased  in  proportion,  then  this  next  generation  will  be  in 
a  still  more  deplethoric  state,  and  will  be  still  more  fertile.  Thus  there  will 
go  on  an  ever-increasing  rate  of  multiplication,  and  an  ever-decreasing  share 
of  food,  for  each  person,  until  the  species  disappears.  Suppose,  on  the 
other  hand,  the  members  of  a  species  to  be  in  an  unusually  plethoric  state. 
Their  rate  of  multiplication,  ordinarily  sufficient  to  maintain  their  numbers, 
will  become  insufficient  to  maintain  their  numbers.  In  the  next  generation, 
therefore,  there  will  be  fewer  to  eat  the  already  abundant  food,  which  be- 
coming relatively  still  more  abundant,  will  render  the  fewer  members  of  the 
species  still  mor»  plethoric,  and  still  less  fertile,  than  their  parents.  And  the 
actions  and  reactions  continuing,  the  species  will  presently  die  out  from  abso- 
lute barrenness. 


MULTIPLICATION  OF  THE  HUMAN  RACE.  513 

deficiency  of  reproductive  power  among  them  may  be  reason- 
ably attributed  to  the  overtaxing  of  their  brains — an  over- 
taxing which  produces  a  serious  reaction  on  the  physique. 
This  diminution  of  reproductive  power  is  not  shown  only  by 
the  greater  frequency  of  absolute  sterility;  nor  is  it  shown 
only  in  the  earlier  cessation  of  child-bearing;  but  it  is  also 
shown  in  the  very  frequent  inability  of  such  women  to  suckle 
their  infants.  In  its  full  sense,  the  reproductive  power  means 
the  power  to  bear  a  well-developed  infant  and  to  supply  that 
infant  with  the  natural  food  for  the  natural  period.  Most  of 
the  flat-chested  girls  who  survive  their  high-pressure  educa- 
tion, are  incompetent  to  do  this.  Were  their  fertility  mea- 
sured by  the  number  of  children  they  could  rear  without 
artificial  aid,  they  would  prove  relatively  very  infertile. 

The  cost  of  reproduction  to  males  being  so  much  less 
than  it  is  to  females,  the  antagonism  between  Genesis  and 
Individuation  is  not  often  shown  in  men  by  suppression  of 
generative  power  consequent  on  unusual  expenditure  in 
bodily  action.  Nevertheless,  there  are  indications  that  this 
results  in  extreme  cases.  We  read  that  the  ancient  athletes 
rarely  had  children;  and  among  such  of  their  modern  repre- 
sentatives as  acrobats,  an  allied  relation  of  cause  and  effefct 
is  alleged.  Indirectly  this  truth,  or  rather  its  converse, 
appears  to  have  been  ascertained  by  those  who  train  men 
for  feats  of  strength — they  find  it  needful  to  insist  on  con- 
tinence. 

Special  proofs  that  in  men  great  cerebral  expenditure 
diminishes  or  destroys  generative  power,  are  difficult  to 
obtain.  It  is,  indeed,  asserted  that  intense  application  to 
mathematics,  requiring  as  it  does  extreme  concentration  of 
thought,  is  apt  to  have  this  result;  and  it  is  asserted,  too, 
that  this  result  is  produced  by  the  excessive  emotional  ex- 
citement of  gambling.  Then,  again,  it  is  a  matter  of  common 
remark  how  frequently  men  of  unusual  mental  activity  leave 
no  offspring.  But  facts  of  this  kind  admit  of  another  inter- 
pretation. The  reaction  of  the  brain  on  the  body  is  so  violent 
79 


514  LAWS  OP  MULTIPLICATION. 

— the  overtaxing  of  the  nervous  system  is  so  apt  to  prostrate 
the  heart  and  derange  the  digestion;  that  the  incapacities 
caused  in  these  cases,  are  probably  often  due  more  to  con- 
stitutional disturbance  than  to  the  direct  deduction  which 
excessive  action  entails.  Such  instances  harmonize  with  the 
hypothesis;  but  how  far  they  yield  it  positive  support  we 
cannot  say. 

§  368.  An  objection  must  here  be  guarded  against.  It  is 
likely  to  be  urged  that  since  the  civilized  races  are,  on  the 
average,  larger  than  many  of  the  uncivilized  races ;  and  since 
they  are  also  somewhat  more  complex  as  well  as  more  active ; 
they  ought,  in  conformity  with  the  alleged  general  law,  to 
be  less  prolific.  There  is,  however,  no  evidence  to  prove  that 
they  are  so :  on  the  whole,  they  seem  rather  the  reverse. 

The  reply  is  that  were  all  other  things  equal,  these 
superior  varieties  of  men  should  have  inferior  rates  of  in- 
crease. But  other  things  are  not  equal;  and  it  is  to  the 
inequality  of  other  things  that  this  apparent  anomaly  is 
attributable.  Already  we  have  seen  how  much  more  fertile 
domesticated  animals  are  than  their  wild  kindred;  and  the 
causes  of  this  greater  fertility  are  also  the  causes  of  the 
greater  fertility,  relative  or  absolute,  which  civilized  men 
exhibit  when  compared  with  savages. 

There  is  the  difference  in  amount  of  food.  Australians, 
Fuegians,  and  sundry  races  that  might  be  named  as  having 
low  rates  of  multiplication,  are  obviously  underfed.  The 
sketches  of  natives  contained  in  the  volumes  of  Livingstone, 
Baker,  and  others,  yield  clear  proofs  of  the  extreme  depletion 
common  among  the  uncivilized.  In  quality  as  well 

as  in  quantity,  their  feeding  is  bad.  Wild  fruits,  insects, 
larvae,  vermin,  &c.,  which  we  refuse  with  disgust,  often  enter 
largely  into  their  dietary.  Much  of  this  inferior  food  they 
eat  uncooked;  and  they  have  not  our  elaborate  appliances 
for  mechanically-preparing  it,  and  rejecting  its  useless  parts. 
So  that  they  live  on  matters  of  less  nutritive  value,  which 


MULTIPLICATION  OF  THE  HUMAN  RACE.          515 

cost  more  both  to  masticate  and  to  digest.  Further, 

to  uncivilized  men  supplies  of  food  come  very  irregularly. 
Long  periods  of  scarcity  are  divided  by  short  periods  of 
abundance.  And  though  by  gorging  when  opportunity 
occurs,  something  is  done  towards  compensating  for  previous 
fasting,  yet  the  effects  of  prolonged  starvation  cannot  be 
neutralized  by  occasional  enormous  meals.  Bearing  in  mind, 
too,  that  improvident  as  they  are,  savages  often  bestir  them- 
selves only  under  pressure  of  hunger,  we  may  fairly  consider 
them  as  habitually  ill-nourished — may  see  that  even  the 
poorer  classes  of  civilized  men,  making  regular  meals  on  food 
separated  from  innutritive  matters,  easy  to  masticate  and 
digest,  tolerably  good  in  quality  and  adequate  if  not  abundant 
in  quantity,  are  much  better  nourished. 

Then,  again,  though  a  greater  consumption  in  muscular 
action  appears  to  be  undergone  by  civilized  men  than 
by  savages;  and  though  it  is  probably  true  that  among  our 
labouring  people  the  daily  repairs  cost  more;  yet  in  many 
cases  there  does  not  exist  so  much  difference  as  we  are  apt 
to  suppose.  The  chase  is  very  laborious ;  and  great  amounts 
of  exertion  are  gone  through  by  the  lowest  races  in  seeking 
and  securing  the  odds  and  ends  of  wild  food  on  which  they 
largely  depend.  We  naturally  assume  that  because  bar- 
barians are  averse  to  regular  labour,  their- muscular  action 
is  less  than  our  own.  But  this  is  not  necessarily  true.  The 
monotonous  toil  is  what  they  cannot  tolerate;  and  they  may 
be  ready  to  go  through  as  much  or  more  exertion  when 
it  is  joined  with  excitement.  If  we  remember  that  the 
sportsman  who  gladly  scrambles  up  and  down  rough  hill- 
sides all  day  after  grouse  or  deer,  would  think  himself  hardly 
used  had  he  to  spend  as  much  effort  and  time  in  digging;  we 
shall  see  that  a  savage  who  is  the  reverse  of  industrious, 
may  nevertheless  be  subject  to  a  muscular  waste  not  very 
different  in  amount  from  that  undergone  by  the  indus- 
trious. When  it  is  added  that  a  larger  physiolo- 
gical expenditure  is  entailed  on  the  uncivilized  than  on  the 


516  LAWS  OP  MULTIPLICATION. 

civilized  by  the  absence  of  good  appliances  for  shelter  and 
protection — that  in  some  cases  they  have  to  make  good  a 
greater  loss  of  heat,  and  in  other  cases  suffer  much  wear  from 
irritating  swarms  of  insects;  we  shall  see  that  the  total  cost 
of  self-maintenance  among  them  is  probably  in  many  cases 
little  less,  and  in  some  cases  more,  than  it  is  among  ourselves. 
So  that  though,  on  the  average,  the  civilized  are  probably 
larger  than  the  savage;  and  though  they  are,  in  their 
nervous  systems  at  least,  somewhat  more  complex;  and 
though,  other  things  equal,  they  ought  to  be  the  less 
prolific;  yet  other  things  are  so  unequal  as  to  make  it 
quite  conformable  to  the  general  law  that  they  should  be 
more  prolific.  In  §  365  we  observed  how,  among  inferior 
animals,  higher  evolution  sometimes  makes  self-preservation 
far  easier,  by  opening  the  way  to  resources  previously  un- 
available: so  involving  an  undiminished,  or  even  an  in- 
creased, rate  of  genesis.  And  similarly  we  may  expect  that 
among  races  of  men,  those  whose  slight  further  develop- 
ments have  been  followed  by  habits  and  arts  which  immensely 
facilitate  life,  will  not  exhibit  a  lower  degree  of  fertility,  and 
may  even  exhibit  a  higher. 

§  369.  One  more  objection  has  to  be  met — a  kindred  ob- 
jection to  which  there  is  a  kindred  reply.  Cases  may  be 
named  of  men  conspicuoiis  for  activity,  bodily  and  mental, 
who  were  also  noted,  not  for  less  generative  power  than  usual, 
but  for  more.  As  their  superiorities  indicate  higher  degrees 
of  evolution,  it  may  be  urged  that  such  men  should,  accord- 
ing to  the  theory,  have  lower  degrees  of  reproductive  activity. 
The  fact  that  here,  along  with  increased  powers  of  self-pre- 
servation, there  go  increased  powers  of  race-propagation, 
seems  irreconcilable  with  the  general  doctrine.  Reconcilia- 
tion is  not  difficult  however. 

The  cases  are  analogous  to  some  before  named,  in  which 
more  abundant  food  simultaneously  aggrandizes  the  indi- 


MULTIPLICATION  OF  THE  HUMAN  RACE.          517 

vidual  and  adds  to  the  production  of  new  individuals:  the 
difference  between  the  cases  being,  that  instead  of  a  better 
external  supply  of  materials  there  is  a  better  internal 
utilization  of  materials.  Creatures  of  the  same  species  noto- 
riously differ  in  goodness  of  constitution.  Here  there  is  some 
visceral  defect,  showing  itself  in  feebleness  of  all  the  func- 
tions; while  here  some  peculiarity  of  organic  balance,  some 
high  quality  of  tissue,  some  abundance  or  potency  of  the 
digestive  juices,  gives  to  the  system  a  perpetual  high  tide  of 
rich  blood,  which  serves  at  once  to  enhance  the  vital  activities 
and  to  raise  the  power  of  propagation.  Such  variations, 
however,  are  independent  of  changes  in  the  proportion  be- 
tween Individuation  and  Genesis.  This  remains  the  same, 
while  both  are  increased  or  decreased  by  the  increase  or 
decrease  of  the  common  stock  of  materials. 

An  illustration  will  best  clear  up  any  perplexity.  Let  us 
say  that  the  fuel  burnt  in  the  furnace  of  a  locomotive  steam- 
engine,  answers  to  the  food  which  a  man  consumes.  Let  us 
say  that  the  produced  steam  expended  in  working  the  engine, 
corresponds  to  that  portion  of  absorbed  nutriment  which 
carries  on  the  man's  functions  and  activities.  And  let  us 
say  that  the  steam  blowing  off  at  the  safety-valve, 
answers  to  that  portion  of  the  absorbed  nutriment  which 
goes  to  the  propagation  of  the  race.  Such  being  the  condi- 
tions of  the  case,  several  kinds  of  variations  are  possible. 
All  other  circumstances  remaining  the  same,  there  may  be 
changes  of  proportion  between  the  steam  used  for  working 
the  engine  and  the  steam  that  escapes  by  the  safety-valve. 
There  may  be  a  structural  or  organic  change  of  proportion. 
By  enlarging  the  safety-valve  or  weakening  its  spring,  while 
the  cylinders  are  reduced  in  size,  there  may  be  established  a 
constitutionally-small  power  of  locomotion  and  a  constitu- 
tionally-large amount  of  escape-steam ;  and  inverse  variations 
so  produced,  will  answer  to  the  inverse  variations  between 
Individuation  and  Genesis  which  different  types  of  organisms 


518  LAWS  OF  MULTIPLICATION. 

show  us.  Again,  there  may  be  a  functional  change  of  pro- 
portion. If  the  engine  has  to  draw  a  considerable  load,  the 
abstraction  of  steam  by  the  cylinders  greatly  reduces  the 
discharge  by  the  safety-valve;  and  if  a  high  velocity  is  kept 
up,  the  discharge  from  the  safety-valve  entirely  ceases.  Con- 
versely, if  the  velocity  is  low,  the  escape-steam  bears  a  large 
ratio  to  the  steam  consumed  by  the  motor  apparatus ;  and  if 
the  engine  becomes  stationary  the  whole  of  the  steam  escapes 
by  the  safety-valve.  This  inverse  variation  answers  to  that 
which  we  have  traced  between  Expenditure  and  Genesis,  as 
displayed  in  the  contrasts  between  species  of  the  same  type 
but  unlike  activities,  and  in  the  contrasts  between  active  and 
inactive  individuals  of  the  same  species.  But  now  beyond 
these  inverse  variations  between  the  quantities  of  consumed 
steam  and  escape-steam,  which  are  structurally  and  function- 
ally caused,  there  are  coincident  variations,  producible  in  both 
by  changes  in  the  quantity  of  steam  supplied — changes  which 
may  be  caused  in  several  ways.  In  the  first  place,  the  fuel 
thrown  into  the  furnace  may  be  increased  or  made  better. 
Other  things  equal,  there  will  result  a  more  active  locomo- 
tion as  well  as  a  greater  escape ;  and  this  will  answer  to  that 
simultaneous  addition  to  its  individual  vigour  and  its  repro- 
ductive activity,  caused  in  an  animal  by  a  larger  quantity,  or 
a  superior  quality,  of  food.  In  the  second  place,  the  steam 
generated  may  be  economized.  Loss  by  radiation  from  the 
boiler  may  be  lessened  by  a  covering  of  non-conducting  sub- 
stances; and  part  of  the  steam  thus  prevented  from  con- 
densing, will  go  to  increase  the  working  power  of  the  engine, 
while  part  will  be  added  to  the  quantity  blowing  off.  This 
variation  corresponds  to  that  simultaneous  addition  to  bodily 
vigour  and  propagative  power,  which  results  in  animals  that 
have  to  expend  less  in  keeping  up  their  temperatures.  In 
the  third  place,  by  improvement  of  the  steam-generating 
apparatus,  more  steam  may  be  obtained  from  a  given  weight 
of  fuel.  A  better-formed  evaporating  surface,  or  boiler  tubes 
which  conduct  more  rapidly,  or  an  increased  number  of  them 


MULTIPLICATION  OF  THE  HUMAN  RACE.          519 

may  cause  a  larger  absorption  of  heat  from  the  burning  mass 
or  the  hot  gases  it  gives  off;  and  the  extra  steam  generated 
by  this  extra  heat  will,  as  before,  augment  both  the  motive 
force  and  the  emission  through  the  safety-valve.  And  this 
last  case  of  coincident  variation,  is  parallel  to  the  case  with 
which  we  are  here  concerned — the  augmentation  of  individual 
expenditure  and  of  reproductive  energy,  that  may  be  caused 
by  a  superiority  of  some  organ  on  which  the  utilizing  or 
economizing  of  materials  depends. 

Manifestly,  therefore,  an  increased  expenditure  for  Gene- 
sis, or  an  increased  expenditure  for  Individuation,  may  arise 
in  one  of  two  quite  different  ways — either  by  diminution  of 
the  antagonistic  expenditure,  or  by  addition  to  the  store  which 
supplies  both  expenditures;  and  confusion  results  from  not 
distinguishing  between  these.  Given  the  ratio  4  to  20,  as 
expressive  of  the  relative  costs  of  Genesis  and  Individuation ; 
then  the  expenditure  for  Genesis  may  be  raised  to  5  while  the 
expenditure  for  Individuation  is  raised  to  25,  without  any 
alteration  of  type,  merely  by  favourable  circumstances  or 
superiority  of  constitution.  On  the  other  hand,  circumstances 
remaining  the  same,  the  expenditure  for  Genesis  may  be 
raised  from  4  to  5,  by  lowering  the  expenditure  for  Indi- 
viduation from  20  to  19:  which  change  of  ratio  may  be 
either  functional  and  temporary,  or  structural  and  per- 
manent. And  only  when  it  is  the  last  does  it  illustrate  that 
inverse  variation  between  degree  of  evolution  and  degree  of 
procreative  dissolution,  which  we  have  everywhere  seen. 

§  370.  There  is  no  reason  to  suppose,  then,  that  the  laws 
of  multiplication  which  hold  of  other  beings,  do  not  hold  of 
the  human  being.  On  the  contrary,  there  are  special  facts 
which  unite  with  general  implications  to  show  that  these 
laws  do  hold  of  the  human  being.  The  absence  of  direct 
evidence  in  some  cases  where  it  might  be  looked  for,  we  find 
fully  explained  when  all  the  factors  are  taken  into  account. 
And  certain  seemingly-adverse  facts  prove,  on  examination, 


520  LAWS  OF  MULTIPLICATION. 

to  be  facts  belonging  to  a  different  category  from  that  in 
which  they  are  placed,  and  harmonize  with  the  rest  when 
rightly  interpreted. 

The  conformity  of  human  fertility  to  the  laws  of  multipli- 
cation in  general,  being  granted,  it  remains  to  inquire  what 
effects  must  be  caused  by  permanent  changes  in  men's  natures 
and  circumstances.  Thus  far  we  have  observed  how,  by  their 
exceptionally-high  evolution  and  exceptionally-low  fertility, 
mankind  display  the  inverse  variation  between  Individuation 
and  Genesis,  in  one  of  its  extremes.  And  we  have  also  ob- 
served how  mankind,  like  other  kinds,  are  functionally  changed 
in  their  rates  of  multiplication  by  changes  of  conditions.  But 
we  have  not  observed  how  alteration  of  structure  in  Man 
entails  alteration  of  fertility.  The  influence  of  this  factor  is 
so  entangled  with  the  influences  of  other  factors  which  are 
for  the  present  more  potent,  that  we  cannot  recognize  it. 
Here,  if  we  proceed  at  all,  we  must  proceed  deductively. 


[NOTE. — From  among  the  publications  of  the  American 
Academy  of  Political  and  Social  Science,  there  was  sent  to 
me  some  years  ago  an  essay  entitled  "  The  Significance  of  a 
Decreasing  Birth  Kate"  by  (Miss)  J.  L.  Brownell,  Fellow  in 
Political  Science,  Bryn  Mawr  College.  This  essay  contains 
a  number  of  elaborate  comparisons  drawn  from  the  vital 
statistics  of  the  tenth  United  States  Census.  The  results 
of  these  comparisons  are  thus  summed  up : — 

"  1.  Whether  or  not  it  be  true  that  the  means  spoken  of  by  Dr. 
Billings,  M.  Dumont,  M.  Levasseur,  and  Dr.  Edson  has  become  nn 
important  factor  in  the  diminishing  birth-rate  of  civilized  countries, 
it  is  evident  that  it  is  not  the  only  factor,  and  that,  quite  apart  from 
voluntary  prevention,  there  is  a  distinct  problem  to  be  investigated. 
This  is  shown  by  the  fact  that  the  white  and  the  colored  birth-rate 
vary  together. 

"  2.  Mr.  Spencer's  generalization  that  the  birth-rate  diminishes  ns 
the  rate  of  individual  evolution  increases  is  confirmed  by  a  comparison 
of  the  birth-rates  with  the  death-rates  from  nervous  diseases,  and  also 


MULTIPLICATION  OF  THE   HUMAN  RACE.          521 

with  the  density  of  population,  the  values  of  agricultural  and  manu- 
factured products,  and  the  mortgage  indebtedness." 

Of  course  multitudinous  differences  of  race,  class,  mode 
of  living,  occupation,  locality,  make  it  difficult  to  draw  posi- 
tive inferences  from  the  data ;  but  the  inferences  above  drawn 
are  held  to  remain  outstanding  after  allowing  for  all  the 
qualifying  conditions.] 


CHAPTER  XIII. 

HUMAN    POPULATION    IN   THE    FUTURE. 

§  371.  ANY  further  evolution  in  the  most-highly  evolved 
of  terrestrial  beings,  Man,  must  be  of  the  same  nature  as 
evolution  in  general.  Structurally  considered,  it  may  consist 
in  greater  integration,  or  greater  differentiation,  or  both — 
augmented  bulk,  or  increased  heterogeneity  and  definiteness, 
or  a  combination  of  the  two.  Functionally  considered,  it 
may  consist  in  a  larger  sum  of  actions,  or  more  multiplied 
varieties  of  actions,  or  both — a  larger  amount  of  sensible  and 
insensible  motion  generated,  or  motions  more  numerous  in 
their  kinds  and  more  intricate  and  exact  in  their  co-ordina- 
tions, or  motions  that  are  greater  alike  in  quantity,  com- 
plexity, and  precision. 

Expressing  the  change  in  terms  of  that  more  special 
evolution  displayed  by  organisms;  we  may  say  that  it  must 
be  one  which  further  adapts  the  moving  equilibrium  of 
organic  actions.  As  was  pointed  out  in  First  Principles, 
§  173,  "  the  maintenance  of  such  a  moving  equilibrium,  re- 
quires the  habitual  genesis  of  internal  forces  corresponding 
in  number,  directions,  and  amounts  to  the  external  incident 
forces — as  many  inner  functions,  single  or  combined,  as  there 
are  single  or  combined  outer  actions  to  be  met."  And  it 
was  also  pointed  out  that  "  the  structural  complexity  accom- 
panying functional  equilibration,  is  definable  as  one  in  which 
there  are  as  many  specialized  parts  as  are  capable,  separately 
and  jointly,  of  counteracting  the  separate  and  joint  forces 
522 


HUMAN  POPULATION  IN  THE  FUTURE.  523 

amid  which  the  organism  exists."  Clearly,  then,  since  all 
incompletenesses  in  Man  as  now  constituted,  are  failures  to 
meet  certain  of  the  outer  actions  (mostly  involved,  remote, 
irregular),  to  which  he  is  exposed;  every  advance  implies 
additional  co-ordinations  of  actions  and  accompanying  com- 
plexities of  organization. 

Or,  to  specialize  still  further  this  conception  of  future  pro- 
gress, we  may  consider  it  as  an  advance  towards  completion 
of  that  continuous  adjustment  of  internal  to  external  rela- 
tions, which  Life  shows  us.  In  Part  I.  of  this  work,  where 
it  was  shown  that  the  correspondence  between  inner  and 
outer  actions  which  under  its  phenomenal  aspect,  we  call 
Life,  is  a  particular  kind  of  what,  in  terms  of  Evolution,  we 
called  a  moving  equilibrium;  it  was  shown  that  the  degree 
of  life  varies  as  the  degree  of  correspondence.  Greater  evo- 
lution or  higher  life  implies,  then,  such  modifications  of 
human  nature  as  shall  make  more  exact  the  existing  corre- 
spondences, or  shall  establish  additional  correspondences,  or 
both.  Connexions  of  phenomena  of  a  rare,  distant,  unobtru- 
sive, or  intricate  kind,  which  we  either  suffer  from  or  do  not 
take  advantage  of,  have  to  be  responded  to  by  new  connexions 
of  ideas,  and  acts  properly  combined  and  proportioned :  there 
must  be  increase  of  knowledge,  or  skill,  or  power,  or  of  all 
these.  And  to  effect  this  more  extensive,  more  varied,  and 
more  accurate,  co-ordination  of  actions,  there  must  be  organi- 
zation of  still  greater  heterogeneity  and  definiteness. 

§  372.  Let  us,  before  proceeding,  consider  in  what  par- 
ticular ways  this  further  evolution,  this  higher  life,  this 
greater  co-ordination  of  actions,  may  be  expected  to  show 
itself. 

Will  it  be  in  strength?  Probably  not  to  any  considerable 
degree.  Mechanical  appliances  are  fast  supplanting  brute 
force,  and  doubtless  will  continue  doing  this.  Though  at 
present  civilized  nations  largely  depend  for  self-preservation 
on  vigour  of  limb,  and  are  likely  to  do  so  while  wars  con- 


524  LAWS  OF  MULTIPLICATION. 

tinue;  yet  that  progressive  adaptation  to  the  social  state 
which  must  at  last  bring  wars  to  an  end,  will  leave  the 
amount  of  muscular  power  to  adjust  itself  to  the  requirements 
of  a  peaceful  regime.  Though,  taking  all  things  into  account, 
the  muscular  power  then  required  may  not  be  less  than  now, 
there  seems  no  reason  why  more  should  be  required. 

Will  it  be  swiftness  or  agility  ?  Probably  not.  In  savages 
these  are  important  elements  of  the  ability  to  maintain 
life;  but  in  civilized  men  they  aid  self-preservation  in  quite 
minor  degrees,  and  there  seems  no  circumstance  likely  to 
necessitate  an  increase  of  them.  By  games  and  gymnastic 
competitions,  such  attributes  may  indeed  be  artificially  in- 
creased; but  no  artificial  increase  which  does  not  bring  a 
proportionate  advantage  can  be  permanent;  since,  other 
things  equal,  individuals  and  societies  that  devote  the  same 
amounts  of  energy  in  ways  which  subserve  life  more  effectu- 
ally, must  by  and  by  predominate. 

Will  it  be  in  mechanical  skill,  that  is,  in  the  better- 
co-ordination  of  complex  movements?  Most  likely  in  some 
degree.  Awkwardness  is  continually  entailing  injuries  and 
deaths.  Moreover  the  complicated  tools  which  civilization 
brings  into  use,  are  constantly  requiring  greater  delicacy  of 
manipulation.  All  the  arts,  industrial  and  esthetic,  as  they 
develop,  imply  a  corresponding  development  of  perceptive  and 
executive  faculties  in  men :  the  two  act  and  react. 

Will  it  be  in  intelligence?  Largely,  no  doubt.  There  is 
ample  room  for  advance  in  this  direction,  and  ample  demand 
for  it.  Our  lives  are  universally  shortened  by  our  ignorance. 
In  attaining  complete  knowledge  of  our  own  natures  and  of 
the  natures  of  surrounding  things — in  ascertaining  the  con- 
ditions of  existence  to  which  we  must  conform,  and  in  dis- 
covering means  of  conforming  to  them  under  all  variations 
of  seasons  and  circumstances;  we  have  abundant  scope  for 
intellectual  progress. 

Will  it  be  in  morality,  that  is,  in  greater  power  of  self- 


HUMAN  POPULATION  IN  THE  FUTURE.  525 

regulation  ?  Largely  also :  perhaps  most  largely.  Right  con- 
duct is  usually  come  short  of  more  from  defect  of  will  than 
defect  of  knowledge.  For  the  right  co-ordination  of  those 
complex  actions  which  constitute  human  life  in  its  civilized 
form,  there  goes  not  only  the  pre-requisite — recognition  of 
the  proper  course:  but  the  further  pre-requisite — a  due 
impulse  to  pursue  that  course.  On  calling  to  mind  our 
daily  failures  to  fulfil  often-repeated  resolutions,  we  shall 
perceive  that  lack  of  the  needful  desire,  rather  than  lack  of 
the  needful  insight,  is  the  chief  cause  of  faulty  action.  A 
further  endowment  of  those  feelings  which  civilization  is 
developing  in  us — sentiments  responding  to  the  requirements 
of  the  social  state — emotive  faculties  that  find  their  gratifi- 
cations in  the  duties  devolving  on  us — must  be  acquired 
before  the  crimes,  excesses,  diseases,  improvidences,  dishones- 
ties, and  cruelties,  that  now  so  greatly  diminish  the  duration 
of  life,  can  cease. 

Thus,  looking  at  the  several  possibilities,  and  asking 
what  direction  this  further  evolution,  this  more  complete 
moving  equilibrium,  this  better  adjustment  of  inner  to  outer 
relations,  this  more  perfect  co-ordination  of  actions,  is  likely 
to  take;  we  conclude  that  it  must  take  mainly  the  direction 
of  a  higher  intellectual  and  emotional  development. 

§  373.  This  conclusion  we  shall  find  equally  forced  on  us 
if  we  inquire  for  the  causes  which  are  to  bring  about  such 
results.  No  more  in  the  case  of  Man  than  in  the  case  of 
any  other  being,  can  we  presume  that  evolution  has  taken 
place,  or  will  hereafter  take  place,  spontaneously.  In  the 
past,  at  present,  and  in  the  future,  all  modifications,  func- 
tional and  organic,  have  been,  are,  and  must  be,  immediately 
or  remotely  consequent  on  surrounding  conditions.  What, 
then,  are  those  changes  in  the  environment  to  which,  by 
direct  or  indirect  equilibration,  the  human  organism  has  been 
adjusting  itself,  is  adjusting  itself  now,  and  will  continue  to 


526  LAWS  OF  MULTIPLICATION. 

adjust  itself?  And  how  do  they  necessitate  a  higher  evolu- 
tion of  the  organism  ? 

Civilization,  everywhere  having  for  its  antecedent  the  in- 
crease of  population,  and  everywhere  having  for  one  of  its 
consequences  a  decrease  of  certain  race-destroying  forces,  has 
for  a  further  consequence  an  increase  of  certain  other  race- 
destroying  forces.  Danger  of  death  from  predatory  animals 
lessens  as  men  grow  more  numerous.  Though,  as  they  spread 
over  the  Earth  and  divide  into  tribes,  men  become  wild 
beasts  to  one  another,  yet  the  danger  of  death  from  this 
cause  also  diminishes  as  tribes  coalesce  into  nations.  But  the 
danger  of  death  which  does  not  diminish,  is  that  produced  by 
augmentation  of  numbers  itself — the  danger  from  deficiency 
of  food.  Supposing  human  nature  to  remain  unchanged,  the 
mortality  hence  resulting  would,  on  the  average,  rise  as 
human  beings  multiplied.  If  mortality,  under  such  condi- 
tions, does  not  rise,  it  must  be  because  the  supply  of  food 
also  augments;  and  this  implies  some  change  in  human 
habits  wrought  by  stress  of  human  needs.  Here,  then,  is 
the  permanent  cause  of  modification  to  which  civilized  men 
are  exposed.  Though  the  intensity  of  its  action  is  ever  being 
mitigated  in  one  direction  by  greater  production  of  food,  it 
is,  in  the  other  direction,  ever  being  added  to  by  the  greater 
production  of  individuals.  Manifestly,  the  wants  of  their 
redundant  numbers  constitute  the  only  stimulus  mankind 
have  to  obtain  more  necessaries  of  life.  Were  not  the  demand 
beyond  the  supply,  there  would  be  no  motive  to  increase  the 
supply.  And  manifestly,  this  excess  of  demand  over  supply 
is  perennial:  this  pressure  of  population,  of  which  it  is  the 
index,  cannot  be  eluded.  Though  by  the  emigration  that 
takes  place  when  the  pressure  arrives  at  a  certain  intensity, 
temporary  relief  is  from  time  to  time  obtained;  yet  as,  by 
this  process,  all  habitable  countries  must  become  peopled,  it 
follows  that  in  the  end  the  pressure,  whatever  it  may  then 
be,  must  be  borne  in  full. 

This  constant  increase  of  people  beyond  the  means  of  sub- 


HUMAN  POPULATION  IN  THE  FUTURE.  527 

sistence  causes,  then,  a  never-ceasing  requirement  for  skill, 
intelligence,  and  self-control — involves,  therefore,  a  constant 
exercise  of  these  and  gradual  growth  of  them.  Every  indus- 
trial improvement  is  at  once  the  product  of  a  higher  form 
of  humanity,  and  demands  that  higher  form  of  humanity  to 
carry  it  into  practice.  The  application  of  science  to  the  arts, 
is  the  bringing  to  bear  greater  intelligence  for  satisfying  our 
wants,  and  implies  continued  progress  of  that  intelligence. 
To  get  more  produce  from  the  acre,  the  farmer  must  study 
chemistr}r,  must  adopt  new  mechanical  appliances,  and  must, 
by  the  multiplication  of  processes,  cultivate  both  his  own 
powers  and  the  powers  of  his  labourers.  To  meet  the 
requirements  of  the  market,  the  manufacturer  is  per- 
petually improving  his  old  machines  and  inventing  new 
ones;  and  by  the  premium  of  high  wages  incites  artizans  to 
acquire  greater  skill.  The  daily-widening  ramifications  of 
commerce  entail  on  the  merchant  a  need  for  more  know- 
ledge and  more  complex  calculations;  while  the  lessening 
profits  of  the  ship-owner  force  him  to  build  more  scientifi- 
cally, to  get  captains  of  higher  intelligence  and  better  crews. 
In  all  cases  pressure  of  population  is  the  original  cause. 
Were  it  not  for  the  competition  this  entails,  more  thought 
and  energy  would  not  daily  be  spent  on  the  business  of  life ; 
and  growth  of  mental  power  would  not  take  place. 
Difficulty  in  getting  a  living  is  alike  the  incentive  to  a 
higher  education  of  children,  and  to  a  more  intense  and 
long-continued  application  in  adults.  In  the  mother  it 
prompts  foresight,  economy,  and  skilful  house-keeping ;  in  the 
father,  laborious  days  and  constant  s^lf-denial.  Nothing  but 
necessity  could  make  men  submit  to  this  discipline;  and 
nothing  but  this  discipline  could  produce  a  continued  pro- 
gression. 

In  this  case,  as  in  many  others,  Nature  secures  each  step 
in  advance  by  a  succession  of  trials;  which  are  perpetually 
repeated,  and  cannot  fail  to  be  repeated,  until  success  is 
achieved.  All  mankind  in  turn  subject  themselves  more  or 


528  LAWS  OF  MULTIPLICATION. 

less  to  the  discipline  described;  they  either  may  or  may  not 
advance  under  it;  but,  in  the  nature  of  things,  only  those 
who  do  advance  under  it  eventually  survive.  For,  neces-_ 
sarily,  families  and  races  whom  this  increasing  difficulty  of 
getting  a  living  which  excess  of  fertility  entails,  does  not 
stimulate  to  improvements  in  production — that  is,  to  greater 
mental  activity — are  on  the  high  road  to  extinction;  and 
must  ultimately  be  supplanted  by  those  whom  the  pressure 
does  so  stimulate.  This  truth  we  have  recently  seen  exem- 
plified in  Ireland.  And  here,  indeed,  without  further  illus- 
tration, it  will  be  seen  that  premature  death,  under  all  its 
forms  and  from  all  its  causes,  cannot  fail  to  work  in  the 
same  direction.  For  as  those  prematurely  carried  off  must, 
in  the  average  of  cases,  be  those  in  whom  the  power  of  self- 
preservation  is  the  least,  it  unavoidably  follows  that  those 
left  behind  to  continue  the  race,  must  be  those  in  whom  the 
power  of  self-preservation  is  the  greatest — must  be  the  select 
of  their  generation.  So  that,  whether  the  dangers  to  exist- 
ence be  of  the  kind  produced  by  excess  of  fertility,  or  of  any 
other  kind,  it  is  clear  that  by  the  ceaseless  exercise  of  the 
faculties  needed  to  contend  with  them,  and  by  the  death  of 
all  men  who  fail  to  contend  with  them  successfully,  there  is 
ensured  a  constant  progress  towards  a  higher  degree  of  skill, 
intelligence,  and  self-regulation — a  better  co-ordination  of 
actions — a  more  complete  life.* 

§  374.  The  proposition  at  which  we  have  thus  arrived  is, 
then,  that  excess  of  fertility,  through  the  changes  it  is  ever 

*  A  good  deal  of  this  chapter  retains  its  original  form ;  and  the  above 
paragraph  is  reprinted  verbatim  from  the  Westminster  Review  for  April, 
1852,  in  which  the  views  developed  in  the  foregoing  hundred  pages  were 
first  sketched  out.  This  paragraph  shows  how  near  one  may  be  to  a  great 
generalization  without  seeing  it.  Though  the  struggle  for  life  is  the  alleged 
motive  force ;  though  the  process  of  natural  selection  is  recognized ;  and 
though  to  it  is  ascribed  a  share  in  the  evolution  of  a  higher  type ;  yet  the 
conception  is  not  that  which  Mr.  Darwin  has  worked  out  with  such  wonder- 
ful skill  and  knowledge.  In  the  first  place,  natural  selection  is  here  de- 
scribed only  as  furthering  direct  adaptation — only  as  aiding  progress  by  the 
preservation  of  individuals  in  whom  functionally-produced  modifications  have 


HUMAN  POPULATION  IN  THE  FUTURE.  529 

working  in  Man's  environment,  is  itself  the  cause  of  Man's 
further  evolution ;  and  the  obvious  corollary  here  to  be  drawn 
is,  that  Man's  further  evolution  so  brought  about,  itself 
necessitates  a  decline  in  his  fertility. 

All  future  progress  in  civilization  which  the  never- 
ceasing  pressure  of  population  must  produce,  will  be  accom- 
panied by  an  enhanced  cost  of  Individuation,  both  in 
structure  and  function;  and  more  especially  in  nervous 
structure  and  function.  The  peaceful  struggle  for  existence 
in  societies  ever  growing  more  crowded  and  more  compli- 
cated, must  have  for  its  concomitant  an  increase  of  the  great 
nervous  centres  in  mass,  in  complexity,  in  activity.  That 
larger  body  of  emotion  needed  as  a  fountain  of  energy  for 
men  who  have  to  hold  their  places  and  rear  their  families 
under  the  intensifying  competition  of  social  life,  is,  other 
things  equal,  the  correlative  of  larger  brain.  Those  higher 
feelings  presupposed  by  the  better  self-regulation  which,  in 
a  better  society,  can  alone  enable  the  individual  to  leave  a 
persistent  posterity,  are,  other  things  equal,  the  correlatives 
of  a  more  complex  brain;  as  are  also  those  more  numerous, 
more  varied,  more  general,  and  more  abstract  ideas,  which 
must  also  become  increasingly  requisite  for  successful  life  as 
society  advances.  And  the  genesis-  of  this  larger  quantity  of  ' 

gone  on  most  favourably.  In  the  second  place,  there  is  no  trace  of  the  idea 
that  natural  selection  may  by  co-operation  with  the  cause  assigned,  or  with 
other  causes,  produce  divergfnces  of  structure ;  and  of  course,  in  the  absence 
of  this  idea,  there  is  no  implication  that  natural  selection  has  anything  to  do 
with  the  origin  of  species.  And  in  the  third  place,  the  all-important  factor 
of  variation — "  spontaneous,"  or  incidental  as  we  may  otherwise  call  it — is 
wholly  ignored.  Though  use  and  disuse  are,  I  think,  much  more  potent 
caiises  of  organic  modification  than  Mr.  Darwin  supposes — though,  while  pur- 
suing the  inquiry  in  detail,  I  have  been  led  to  believe  that  direct  equilibration 
has  played  a  more  active  part  even  than  I  had  myself  at  one  time  thought ; 
yet  I  hold  Mr.  Darwin  to  have  shown  beyond  question,  that  a  great  part  of 
the  facts — perhaps  the  greater  part — are  explicable  only  as  resulting  from 
the  survival  of  individuals  which  have  deviated  in  some  indirectly-caused  way 
from  the  ancestral  type.  Thus,  the  above  paragraph  contains  merely  a  pass- 
ing recognition  of  the  selective  process ;  and  indicates  no  suspicion  of  the 
enormous  range  of  its  effects,  or  of  the  conditions  under  which  a  large  part 
of  its  effects  are  produced. 
80 


530  LAWS  OF  MULTIPLICATION. 

feeling  and  thought,  in  a  brain  thus  augmented  in  size  and 
developed  in  structure,  is,  other  things  equal,  the  correlative 
of  a  greater  wear  of  nervous  tissue  and  greater  consumption 
of  materials  to  repair  it.  So  that  both  in  original  cost  of  con- 
struction and  in  subsequent  cost  of  working,  the  nervous 
system  must  become  a  heavier  tax  on  the  organism.  Already 
the  brain  of  the  civilized  man  is  larger  by  nearly  thirty  per 
cent,  than  the  brain  of  the  savage.  Already,  too,  it  presents 
an  increased  heterogeneity — especially  in  the  distribution  of 
its  convolutions.  And  further  changes  like  these  which  have 
taken  place  under  the  discipline  of  civilized  life,  we  infer 
will  continue  to  take  place.  But  everywhere  and 

always,  evolution  is  antagonistic  to  procreative  dissolution. 
Whether  it  be  in  greater  growth  of  the  organs  which  sub- 
serve self-maintenance,  whether  it  be  in  their  added  com- 
plexity of  structure,  or  whether  it  be  in  their  higher  activity, 
the  abstraction  of  the  required  materials  implies  a  dimi- 
nished reserve  of  materials  for  race-maintenance.  And  we 
have  seen  reason  to  believe  that  this  antagonism  between 
Individuation  and  Genesis,  becomes  unusually  marked  where 
the  nervous  system  is  concerned,  because  of  the  costliness  of 
nervous  structure  and  function.  In  §  346  was  pointed  out 
the  apparent  connexion  between  high  cerebral  development 
and  prolonged  delay  of  sexual  maturity;  and  in  §§366,  367, 
the  evidence  went  to  show  that  where  exceptional  fertility 
exists  there  is  sluggishness  of  mind,  and  that  where  there 
has  been  during  education  excessive  expenditure  in  mental 
action,  there  frequently  follows  a  complete  or  partial  infer- 
tility. Hence  the  particular  kind  of  further  evolution  which 
Man  is  hereafter  to  undergo,  is  one  which,  more  than  any 
other,  may  be  expected  to  cause  a  decline  in  his  power  of 
reproduction. 

The  higher  nervous  development  and  greater  expenditure 
in  nervous  action,  here  described  as  indirectly  brought  about 
by  increase  of  numbers,  and  as  thereafter  becoming  a  check 
on  the  increase  of  numbers,  must  not  be  taken  to  imply 


HUMAN  POPULATION  IN  THE  FTJTUBE.  531 

an  intenser  strain — a  mentally-laborious  life.  The  greater 
emotional  and  intellectual  power  and  activity  above  con- 
templated, must  be  understood  as  becoming,  by  small  incre- 
ments, organic,  spontaneous,  and  pleasurable.  As,  even  when 
relieved  from  the  pressure  of  necessity,  large-brained  Euro- 
peans voluntarily  enter  on  enterprises  and  activities  which 
the  savage  could  not  keep  up  even  to  satisfy  urgent  wants ; 
so,  their  still  larger-brained  descendants  will,  in  a  still  higher 
degree,  find  their  gratifications  in  careers  entailing  still 
greater  mental  expenditures.  This  enhanced  demand  for 
materials  to  establish  and  carry  on  the  psychical  functions, 
will  be  a  constitutional  demand.  We  must  conceive  the 
type  gradually  so  modified,  that  the  more-developed  nervous 
system  irresistibly  draws  off,  for  its  normal  and  unforced 
activities,  a  larger  proportion  of  the  common  stock  of  nutri- 
ment; and  while  so  increasing  the  intensity,  completeness, 
and  length  of  the  individual  life,  necessarily  diminishing  the 
reserve  applicable  to  the  setting  up  of  new  lives — no  longer 
required  to  be  so  numerous. 

Though  the  working  of  this  process  will  doubtless  be 
interfered  with  and  modified  in  the  future,  as  it  has  been 
in  the  past,  by  the  facilitations  of  living  which  civilization 
brings;  yet  nothing  beyond  temporary  interruptions  can  so 
be  caused.  However  much  the  industrial  arts  may  be  im- 
proved, there  must  be  a  limit  to  the  improvement;  while, 
with  a  rate  of  multiplication  in  excess  of  the  rate  of  mor- 
tality, population  must  continually  tread  on  the  heels  of 
production.  So  that  though,  during  the  earlier  stages  of 
civilization,  an  increased  amount  of  food  may  accrue  from  a 
given  amount  of  labour,  there  must  come  a  time  when  this 
relation  will  be  reversed,  and  when  every  additional  incre- 
ment of  food  will  be  obtained  by  a  more  than  proportionate 
labour :  the  disproportion  growing  ever  higher,  and  the  dimi- 
nution of  the  reproductive  power  becoming  greater. 

§  375.  There  now  remains  but  to  inquire  towards  what 


532  LAWS  OP  MULTIPLICATION. 

limit  this  progress  tends.  So  long  as  the  fertility  of  the 
race  is  more  than  sufficient  to  balance  the  diminution  by 
deaths,  population  must  continue  to  increase.  So  long  as 
population  continues  to  increase,  there  must  be  pressure  on 
the  means  of  subsistence.  And  so  long  as  there  is  pressure 
on  the  means  of  subsistence,  further  mental  development 
must  go  on,  and  further  diminution  of  fertility  must  result; 
provided  that  the  actions  and  reactions  which  have  been 
described  are  not  artificially  interfered  with.  I  append  this 
qualifying  clause  advisedly,  and  especially  emphasize  it,  be- 
cause these  actions  and  reactions  have  been  hitherto,  and 
are  now,  greatly  interfered  with  by  governments,  and  the 
continuance  of  the  interferences  may  retard,  if  not  stop, 
that  further  evolution  which  would  else  go  on. 

I  refer  to  those  hindrances  to  the  survival  of  the  fittest 
which  in  earlier  times  resulted  from  the  undiscriminating 
charities  of  monasteries  and  in  later  times  from  the  opera- 
tion of  Poor  Laws.  Of  course  if  the  competition  which 
increasing  pressure  of  population  entails,  is  prevented  from 
acting  on  a  considerable  part  of  the  community,  such  part, 
saved  from  the  needed  intellectual  and  moral  stress,  will  not 
undergo  any  further  mental  development;  and  must  ever 
tend  to  leave  a  posterity,  and  an  increasing  posterity,  in 
which  none  of  that  higher  individuation  which  checks 
genesis  takes  place.  Such  State-meddlings  with  the  natural 
play  of  actions  and  reactions  produce  a  further  evil  equally 
great  or  greater.  For  those  who  are  not  self-maintained,  or 
but  partially  self-maintained,  are  supplied  with  the  means 
they  lack  by  the  better  members  of  the  community;  and 
these  better  members  have  thus  not  only  to  support  them- 
selves and  their  offspring,  but  also  to  support  or  aid  the 
inferior  members  and  their  offspring.  The  under-working  of 
one  part  is  accompanied  by  the  over-working  of  the  other 
part — by  a  working  which  at  each  stage  of  progress  exceeds 
that  which  the  normal  conditions  necessitate,  and  results 
sometimes  in  illness,  premature  age,  or  death,  or  in  lessened 


HUMAN  POPULATION  IN  THE  FUTURE.  533 

number  of  children,  or  in  imperfect  rearing  of.  children :  the 
bad  are  fostered  and  the  good  are  repressed. 

It  does  not  follow  that  the  struggle  for  life  and  the  sur- 
vival of  the  fittest  must  be  left  to  work  out  their  effects 
without  mitigation.  It  is  contended  only  that  there  shall 
not  be  a  forcible  burdening  of  the  superior  for  the  support 
of  the  inferior.  Such  aid  to  the  inferior  as  the  superior 
voluntarily  yield,  kept  as  it  will  be  within  moderate  limits, 
may  be  given  with  benefit  to  both — relief  to  the  one,  moral 
culture  to  the  other.  And  aid  willingly  given  (little  to  the 
least  worthy  and  more  to  the  most  worthy)  will  usually  be 
so  given  as  not  to  further  the  increase  of  the  unworthy. 
For  in  proportion  as  the  emotional  nature  becomes  more 
evolved,  and  there  grows  up  a  higher  sense  of  parental  re- 
sponsibility, the  begetting  of  children  that  cannot  be  properly 
reared  will  be  universally  held  intolerable.  If,  as  we  see, 
public  opinion  in  many  places  and  times  becomes  coercive 
enough  to  force  men  to  fight  duels,  we  can  scarcely  doubt  that 
at  a  higher  stage  of  evolution  it  may  become  so  coercive  as  to 
prevent  men  from  marrying  improvidently.  If  the  frowns 
of  their  fellows  can  make  men  commit  immoral  acts,  surely 
they  may  make  men  refrain  from  immoral  acts— especially 
when  the  actors  themselves  feel  that  the  threatened  frowns 
would  be  justified.  Hence  with  a  higher  moral  nature  will 
come  a  restriction  on  the  multiplication  of  the  inferior. 

In  brief,  the  sole  requirement  is  that  there  shall  be  no 
extensive  suspension  of  that  natural  relation  between  merit 
and  benefit  which  constitutes  justice.  Holding,  then,  that 
this  all-essential  condition  will  itself  come  to  be  recognized 
and  enforced  by  a  more  evolved  humanity,  let  us  consider 
what  is  the  goal  towards  which  the  restraint  on  genesis  by 
individuation  progresses. 

§  3750.  Supposing  the  Sun's  light  and  heat,  on  which  all 
terrestrial  life  depends,  to  continue  abundant  for  a  period 
long  enough  to  allow  the  entire  evolution  we  are  contem- 


534  LAWS  OP  MULTIPLICATION. 

plating;  there  are  still  certain  changes  which  must  prevent 
such  complete  adjustment  of  human  nature  to  surrounding 
conditions,  as  would  permit  the  rate  of  multiplication  to 
become  equal  to  the  rate  of  mortality.  As  before  pointed  out 
(§  148),  during  an  epoch  of  21,000  years  each  hemisphere  goes 
through  a  cycle  of  temperate  seasons  and  seasons  extreme  in 
their  heat  and  cold — variations  which  are  themselves  alter- 
nately exaggerated  and  mitigated  in  the  course  of  far  longer 
cycles;  and  we  saw  that  these  cause  perpetual  ebbings  and 
Sowings  of  species  over  different  parts  of  the  Earth's  surface. 
Further,  by  slow  but  inevitable  geologic  changes,  especially 
those  of  elevation  and  subsidence,  the  climate  and  physical 
characters  of  every  habitat  are  modified;  while  old  habitats 
are  destroyed  and  new  are  formed.  This,  too,  we  noted  as 
a  constant  cause  of  migrations  and  of  resulting  alterations 
of  environment.  Now  though  the  human  race  differs  from 
other  races  in  having  a  power  of  artificially  counteracting 
external  changes,  yet  there  are  limits  to  this  power;  and, 
even  were  there  no  limits,  the  changes  could  not  fail  to  work 
their  effects  indirectly,  if  not  directly.  If,  as  is  thought 
probable,  these  astronomic  cycles  entail  recurrent  glacial 
periods  in  each  hemisphere,  then  parts  of  the  Earth  which  are 
at  one  time  thickly  peopled,  will  at  another  time  be  almost 
deserted,  and  vice  versa.  The  geologically-caused  alterations 
of  climate  and  surface,  must  produce  further  slow  re-distri- 
butions of  population;  and  other  currents  of  people,  to  and 
from  different  regions,  will  be  necessitated  by  the  rise  of  suc- 
cessive centres  of  higher  civilization.  Consequently,  man- 
kind cannot  but  continue  to  undergo  changes  of  environ- 
ment, physical  and  moral,  analogous  to  those  which  they 
have  thus  far  been  undergoing.  Such  changes  may  eventu- 
ally become  slower  and  less  marked;  but  they  can  never 
cease.  And  if  they  can  never  cease  there  can  never  arise  a 
perfect  adaptation  of  human  nature  to  its  conditions  of  exist- 
ence. To  establish  that  complete  correspondence  between 
inner  and  outer  actions  which  constitutes  the  highest  life  and 


HUMAN  POPULATION  IN  THE  FUTURE.  535 

greatest  power  of  self-preservation,  there  must  be  a  prolonged 
converse  between  the  organism  and  circumstances  which  re- 
main the  same.  If  the  external  relations  are  being  altered 
while  the  internal  relations  are  being  adjusted  to  them,  the 
adjustment  can  never  become  exact.  And  in  the  absence  of 
exact  adjustment,  there  cannot  exist  that  theoretically-highest 
power  of  self-preservation  with  which  there  would  co-exist 
the  theoretically-lowest  power  of  race-production. 

Hence  though  the  number  of  premature  deaths  may  ulti- 
mately become  very  small,  it  can  never  become  so  small  as 
to  allow  the  average  number  of  offspring  from  each  pair  to 
fall  so  low  as  two.  Some  average  number  between  two  and 
three  may  be  inferred  as  the  limit — a  number,  however,  which 
is  not  likely  to  be  quite  constant,  but  may  be  expected  at 
one  time  to  increase  somewhat  and  afterwards  to  decrease 
somewhat,  according  as  variations  in  physical  and  social  con- 
ditions lower  or  raise  the  cost  of  self-preservation. 

To  this  qualification  must  be  added  a  further  qualifica- 
tion. The  foregoing  argument  tacitly  assumes  that  the 
causes  described  will  continuously  operate  on  all  mankind; 
whereas  a  survey  of  the  facts  makes  it  clear  that  some  parts 
only  of  the  Earth's  surface  are  capable  of  bearing  high 
types  of  civilization,  and  consequently  high  types  of  Man. 
There  must  remain  hereafter,  as  there  are  now,  considerable 
parts  of  its  surface  which  can  support  only  groups  of  nomads, 
or  other  groups  obliged  by  their  habitats  to  lead  simple 
and  inferior  kinds  of  life.  Only  by  subjection  to  the  disci- 
pline we  have  been  contemplating  can  there  be  produced  the 
fully-developed  Man;  and  evidently  in  many  parts  of  the 
world  this  discipline  will  continue  to  be  eluded.  Not  only 
must  we  conclude  that  the  varieties  of  our  race  now  liv- 
ing in  desert  regions  and  arctic  climates  will  continue  here- 
after to  do  so,  but  we  may  conclude  that  always,  as  now,  a 
certain  proportion  of  men  who  are  born  in  civilized  societies, 
impatient  of  the  stress  which  pressure  of  population  puts 
on  them,  will  escape  into  unoccupied  or  sparsely-peopled 


530  LAWS  OF  MULTIPLICATION. 

regions,  where  they  can  lead  unrestrained  lives  though  lives 
of  hardship.  Eecognizing  as  we  must  the  probability  that 
in  common  with  all  other  things,  humanity  will  continue  to 
differentiate  and  produce  a  more  heterogeneous  assemblage 
of  types,  we  must  infer  that  only  in  some  of  the  highest  of 
these  will  the  antagonism  of  individuation  and  genesis  have 
the  anticipated  effeqts. 

Eestrictingj^tVselves  to  these,  then,  we  may  conclude 
that  in  the  eid,  [pressure  of  population  and  its  accompany- 
ing evils  will  almost  disappear;  and  will  leave  a  state  of 
things  requiring  from  each  individual,  little  more  than  a 
normal  and  pleasurable  activity^'  3j@ssa.tion  in  the  decrease 
of  fertility  implies  cessation  in  the  development  of  the 
nervous  system;  and  this  implies  a  nervous  system  which 
has  become  equal  to  all  that  is  demanded  of  it — has  not  to  do 
more  than  is  natural  to  it.  But  that  exercise  of  faculties  which 
does  not  exceed  what  is  natural,  constitutes  gratification. 

The  necessary  antagonism  of  Individuation  and  Genesis, 
not  only,  then,  fulfils  the  a  priori  law  of  maintenance  of 
race,  from  the  monad  up  to  Man,  but  ensures  final  attainment 
of  the  highest  form  of  this  maintenance — a  form  in  which 
the  amount  of  life  shall  be  the  greatest  possible  and  the 
births  and  deaths  the  fewest  possiw^T From  the  beginning 
pressure  of  population  has  been  the  proximate  cause  of  pro- 
gress. It  produced  the  original  diffusion  of  the  race.  It 
compelled  men  to  abandon  predatory  habits  and  take  to 
agriculture.  It  led  to  the  clearing  of  the  Earth's  surface. 
It  forced  men  into  the  social  state;  made  social  organiza- 
tion inevitable ;  and  has  developed  the  social  sentiments.  It 
has  stimulated  to  progressive  improvements  in  production, 
and  to  increased  skill  and  intelligence.  It  is  daily  thrusting 
us  into  closer  contact  and  more  mutually-dependent  relation- 
ships. And  after  having  caused,  as  it  ultimately  must,  the 
due  peopling  of  the  globe,  and  the  raising  of  its  habitable 
parts  into  the  highest  state  of  culture — after  having  perfected 
all  processes  for  the  satisfaction  of  human  wants — after 


HUMAN  POPULATION  IN  THE  FUTUEE.  537 

having,  at  the  same  time,  developed  the  intellect  into  com- 
petence for  its  work,  and  the  feelings  into  fitness  for  social 
life — after  having  done  all  this,  the  pressure  of  population 
must  gradually  approach  to  an  end — an  end,  however,  which 
for  the  reasons  given  it  cannot  absolutely  reach,  j  -*'''*'/•* 

§  377.  In  closing  the  argument  let  us  not  overlook  the 
self-sufficingness  of  those  universal  processes  by  which  the 
results  reached  thus  far  have  been  wrought  out,  and  which 
may  be  expected  to  work  out  these  future  results. 

Evolution  under  all  its  aspects,  general  and  special,  is  an 
advance  towards  equilibrium.  We  have  seen  that  the  theo- 
retical limit  towards  which  the  integration  and  differentia- 
tion of  every  aggregate  advances,  is  a  state  of  balance  be- 
tween all  the  forces  to  which  its  parts  are  subject,  and  the 
forces  which  its  parts  oppose  to  them  (First  Prin.  §  170). 
And  we  have  seen  that  organic  evolution  is  a  progress  towards 
a  moving  equilibrium  completely  adjusted  to  environing 
actions. 

It  has  been  also  pointed  out  that,  in  civilized  Man,  there  is 
going  on  a  new  class  of  equilibrations — those  between  his  ac- 
tions and  the  actions  of  the  societies  he  forms  (First  Prin. 
§  175).  Social  restraints  and  requirements  are  ever  altering 
his  activities  and  by  consequence  his  nature ;  and  as  fast  as  his 
nature  is  altered,  social  restraints  and  requirements  undergo 
more  or  less  re-adjustment.  Here  the  organism  and  the  con- 
ditions are  both  modifiable;  and  by  successive  conciliations 
of  the  two,  there  is  effected  a  progress  towards  equilibrium. 

More  recently  we  have  seen  that  in  every  species,  there 
establishes  itself  an  equilibrium  of  an  involved  kind  between 
the  total  race-destroying  forces  and  the  total  race-preserving 
forces — an  equilibrium  which  implies  that  where  the  ability 
to  maintain  individual  life  is  small,  the  ability  ta  propagate 
must  be  great,  and  vice  versa.  Whence  it  follows  that  the 
evolution  of  a  race  more  in  equilibrium  with  the  environ- 
ment, is  also  the  evolution  of  a  race  in  which  there  is  a  cor- 


538  LAWS  OF  MULTIPLICATION. 

relative  approach  towards  equilibrium  between  the  number 
of  new  individuals  produced  and  the  number  which  survive 
and  propagate. 

The  final  result  to  be  observed  is  that  in  Man,  all  these 
equilibrations  between  constitution  and  conditions,  between 
the  structure  of  society  and  the  nature  of  its  members, 
between  fertility  and  mortality,  advance  simultaneously 
towards  a  common  climax.  In  approaching  an  equilibrium 
between  his  nature  and  the  ever-varying  circumstances  of  his 
inorganic  environment,  and  in  approaching  an  equilibrium 
between  his  nature  and  all  the  requirements  of  the  social 
state,  Man  is  at  the  same  time  approaching  that  lowest  limit 
of  fertility  at  which  the  equilibrium  of  population  is  main- 
tained by  the  addition  of  as  many  infants  as  there  are  sub- 
tractions by  death  in  old  age.  But  in  a  universe  of  which  all 
parts  are  in  motion  and  every  part  is  consequently  subject  to 
change  of  conditions,  neither  this  equilibrium  nor  any  other 
equilibrium  can  become  complete. 


THE   END. 


APPENDICES. 


APPENDIX  A. 


SUBSTITUTION   OF  AXIAL  FOR   FOLIAR   ORGANS   IN   PLANTS. 


I  APPEND  here  the  evidences  referred  to  in  §  190.  The  most 
numerous  and  striking  I  have  met  with  among  the  Umbelliferce. 
Monstrosities  having  the  alleged  implication,  are  frequent  in  the 
common  Cow-Parsnep — so  frequent  that  they  must  be  familiar  to 
botanists ;  and  wild  Angelica  supplies  many  over-developments  of 
like  meaning.  Omitting  numerous  cases  of  more  or  less  significance, 
I  will  limit  myself  to  two. 

One  of  them  is  that  of  a  terminal  umbel,  in  which  nine  of  the 
outer  umbellules  are  variously  transformed — here  a  single  flower 
being  made  monstrous  by  the  development  of  some  of  its  members 
into  buds ;  there  several  such  malformed  flowers  being  associated 
with  rays  that  bear  imperfect  umbellules ;  and  elsewhere,  flowers 

c 


being  replaced  by  umbellules :  some  of  which  are  perfect,  and  others 
imperfect  only  in  the  shortness  of  the  flower-stalks.  The  annexed 
Fig.  69,  representing  in  a  somewhat  conventionalized  way,  a  part  of 

541 


542  APPENDIX  A. 

the  dried  specimen,  will  give  an  idea  of  this  Angelica.  At  a  is 
shown  a  single  flower  partially  changed  ;  in  the  umbellule  marked 
b,  one  of  the  rays  bears  a  secondary  umbellule ;  and  there  may 
be  seen  at  c  and  d,  several  such  over-developments. 

But  the  most  conclusive  instance  is  that  of  a  Oow-Parsnep,  in 
which  a  single  terminal  umbel,  besides  the  transformations  already 
mentioned,  exhibits  higher  degrees  of  such  transformations.*  The 
components  of  this  complex  growth  are ; — three  central  umbellules, 
abnormal  only  in  minor  points ;  one  umbellule,  external  to  these, 
which  is  partially  changed  into  an  umbel ;  one  rather  more  out  of  the 
centre,  which  is  so  far  metamorphosed  as  to  be  more  an  umbel  than 
an  umbellule  :  nine  peripheral  clusters  formed  by  the  development 
of  umbellules  into  umbels,  some  of  which  are  partially  compounded 
still  further.  Examined  in  detail,  these  structures  present  the  fol- 
lowing facts : — 1.  The  innermost  umbellule  is  normal,  save  in  having 
a  peripheral  flower  of  which  one  member  (apparently  a  petal)  is 
transformed  into  a  flower-bud.  2.  The  next  umbellule,  not  quite  so 
central,  has  one  of  its  peripheral  flowers  made  monstrous  by  the 
growth  of  a  bud  from  the  base  of  the  calyx.  3.  The  third  of  the 
central  umbellules  has  two  abnormal  outer  flowers.  One  of  them 
carries  a  flower-bud  on  its  edge,  in  place  of  a  foliar  member. 
The  other  is  half  flower  and  half  umbellule :  being  composed  of 
three  petals,  three  stamens,  and  five  flower-buds  growing  where 
the  other  petals  and  stamens  should  grow.  4.  Outside  of  these 
umbellules  comes  one  of  the  mixed  clusters.  Its  five  central 
flowers  are  normal.  Surrounding  these  are  several  flowers  trans- 
formed in  different  degrees :  one  having  a  stamen  partially  changed 
into  a  flower  bud.  And  then,  at  the  periphery  of  this  mixed 
cluster,  come  three  complete  umbellules  and  an  incomplete  one  in 
which  some  petals  and  stamens  of  the  original  flower  remain. 
5.  A  mixed  cluster,  in  which  the  umbel-structure  predominates, 
stands  next.  Its  three  central  flowers  are  normal.  Surrounding 
them  are  five  flowers  over-developed  in  various  ways,  like  those 
already  described.  And  on  its  periphery  arc  seven  complete  um- 
bellules in  place  of  flowers ;  besides  an  incomplete  umbellule  that 
contains  traces  of  the  original  flower,  one  of  them  being  a  petal 
imperfectly  twisted  up  into  a  bud.  6.  Of  the  nine  external 
clusters,  in  which  the  development  of  simple  into  compound  um- 
bels is  most  decided,  nearly  all  present  anomalies.  Three  of  them 
have  each  a  central  flower  untransformed ;  and  in  others,  the  central 

*  For  the  information  of  those  who  may  wish  to  examine  metamorphoses 
of  these  kinds,  I  may  here  state  that  I  have  found  nearly  all  the  examples 
described,  in  the  neighbourhood  of  the  sea — the  last-named,  on  the  shore  of 
Locheil,  near  Fort  William.  Whether  it  is  that  I  have  sought  more  dili- 
gently for  cases  when  in  such  localities,  or  whether  it  is  that  the  sea-air 
favours  that  excessive  nutrition  whence  these  transformations  result,  I  am 
unable  to  say. 


SUBSTITUTION  OF  AXIAL  FOR  FOLIAR  ORGANS.   54.3 

umbellule  is  composed  of  two,  three,  or  four  flowers.  7.  But  the 
most  remarkable  fact  is,  that  in  sundry  of  these  peripheral  clusters, 
resulting  from  the  metamorphosis  of  simple  umbels  into  compound 
umbels,  the  like  metamorphosis  is  carried  a  stage  higher.  Some  of 
the  component  rays,  are  themselves  the  bearers  of  compound  umbels 

€ 


instead  of  simple  umbels.  In  Fig.  70,  a  portion  of  the  dried  speci- 
men is  represented.  Two  of  the  central  umbellules  are  marked  a 
and  b  ;  those  marked  c  and  d  are  mixed  clusters;  at  e  and /are 
compound  umbels  replacing  simple  ones ;  and  g  shows  one  of  the 
rays  on  which  the  over-development  goes  still  further. 

Does  not  this  evidence,  enforced  as  it  is  by  much  more  of  like 
kind,  go  far  to  prove  that  foliar  organs  may  be  developed  into  axial 
organs  ?  Even  were  not  the  transitional  forms  traceable,  there  would 
still,  I  think,  be  no  other  legitimate  interpretation  of  the  facts  last 
detailed.  The  only  way  of  eluding  the  conclusion  here  drawn,  is  by 
assuming  that  where  a  cluster  of  flowers  replaces  a  single  flower,  it 
is  because  the  axillary  buds  which  hypothetically  belong  to  the 
several  foliar  organs  of  the  flower,  become  developed  into  axes ;  and 
assuming  this,  is  basing  an  hypothesis  on  another  hypothesis  that  is 
directly  at  variance  with  facts.  The  foliar  organs  of  flowers  do  not 
bear  buds  in  their  axils ;  and  it  would  never  have  been  supposed 
that  such  buds  are  typically  present,  had  it  not  been  for  that 
mistaken  conception  of  "  type  "  which  has  led  to  many  other  errors 
in  Biology.  Goethe  writes :  "  Now  as  we  cannot  realize  the  idea 
of  a  leaf  apart  from  the  node  out  of  which  it  springs,  or  of  a  node 
without  a  bud,  we  may  venture  to  infer,"  &c.  See  here  an  example 
of  a  method  of  philosophizing  not  uncommon  among  the  Germans. 


544  APPENDIX  A. 

The  method  is  this — Survey  a  portion  of  the  facts,  and  draw  from 
them  a  general  conception ;  project  this  general  conception  back 
into  the  objective  world,  as  a  mould  in  which  Nature  casts  her 
products ;  expect  to  find  it  everywhere  fulfilled  ;  and  allege  poten- 
tial fulfilment  where  no  actual  fulfilment  is  visible. 

If  instead  of  imposing  our  ideal  forms  on  Nature,  we  are  con- 
tent to  generalize  the  facts  as  Nature  presents  them,  we  shall  find 
no  warrant  for  the  morphological  doctrine  above  enunciated.  The 
only  conception  of  type  justified  by  the  logic  of  science,  is — that 
correlation  of  parts  which  remains  constant  under  all  modifications 
of  the  structure  to  be  defined.  To  ascertain  this,  we  must  compare 
all  these  modifications,  and  note  what  traits  are  common  to  them. 
On  doing  so  with  the  successive  segments  of  a  phfenogamic  axis, 
we  are  brought  to  a  conclusion  widely  different  from  that  of  Goethe. 
Axillary  buds  are  almost  universally  absent  from  the  cotyledons ; 
they  are  habitually  present  in  the  axils  of  fully-developed  leaves 
higher  up  the  axis  ;  they  are  often  absent  from  leaves  that  are  close 
to  the  flower ;  they  are  nearly  always  absent  from  the  bracts ;  absent 
from  the  sepals ;  absent  from  the  petals ;  absent  from  the  stamens ; 
absent  from  the  carpels.  Thus,  out  of  eight  leading  forms  which 
folia  assume,  one  has  the  axillary  bud  and  seven  are  without  it. 
With  these  facts  before  us,  it  seems  to  me  not  difficult  to  "  realize 
the  idea  "  "  of  a  node  without  a  bud."  If  we  are  not  possessed 
by  a  foregone  conclusion,  the  evidence  will  lead  us  to  infer,  that 
each  node  bears  a  foliar  appendage  and  may  bear  an  axillary  bud. 

Even,  however,  were  it  granted  that  the  typical  segment  of  a 
Phaenogam  includes  an .  axillary  bud,  which  must  be  regarded  as 
always  potentially  present,  no  legitimate  counter-interpretation  of 
the  montrosities  above  described  could  thence  be  drawn.  If  when 
an  umbellule  is  developed  in  place  of  a  flower,  the  explanation  is, 
that  its  component  rays  are  axillary  to  the  foliar  organs  of  the 
flower  superseded  ;  we  may  fairly  require  that  these  foliar  organs  to 
which  they  are  axillary,  shall  be  shown.  But  there  are  none.  In 
the  last  specimen  figured,  the  inner  rays  of  each  such  umbellule  are 
without  them ;  most  of  the  outer  rays  are  also  without  them  ;  and 
in  one  cluster,  only  a  single  ray  has  a  bract  at  its  point  of  origin. 
There  is  a  rejoinder  ready,  however :  the  foliar  organs  are  said  to 
be  suppressed.  Though  Goethe  could  not  "  realize  the  idea  "  "  of 
a  node  without  a  bud,"  those  who  accept  his  typical  form  appear  to 
find  no  difficulty  in  realizing  the  idea  of  an  axillary  bud  without 
anything  to  which  it  is  axillary.  But  letting  this  pass,  suppose  we 
ask  what  is  the  warrant  for  this  assumed  suppression.  Axillary 
buds  normally  occur  where  the  nutrition  is  high  enough  to  produce 
fully-developed  leaves ;  and  when  axillary  buds  are  demonstrably 
present  in  flowers,  they  accompany  foliar  organs  that  are  more  leaf- 
like  than  usual — always  greener  if  not  always  larger.  That  is  to 


SUBSTITUTION  OP  AXIAL  FOR  FOLIAR  ORGANS.   545 

say,  the  normal  and  the  abnormal  axillary  buds,  are  alike  the  con- 
comitants of  foliar  organs  coloured  by  that  chlorophyll  which 
habitually  favours  foliar  development.  How,  then,  can  it  be  sup- 
posed that  when,  out  of  a  flower  there  is  developed  a  cluster  of 
flower-bearing  rays,  the  implied  excess  of  nutrition  causes  the  foliar 
organs  to  abort  ?  It  is  true  that  very  generally  in  a  branched  in- 
florescence, the  bracts  of  the  several  flower- branches  are  very  small 
(their  smallness  being  probably  due  to  that  defective  supply  of 
certain  chlorophyll-forming  matters,  which  is  the  proximate  cause 
of  flowering) ;  and  it  is  true  that,  under  these  conditions,  a  flower- 
ing axis  of  considerable  size,  for  the  development  of  which  chloro- 
phyll is  less  needful,  grows  from  the  axil  of  a  dwarfed  leaf.  But 
the  inference  that  the  foliar  organ  may  therefore  be  entirely  sup- 
pressed, seems  to  me  irreconcilable  with  the  fact,  that  the  foliar 
organ  is  always  developed  to  some  extent  before  the  axillary  bud 
appears.  Until  it  has  been  shown  that  in  some  cases  a  lateral  bud 
first  appears,  and  a  foliar  organ  afterwards  grows  out  beneath  it,  to 
form  its  axil,  the  conception  of  an  axillary  bud  of  which  the  foliar 
organ  is  suppressed,  will  remain  at  variance  with  the  established 
truths  of  development. 


The  above  originally  formed  a  portion  of  §  190.  I  have  trans- 
ferred it  to  the  Appendix,  partly  because  it  contains  too  much 
detail  to  render  it  tit  for  the  general  argument,  and  partly  because 
the  interpretations  being  open  to  some  question,  it  seemed  unde- 
sirable to  risk  compromising  that  argument  by  including  them. 
The  criticisms  passed  upon  these  interpretations  have  not,  how- 
ever, sufficed  to  convince  me  of  their  incorrectness.  Unfortu- 
nately, I  have  since  had  no  opportunity  of  verifying  the  above 
statements  by  microscopic  examinations,  as  I  had  intended. 

Though  unable  to  enforce  the  inference  drawn  by  further  facts 
more  minutely  looked  into,  I  may  add  some  arguments  based  on 
facts  that  are  well  known.  One  of  these  is  the  fact  that  the  so- 
called  axillary  bud  is  not  universally  axillary — is  not  universally 
seated  in  the  angle  made  by  the  axis  and  an  appended  foliar  organ. 
In  certain  plants  the  axillary  bud  is  placed  far  above  the  node, 
half-way  between  it  and  the  succeeding  node.  So  that  not  only  may 
a  segment  of  a  phsenogamic  axis  be  without  the  axillary  bud,  but 
the  axillary  bud,  when  present,  may  be  removed  from  that  place  in 
which,  according  to  Goethe,  it  necessarily  exists.  Another  fact  not 
congruous  with  the  current  doctrine,  is  the  common  occurrence  of 
"  adventitious  "  buds — the  buds  that  are  put  out  from  roots  and  from 
old  stems  or  branches  bare  of  leaves.  The  name  under  which  they  are 
thus  classed,  is  meant  to  imply  that  they  may  be  left  out  of  conside- 
ration. Those,  however,  who  have  not  got  a  theory  to  save  by 
81 


546  APPENDIX  A. 

putting  anomalies  out  of  sight,  may  be  inclined  to  think  that  the 
occurrence  of  buds  where  they  are  avowedly  unconnected  with 
nodes,  and  are  axillary  to  nothing,  tells  very  much  against  the  as- 
sumption that  every  bud  implies  a  node  and  a  corresponding  foliar 
organ.  And  they  may  also  see  that  the  development  of  these  ad- 
ventitious buds  at  places  where  there  is  excess  of  nutritive  mate- 
rials, favours  the  view  above  set  forth.  For  if  a  bud  thus  arises  at 
a  place  where  it  is  not  morphologically  accounted  for,  simply  because 
there  happens  to  be  at  that  place  an  abundance  of  unorganized  pio- 
toplasm ;  then,  clearly,  it  is  likely  that  if  the  mass  of  protoplasm 
from  which  a  leaf  would  usually  arise,  is  greatly  increased  in  mass 
by  excess  of  nutrition,  it  may  develop  into  an  axis  instead  of  a  leaf. 

Many  years  after  this  work  was  published,  I  discovered  among 
my  papers  a  memorandum  which  unfortunately  I  had  overlooked, 
containing  further  evidence  in  support  of  the  foregoing  conclusion. 
With  the  omission  of  an  error  concerning  the  species  of  plant,  I 
reproduce  this  memorandum  just  as  it  stood : — 

"  I  found  at  Dieppe,  July  1,  1860,  in  a  garden  near  the  sea  a 
sample  of  cultivated  wild  flower  (I  thought  it  was  grown  as  an 
ornamental  flower)  in  which  some  of  the  single  flowers  of  the 
umbel  were  developed  into  groups  of  flowers  thus : — 


"  In  the  case  where  the  transformation  was  fully  effected  the 
umbellule  had  six  flowers,  answering  to  the  six  petals  of  the 
original  flowers.  In  other  cases  the  transformation  was  incom- 
plete. There  were  instances  where  but  two  of  the  petals  were 
developed  into  flowers ;  and  the  other  petals  remained  unchanged. 
Others  in  which  three  were  developed ;  and  others  where  four 
were  developed.  In  some  cases,  too,  the  development  of  a  petal 
into  a  flower  was  imperfect,  in  the  absence  of  the  flower  stalk — 
the  flowers  were  sessile  in  the  place  where  the  petals  would 
have  been.  In  one  case  there  was  an  imperfect  flower  sessile ; 
another  imperfect  flower  on  a  short  stalk;  and  three  perfect 
flowers  on  loner  stalks. 


SUBSTITUTION  OF  AXIAL  FOR  FOLIAR  ORGANS.   547 

"  I  was  in  some  doubt  whether  the  petals  or  the  stamens  were 
developed.  In  cases  of  imperfect  transformation  the  petals  at 
the  base  of  the  umbellule  seemed  to  stand  in  the  position  of 
calyx  or  involucrum,  giving  the  idea  that  the  stamens  were  de- 
veloped into  flowers.  But  in  the  case  where  there  were  six  flowers 
developed  there  were  no  petals  at  the  base. 

"  That  it  was  a  matter  of  extra  nutrition  was  shown  by  this : — 

"  1.  That  they  were  cultivated  as  garden  flowers. 

"  2.  That  where  there  was  one  perfectly  developed  umbellule, 
it  was  the  only  one  in  the  umbel. 

"  3.  That  where  there  were  three  umbellules  they  were  all  im- 
perfect. 

"  4.  That  in  this  imperfect  umbellule  the  perfect  flowers  were 
on  long  stalks  and  the  imperfect  ones  sessile. 

"  5.  That  the  umbellules  were  on  stalks  both  longer  and 
thicker  than  those  of  single  flowers." 

[Concerning  the  foregoing  argument  at  large  an  expert 
writes  : — "  The  abnormalities  you  describe  certainly  show  that 
an  axis  may  arise  abnormally  in  the  place  of  a  normal  leaf- 
structure,  and  every  modern  botanist  would  be  in  agreement 
with  you  in  your  criticism  of  the  older  form  of  the  doctrine  of 
axillary  buds.  I  think  we  are  largely  emancipated  from  the 
dextrous  juggling  with  the  arrangements  and  relations  of  organs 
which  used  to  pass  current  as  morphology. 

"You  have  quoted  sufficient  evidence  in  the  text  (§  190)  to 
establish  the  conclusion  that  no  sharp  line  can  be  drawn  between 
axes  and  leaf-structure ;  and  a  very  great  deal  more  could  be 
added  in  the  same  sense.  Petioles  for  instance,  exist  which  the 
most  highly  trained  histological  observer  could  not  distinguish 
from  stems. 

"  But  I  must  demur  to  the  suggestion  that  the  replacement  of 
one  by  the  other  is  primarily  a  question  of  nutrition.  We  are 
as  ignorant  as  ever  of  the  proximate  cause  of  the  production 
of  a  leaf  or  a  shoot  at  a  certain  spot  in  meristematic  tissue." 

To  this  last  remark  I  had  at  first  made  only  the  reply  that  the 
plants  exhibiting  the  abnormalities  were  in  all  cases  excessively 
luxuriant  in  their  growths;  but  to  this  I  am  now  able  to  add  a 
more  definite  reply.  The  expert  from  whom  I  have  just  quoted, 
had  read  this  appendix  before  there  had  been  made  to  it  the 
above  addition  describing  the  flower  from  Dieppe ;  and  I  was  not 
myself  aware,  until  I  came  to  read  over  this  addition,  what  clear 
evidence  it  contains  that  extra  nutrition  was  the  cause  of  these 
transformations  of  foliar  structures  into  axial  structures;  but 
the  above  paragraphs  1,  2,  3,  4,  5,  contain  different  evidences 
conspiring  to  prove  this.] 


APPENDIX    B. 


A   CRITICISM   ON  PROF.   OWEN'S  THEORY   OF  THE 
VERTEBRATE  SKELETON. 

[From  the  BRITISH  &  FOREIGN  MEDICO-CHIRURGICAL  REVIEW  FOR  OCT.,  1858.] 


I.  On  the  Archetype  and  Homologies  of  the  Vertebrate  Skeleton. 
By  RICHARD  OWEN,  F.R.S. — London,  1848.  pp.  172. 

IT.  Principes  dj  Osteologie  Comparee,  ou  Recherches  sur  V Arche- 
type et  les  Homologies  du  Squelette  Vertebre.  Par  RICHARD 
OWEN. — Paris. 

Principles  of  Comparative  Osteology  ;  or,  Researches  on  the  Arche- 
type and  the  Homologies  of  the  Vertebrate  Skeleton.  By  RICHARD 
OWEN. 

III.  On  the  Nature  of  Limbs.  A  Discourse  delivered  on  Friday, 
February  9,  at  an  Evening  Meeting  of  the  Royal  Institution  of 
Great  Britain.  By  RICHARD  OWEN,  F.R.S.— London,  1849. 
pp.  119. 

JUDGING  whether  another  proves  his  position  is  a  widely  different 
thing  from  proving  your  own.  To  establish  a  general  law  requires 
an  extensive  knowledge  of  the  phenomena  to  he  generalized ;  hut  to 
decide  whether  an  alleged  general  law  is  established  by  the  evidence 
assigned,  requires  merely  an  adequate  reasoning  faculty.  Especially 
is  such  a  decision  easy  where  the  premises  do  not  warrant  the  con- 
clusion. It  may  be  dangerous  for  one  who  has  but  little  previous 
acquaintance  with  the  facts,  to  say  that  a  generalization  is  demon- 
strated ;  seeing  that  the  argument  may  be  one-sided :  there  may  be 
many  facts  unknown  to  him  which  disprove  it.  But  it  is  not 
dangerous  to  give  a  negative  verdict  when  the  alleged  demonstra- 
548 


A  CRITICISM  ON  PROF.   OWEN'S  THEORY.          549 

tion  is  manifestly  insufficient.  If  the  data  put  before  him  do  not 
bear  out  the  inference,  it  is  competent  for  every  logical  reader  to 
say  so. 

From  this  stand-point,  then,  we  venture  to  criticize  some  of 
Professor  Owen's  osteological  theories.  For  his  knowledge  of 
comparative  osteology  we  have  the  highest  respect.  We  believe 
that  no  living  man  has  so  wide  and  detailed  an  acquaintance  with 
the  bony  structure  of  the  Vertebrata.  Indeed,  there  probably  has 
never  been  any  one  whose  information  on  the  subject  was  so  nearly 
exhaustive.  Moreover,  we  confess  that  nearly  all  we  know  of  this 
department  of  biology  has  been  learnt  from  his  lectures  and  writ- 
ings. We  pretend  to  no  independent  investigations,  but  merely  to 
such  knowledge  of  the  phenomena  as  he  has  furnished  us  with. 
Our  position,  then,  is  such  that,  had  Professor  Owen  simply  enun- 
ciated his  generalizations,  we  should  have  accepted  them  on  his 
authority.  But  he  has  brought  forward  evidence  to  prove  them. 
By  so  doing  he  has  tacitly  appealed  to  the  judgments  of  his  readers 
and  hearers — has  practically  said,  "  Here  are  the  facts  ;  do  they 
not  warrant  these  conclusions  ? "  And  all  we  propose  to  do,  is  to 
consider  whether  the  conclusions  are  warranted  by  the  facts  brought 
forward. 

Let  us  first  limit  the  scope  of  our  criticisms.  On  that  division 
of  comparative  osteology  which  deals  with  what  Professor  Owen 
distinguishes  as  "  special  homologies,"  we  do  not  propose  to  enter. 
That  the  wing  of  a  bird  is  framed  upon  bones  essentially  parallel  to 
those  of  a  mammal's  fore-limb ;  that  the  cannon-bone  of  a  horse's 
leg  answers  to  the  middle  metacarpal  of  the  human  hand ;  that 
various  bones  in  the  skull  of  a  fish  are  homologous  with  bones  in 
the  skull  of  a  man — these  and  countless  similar  facts,  we  take  to  be 
well  established.  It  may  be,  indeed,  that  the  doctrine  of  special 
homologies  is  at  present  carried  too  far.  It  may  be  that,  just  as 
the  sweeping  generalization  at  one  time  favoured,  that  the  embryonic 
phases  of  the  higher  animals  represent  the  adult  forms  of  lower 
ones,  has  been  found  untrue  in  a  literal  sense,  and  is  acceptable 
only  in  a  qualified  sense ;  so  the  sweeping  generalization  that  the 
skeletons  of  all  vertebrate  animals  consist  of  homologous  parts, 
will  have  to  undergo  some  modification.  But  that  this  generaliza- 
tion is  substantially  true,  all  comparative  anatomists  agree. 

The  doctrine  which  we  are  here  to  consider,  is  quite  a  separate 
one — that  of  "  general  homologies."  The  truth  or  falsity  of  this 
may  be  decided  on  quite  apart  from  that  of  the  other.  Whether 
certain  bones  in  one  vertebrate  animal's  skeleton  correspond  with 
certain  bones  in  another's,  or  in  every  other's,  is  one  question ;  and 
whether  the  skeleton  of  every  vertebrate  animal  is  divisible  into  a 
series  of  segments,  each  of  which  is  modelled  after  the  same  type, 
is  another  question.  While  the  first  is  answered  in  the  affirmative, 


550  APPENDIX  B. 

the  last  may  be  answered  in  the  negative ;  and  we  propose  to  give 
reasons  why  it  should  be  answered  in  the  negative. 

In  so  far  as  his  theory  of  the  skeleton  is  concerned,  Professor 
Owen  is  an  avowed  disciple  of  Plato.  At  the  conclusion  of  his 
Archetype  and  Homologies  of  the  Vertebrate  Skeleton,  he  quotes  ap- 
provingly the  Platonic  hypothesis  of  tSt'cu,  "  a  sort  of  models,  or 
moulds  in  which  matter  is  cast,  and  which  regularly  produce  the 
same  number  and  diversity  of  species."  The  vertebrate  form  in 
general  (see  diagram  of  the  Archetypus),  or  else  the  form  of  each 
kind  of  vertebrate  animal  (see  p.  172,  where  this  seems  implied), 
Professor  Owen  conceives  to  exist  as  an  u  idea  " — an  "  arche- 
typal exemplar  on  which  it  has  pleased  the  Creator  to  frame 
certain  of  his  living  creatures."  Whether  Professor  Owen  holds 
that  the  typical  vertebra  also  exists  as  an  "  idea,"  is  not  so 
certain.  From  the  title  given  to  his  figure  of  the  "  ideal  typical 
vertebra,"  it  would  seem  that  he  does;  and  at  p.  40  of  his 
Nature  of  Limbs,  and  indeed  throughout  his  general  argument,  this 
supposition  is  implied.  But  on  the  last  two  pages  of  the  Archetype 
and  Homologies,  it  is  distinctly  alleged  that  "  the  repetition  of  simi- 
lar segments  in  a  vertebral  column,  and  of  similar  elements  in  a 
vertebral  segment,  is  analogous  to  the  repetition  of  similar  crystals 
as  the  result  of  polarizing  force  in  the  growth  of  an  inorganic 
body ;  "  it  is  pointed  out  that,  "  as  we  descend  the  scale  of  animal 
life,  the  forms  of  the  repeated  parts  of  the  skeleton  approach  more 
and  more  to  geometrical  figures ; "  and  it  is  inferred  that  "  the 
Platonic  iSe'a  or  specific  organizing  principle  or  force,  would  seem 
to  be  in  antagonism  with  the  general  polarizing  force,  and  to  sub- 
due and  mould  it  in  subserviency  to  the  exigencies  of  the  resulting 
specific  form."  If  Professor  Owen's  doctrine  is  to  be  understood 
as  expressed  in  these  closing  paragraphs  of  his  Archetype  and  Homo- 
logies— if  he  considers  that  "  the  iSe'a  "  "  which  produces  the  diver- 
sity of  form  belonging  to  living  bodies  of  the  same  materials,"  is 
met  by  the  "  counter-operation  "  of  "  the  polarizing  force  pervading 
all  space,"  which  produces  "  the  similarity  of  forms,  the  repetition 
of  parts,  the  signs  of  unity  of  organization,"  and  which  is  "  sub- 
dued" as  we  ascend  "  in  the  scale  of  being ; "  then  we  may  pass  on 
with  the  remark  that  the  hypothesis  is  too  cumbrous  and  involved 
to  have  much  vraisemblance.  If,  on  the  other  hand,  Professor  Owen 
holds,  as  every  reader  would  suppose  from  the  general  tenor  of  his 
reasonings,  that  not  only  docs  there  exist  an  archetypal  or  ideal 
vertebrate  skeleton,  but  that  there  also  exists  an  archetypal  or 
ideal  vertebra;  then  he  carries  the  Platonic  hypothesis  much 
further  than  Plato  does.  Plato's  argument,  that  before  any  species 
of  object  was  created  it  must  have  existed  as  an  idea  of  the  Creative 
Intelligence,  and  that  hence  all  objects  of  such  species  must  bo 
• 


A  CRITICISM  ON  PROP.   OWEN'S  THEORY.          551 

copies  of  this  original  idea,  is  tenable  enough  from  the  anthropo- 
morphic point  of  view.  But  while  those  who,  with  Plato,  think  tit 
to  base  their  theory  of  creation  upon  the  analogy  of  a  carpenter 
designing  and  making  a  table,  must  yield  assent  to  Plato's  inference, 
they  are  by  no  means  committed  to  Professor  Owen's  expansion  of 
it.  To  say  that  before  creating  a  vertebrate  animal,  God  must 
have  had  the  conception  of  one,  does  not  involve  saying  that  God 
gratuitously  bound  himself  to  make  a  vertebrate  animal  out  of  seg- 
ments all  moulded  after  one  pattern.  As  there  is  no  conceivable 
advantage  in  this  alleged  adhesion  to  a  fundamental  pattern — as, 
for  the  fulfilment  of  the  intended  ends,  it  is  not  only  needless,  but 
often,  as  Professor  Owen  argues,  less  appropriate  than  some  other 
construction  would  be  (see  Nature  of  Limbs,  pp.  39,  40),  to  sup- 
pose the  creative  processes  thus  regulated,  is  not  a  little  startling. 
Even  those  whose  conceptions  are  so  anthropomorphic  as  to  think 
they  honour  the  Creator  by  calling  him  "  the  Great  Artificer,"  will 
scarcely  ascribe  to  him  a  proceeding  which,  in  a  human  artificer, 
they  would  consider  a  not  very  worthy  exercise  of  ingenuity. 

'But  whichever  of  these  alternatives  Professor  Owen  contends 
for — whether  the  typical  vertebra  is  that  more  or  less  crystalline 
figure  which  osseous  matter  ever  tends  to  assume  in  spite  of  "  the 
tSeo.  or  organizing  principle,"  or  whether  the  typical  vertebra  is 
itself  an  "  iSea  or  organizing  principle  " — there  is  alike  implied 
the  belief  that  the  typical  vertebra  has  an  abstract  existence  apart 
from  actual  vertebrae.  It  is  a  form  which,  in  every  endoskeleton, 
strives  to  embody  itself  in  matter — a  form  which  is  potentially 
present  in  each  vertebra ;  which  is  manifested  in  each  vertebra 
with  more  or  less  clearness ;  but  which,  in  consequence  of  antago- 
nizing forces,  is  nowhere  completely  realized.  Apart  from  the 
philosophy  of  this  hypothesis,  let  us  here  examine  the  evidence 
which  is  thought  to  justify  it. 

And  first  as  to  the  essential  constituents  of  the  "  ideal  typical 
vertebra."  Exclusive  of  "diverging  appendages"  which  it  "may 
also  support,"  "it  consists  in  its  typical  completeness  of  the  follow- 
ing elements  and  parts  "  : — A  centrum  round  which  the  rest  are 
arranged  in  a  somewhat  radiate  manner;  above  it  two  neurapopkyscs 
— converging  as  they  ascend,  and  forming  with  the  centrum  a  trian- 
guloid  space  containing  the  neural  axis ;  aneural  spine,  surmounting 
the  two  neurapophyses,  and  with  them  completing  the  neural  arch  ; 
below  the  centrum  two  hcemapophyses  and  a  hcemal  spine,  forming  a 
haemal  arch  similar  to  the  neural  arch  above,  and  enclosing  the 
hjpmal  axis;  two  pleurapophyses  radiating  horizontally  from  the 
sides  of  the  centrum ;  and  two  parapophyses  diverging  from  the 
centrum  below  the  pleurapophyses.  "  These,"  says  Professor 
Owen,  "  being  usually  developed  from  distinct  and  independent 


552  APPENDIX  B. 

centres,  I  have  termed  '  autogenous  elements.'  "  The  remaining 
elements,  which  he  classes  as  "  exogenous,"  because  they  "  shoot 
out  as  continuations  from  some  of  the  preceding  elements,"  are 
the  diapophyses  diverging  from  the  upper  part  of  the  centrum  as 
the  parapophyses  do  below,  and  the  zygapophyses  which  grow  out 
of  the  distal  ends  of  the  neurapophyses  and  htcmapophyses. 

If,  now,  these  are  the  constituents  of  the  vertebrate  segment  "  in 
its  typical  completeness ; "  and  if  the  vertebrate  skeleton  consists 
of  a  succession  of  such  segments ;  we  ought  to  have  in  these  con- 
stituents, representatives  of  all  the  elements  of  the  vertebrate 
skeleton — at  any  rate,  all  its  essential  elements.  Are  we  then  to 
conclude  that  the  "  diverging  appendages,"  which  Professor  Owen 
regards  as  rudimental  limbs,  and  from  certain  of  which  he  considers 
actual  limbs  to  be  developed,  are  typically  less  important  than  some 
of  the  above-specified  exogenous  parts — say  the  zygapophyses  ? 

That  the  meaning  of  this  question  may  be  understood,  it  will  be 
needful  briefly  to  state  Professor  Owen's  theory  of  The  Nature  of 
Limbs ,'  and  such  criticisms  as  we  have  to  make  on  it  must  be  in- 
cluded in  the  parenthesis.  In  the  first  place,  he  aims  to  show  that 
the  scapular  and  pelvic  arches,  giving  insertion  to  the  fore  and  hind 
limbs  respectively,  are  displaced  and  modified  haemal  arches, 
originally  belonging  in  the  one  case  to  the  occipital  vertebra,  and  in 
the  other  case  to  some  trunk- vertebra  not  specified.  In  support  of 
this  assumption  of  displacement,  carried  in  some  cases  to  the  extent 
of  twenty-seven  vertebra?,  Professor  Owen  cites  certain  acknow- 
ledged displacements  which  occur  in  the  human  skeleton  to  the  ex- 
tent of  half  a  vertebra — a  somewhat  slender  justification.  But  for 
proof  that  such  a  displacement  has  taken  place  in  the  scapular  arch, 
he  chiefly  relies  on  the  fact  that  in  fishes,  the  pectoral  fins,  which 
are  the  homologues  of  the  fore-limbs,  are  directly  articulated  to 
certain  bones  at  the  back  of  the  head,  which  he  alleges  are  parts 
of  the  occipital  vertebra.  This  appeal  to  the  class  of  fishes  is 
avowedly  made  on  the  principle  that  these  lowest  of  the  Vertebrata 
approach  closest  to  archetypal  regularity,  and  may  therefore  be 
expected  to  show  the  original  relations  of  the  bones  more  nearly. 
Simply  noting  the  facts  that  Professor  Owen  does  not  give  us  any 
transitional  forms  between  the  alleged  normal  position  of  the 
scapular  arch  in  fishes,  and  its  extraordinary  displacement  in  the 
higher  Vertebrata  ;  and  that  he  makes  no  reference  to  the  embryonic 
phases  of  the  higher  Vertebrata,  which  might  be  expected  to  ex- 
hibit the  progressive  displacement ;  we  go  on  to  remark  that,  in 
the  case  of  the  pelvic  arch,  he  abandons  his  principle  of  appealing 
to  the  lowest  vertebrate  forms  for  proof  of  the  typical  structure. 
In  fishes,  the  rudimentary  pelvis,  widely  removed  from  the  spinal 
column,  shows  no  signs  of  having  belonged  to  any  vertebra ;  and 
here  Professor  Owen  instances  the  perennibranchiate  Batrachia  as 


A  CRITICISM  ON  PROF.  OWEN'S  THEORY.          553 

exhibiting  the  typical  structure  :  remarking  that  "  mammals,  birds, 
and  reptiles  show  the  rule  of  connexion,  and  fishes  the  exception." 
Thus  in  the  case  of  the  scapular  arch,  the  evidence  afforded  by 
fishos  is  held  of  great  weight,  because  of  their  archetypal  regularity ; 
while  in  the  case  of  the  pelvic  arch,  their  evidence  is  rejected  as 
exceptional.  But  now,  having,  as  he  considers,  shown  that  these 
bony  frames  to  which  the  limbs  are  articulated  are  modified  haemal 
arches,  Professor  Owen  points  out  that  the  haemal  arches  habitually 
bear  certain  "  diverging  appendages  ;  "  and  he  aims  to  show  that 
the  "  diverging  appendages  "  of  the  scapular  and  pelvic  arches  re- 
spectively, are  developed  into  the  fore  and  hind  limbs.  There  are 
several  indirect  ways  in  which  we  may  test  the  probability  of  this 
conclusion.  If  these  diverging  appendages  are  "  rudimental  limbs  " 
— "  future  possible  or  potential  arms,  legs,  wings,  or  feet,"  we  may 
fairly  expect  them  always  to  bear  to  the  haemal  arches  a  relation 
such  as  the  limbs  do.  But  they  by  no  means  do  this.  "  As  the 
vertebras  approach  the  tail,  these  appendages  are  often  transferred 
gradually  from  the  pleurapophysis  to  the  parapophysis,  or  even  to 
the  centrum  and  neural  arch."  (Arch,  and  Horn.,  p.  93.)  Again, 
it  might  naturally  be  assumed  that  in  the  lowest  vertebrate  forms, 
where  the  limbs  are  but  little  developed,  they  would  most  clearly 
display  their  alliance  with  the  appendages,  or  "  rudimental  limbs," 
by  the  similarity  of  their  attachments.  Instead  of  this,  however, 
Professor  Owen's  drawings  show  that  whereas  the  appendages  are 
habitually  attached  to  the  pleurapophyses,  the  limbs,  in  their  earliest 
and  lowest  phase,  alike  in  fishes  and  in  the  Lepidosiren,  are  articu- 
lated to  the  haemapophyses.  Most  anomalous  of  all,  however,  is 
the  process  of  development.  When  we  speak  of  one  thing  as  being 
developed  out  of  another,  we  imply  that  the  parts  next  to  the  germ 
are  the  first  to  appear,  and  the  most  constant.  In  the  evolution  of 
a  tree  out  of  a  seed,  there  come  at  the  outset  the  stem  and  the 
radicle ;  afterwards  the  branches  and  divergent  roots ;  and  still 
later  the  branchlets  and  rootlets ;  the  remotest  parts  being  the  latest 
and  most  inconstant.  If,  then,  a  limb  is  developed  out  of  a  "  di- 
verging appendage  "  of  the  haemal  arch,  the  earliest  and  most  con- 
stant bones  should  be  the  humerus  and  femur ;  next  in  order  of 
time  and  constancy  should  come  the  coupled  bones  based  on  these  ; 
while  the  terminal  groups  of  bones  should  be  the  last  to  make  their 
appearance,  and  the  most  liable  to  be  absent.  Yet,  as  Professor 
Owen  himself  shows,  the  actual  mode  of  development  is  the  very  re- 
verse of  this.  At  p.  1 6  of  the  Archetype  and  If  ontologies,  he  says : — 

"  The  earlier  stages  in  the  development  of  all  locomotive  extremities  are 
permanently  retained  or  represented  in  the  paired  fins  of  fishes.  First  the 
essential  part  of  the  member,  the  hand  or  foot,  appears :  then  the  fore-arm  or 
log,  both  much  shortened,  flattened,  and  expanded,  as  in  all  fins  and  all  em- 
bryonic rudiments  of  limbs  :  finally  come  the  humeral  and  femoral 
but  this  stage  I  have  not  found  attained  in  any  fish." 


554  APPENDIX  B. 

That  is  to  say,  alike  in  ascending  through  the  Vertebrata  gene- 
rally, and  in  tracing  up  the  successive  phases  of  a  mammalian  em- 
bryo, the  last-developed  and  least  constant  division  of  the  limb,  is 
that  basic  one  by  which  it  articulates  with  the  haemal  arch.  It 
seems  to  us  that,  so  far  from  proving  his  hypothesis,  Professor 
Owen's  own  facts  tend  to  show  that  limbs  do  not  belong  to  the 
vertebrae  at  all ;  that  they  make  their  first  appearance  peripherally  ; 
that  their  development  is  centripetal ;  and  that  they  become  fixed 
to  such  parts  of  the  vertebrate  axis  as  the  requirements  of  the  case 
determine. 

But  now,  ending  here  this  digressive  exposition  and  criticism, 
and  granting  the  position  that  limbs  "  are  developments  of  costal 
appendages,"  let  us  return  to  the  question  above  put — Why  are  not 
these  appendages  included  as  elements  of  the  "  ideal  typical  ver- 
tebra ?"  It  cannot  be  because  of  their  comparative  inconstancy  ; 
for  judging  from  the  illustrative  figures,  they  seem  to  be  as  con- 
stant as  the  haemal  spine,  which  is  one  of  the  so-called  autogenous 
elements :  in  the  diagram  of  the  Archetypus,  the  appendage  is  re- 
presented as  attached  to  every  vertebrate  segment  of  the  head  and 
trunk,  which  the  haemal  spine  is  not.  It  cannot  be  from  their  com- 
parative unimportance ;  seeing  that  as  potential  limbs  they  are 
essential  parts  of  nearly  all  the  Vertelrata — much  more  obviously 
so  than  the  diapophyses  are.  If,  as  Professor  Owen  argues,  "  the 
divine  mind  which  planned  the  archetype  also  foreknew  all  its 
modifications ; "  and  if,  among  these  modifications,  the  development 
of  limbs  out  of  diverging  appendages  was  one  intended  to  charac- 
terize all  the  higher  Vertebrata  ;  then,  surely,  these  diverging  ap- 
pendages must  have  been  parts  of  the  "  ideal  typical  vertebra." 
Or,  if  the  "  ideal  typical  vertebra  "  is  to  be  understood  as  a  crystal- 
line form  in  antagonism  with  the  organizing  principle ;  then  why 
should  not  the  appendages  be  included  among  its  various  offshoots  ? 
We  do  not  ask  this  question  because  of  its  intrinsic  importance. 
We  ask  it  for  the  purpose  of  ascertaining  Professor  Owen's  method 
of  determining  what  are  true  vertebral  constituents.  He  presents 
us  with  a  diagram  of  the  typical  vertebra,  in  which  are  included 
certain  bones,  and  from  which  are  excluded  certain  others.  If  re- 
lative constancy  is  the  criterion,  then  there  arises  the  question — 
What  degree  of  constancy  entitles  a  bone  to  be  included  ?  If  re- 
lative importance  is  the  criterion,  there  comes  not  only  the  question 
— What  degree  of  importance  suffices  ?  but  the  further  question 
— How  is  importance  to  be  measured  ?  If  neither  of  these  is  the 
criterion,  then  what  is  it  ?  And  if  there  is  no  criterion,  does  it 
not  follow  that  the  selection  is  arbitrary  ? 

This  question  serves  to  introduce  a  much  wider  one : — Has  the 
"  ideal  typical  vertebra  "  any  essential  constituents  at  all  ?  It  might 


A  CRITICISM   ON  PROF.   OWEN'S  THEORY.          555 

naturally  be  supposed  that  though  some  bones  are  so  rarely 
developed  as  not  to  seem  worth  including,  and  though  some  that 
are  included  are  very  apt  to  be  absent,  yet  that  certain  others  are 
invariable  :  forming,  as  it  were,  the  basis  of  the  ideal  type.  Let 
us  see  whether  the  facts  bear  out  this  supposition.  In  his  "  summary 
of  modifications  of  corporal  vertebrae  "  (p.  96),  Professor  Owen 
says — "  The  haemal  spine  is  much  less  constant  as  to  its  existence, 
and  is  subject  to  a  much  greater  range  of  variety,  when  present, 
than  its  vertical  homotype  above,  which  completes  the  neural  arch." 
Again  he  says — "  The  hcemapophyses,  as  osseous  elements  of  a 
vertebra,  are  less  constant  than  the  pleurapophyses."  And  again — 
"  The  pleurapophyses  are  less  constant  elements  than  the  neurapo- 
physes."  And  again — "  Amongst  air-breathing  vertebrates  the 
pleurapophyses  of  the  trunk  segments  are  present  only  in  those  spe- 
cies in  which  the  septum  of  the  heart's  ventricle  is  complete  and  im- 
perf  orate,  and  here  they  are  exogenous  and  confined  to  the  cervical 
and  anterior  thoracic  vertebrae."  And  once  more,  both  the  neura- 
pophyses  and  the  neural  spine  "  are  absent  under  both  histological 
conditions,  at  the  end  of  the  tail  in  most  air-breathing  vertebrates, 
where  the  segments  are  reduced  to  their  central  elements."  That 
is  to  say,  of  all  the  peripheral  elements  of  the  "  ideal  typical  ver- 
tebra," there  is  not  one  which  is  always  present.  It  will  be  ex- 
pected, however,  that  at  any  rate  the  centrum  is  constant :  the 
bone  which  "  forms  the  axis  of  the  vertebral  column,  and  com- 
monly the  central  bond  of  union  of  the  peripheral  elements  of  the 
vertebrate  (p.  97),  is  of  course  an  invariable  element.  No :  not 
even  this  is  essential. 

"  The  centrums  do  not  pass  beyond  the  primitive  stage  of  the  notochord 
(undivided  column)  in  the  existing  lepidosiren,  and  they  retained  the  like 
rudimental  state  in  every  fish  whose  remains  have  been  found^in  strata 
earlier  than  the  permian  aera  in  Geology,  though  the  number  of  vertebrae  is 
frequently  indicated  in  Devonian  and  Silurian  ichthyolites  by  the  fossilized 
neur-  and  hsemapophyses  and  their  spines  "  (p.  96). 

Indeed,  Professor  Owen  himself  remarks  that  "  the  neurapo- 
physes  are  more  constant  as  osseous  or  cartilaginous  elements  of  the 
vertebrae  than  the  centrums  "  (p.  97).  Thus,  then,  it  appears  that 
the  several  elements  included  in  the  "  ideal  typical  vertebra  "  have 
various  degrees  of  constancy,  and  that  no  one  of  them  is  essential. 
There  is  no  one  part  of  a  vertebra  which  invariably  answers  to  its 
exemplar  in  the  pattern-group.  How  does  this  fact  consist  with  the 
hypothesis  ?  If  the  Creator  saw  fit  to  make  the  vertebrate  skeleton 
out  of  a  series  of  segments,  all  formed  on  essentially  the  same  model 
— if,  for  the  maintenance  of  the  type,  one  of  these  bony  segments 
is  in  many  cases  formed  out  of  a  coalesced  group  of  pieces,  where, 
as  Professor  Owen  argues,  a  single  piece  would  have  served  as  well 
or  better;  then  we  ought  to  find  this  typical  repetition  of  parts 


556  APPENDIX  B. 

uniformly  manifested.  Without  any  change  of  shape,  it  would  ob- 
viously have  been  quite  possible  for  every  actual  vertebra  to  have 
contained  all  the  parts  of  the  ideal  one — rudimentally  where  they 
were  not  wanted.  Even  one  of  the  terminal  bones  of  a  mammal's 
tail  might  have  been  formed  out  of  the  nine  autogenous  pieces, 
united  by  suture  but  admitting  of  identification.  As,  however, 
there  is  no  such  uniform  typical  repetition  of  parts,  it  seems  to  us 
that  to  account  for  the  typical  repetition  which  does  occur,  by  sup- 
posing the  Creator  to  have  fixed  on  a  pattern- vertebra,  is  to  ascribe 
to  him  the  inconsistency  of  forming  a  plan  and  then  abandoning  it. 
If,  on  the  other  hand,  Professor  Owen  means  that  the  "  ideal 
typical  vertebra  "  is  a  crystalline  form  in  antagonism  with  "  the 
idea  or  organizing  principle  ;  "  then  we  might  fairly  expect  to  find 
it  most  clearly  displaying  its  crystalline  character,  and  its  full  com- 
plement of  parts,  in  those  places  where  the  organizing  principle 
may  be  presumed  to  have  "subdued"  it  to  the  smallest  extent. 
Yet  in  the  Vertebrata  generally,  and  even  in  Professor  Owen's 
Archetypus,  the  vertebrae  of  the  tail,  which  must  be  considered  as, 
if  anything,  less  under  the  influence  of  the  organizing  principle 
than  those  of  the  trunk,  do  not  manifest  the  ideal  form  more  com  - 
pletely.  On  the  contrary,  as  we  approach  the  end  of  the  tail,  the 
successive  segments  not  only  lose  their  remaining  typical  elements, 
but  become  as  uncrystalline-looking  as  can  be  conceived. 

Supposing,  however,  that  the  assumption  of  suppressed  or  unde- 
veloped elements  be  granted — supposing  it  to  be  consistent  with 
the  hypothesis  of  an  "  ideal  typical  vertebra,"  that  the  constituent 
parts  may  severally  be  absent  in  greater  or  less  number,  sometimes 
leaving  only  a  single  bone  to  represent  them  all ;  may  it  not  be  that 
such  parts  as  are  present,  show  their  respective  typical  natures  by 
some  constant  character :  say  their  mode  of  ossification  ? 

To  this  question  some  parts  of  the  Archetype  and  Homologies 
seem  to  reply,  "  Yes ;  "  while  others  clearly  answer,  "  No."  Criticis- 
ing the  opinions  of  Geoffrey  St.  Hilaire  and  Cuvier,  who  agreed  in 
thinking  that  ossification  from  a  separate  centre  was  the  test  of  a 
separate  bone,  and  that  thus  there  were  as  many  elementary  bones 
in  the  skeleton  as  there  were  centres  of  ossification,  Professor  Owen 
points  out  that,  according  to  this  test,  the  human  femur,  which  is 
ossified  from  four  centres,  must  be  regarded  as  four  bones ;  while 
the  femur  in  birds  and  reptiles,  which  is  ossified  from  a  single 
centre,  must  be  regarded  as  a  single  bone.  Yet,  on  the  other  hand, 
he  attaches  weight  to  the  fact  that  the  skull  of  the  human  foetus 
presents  "  the  same  ossific  centres  "  as  do  those  of  the  embryo  kan- 
garoo and  the  young  bird.  (Nature  of  Limbs,  p.  40.)  And  at  p. 
104  of  the  Homologies,  after  giving  a  number  of  instances,  he  says — 

"These  and  the  like  correspondences  between  the  points  of  ossiQcation  of 


A  CRITICISM  ON  PROP.   OWEN'S  THEORY.  557 

the  human  foetal  skeleton,  and  the  separate  bones  of  the  adult  skeletons  of 
inferior  animals,  are  pregnant  with  interest,  and  rank  among  the  most  striking 
illustrations  of  unity  of  plan  in  the  vertebrate  organization." 

It  is  true  that  on  the  following  page  he  seeks  to  explain  this 
seeming  contradiction  by  distinguishing 

"  between  those  centres  of  ossification  that  have  homological  relations,  and 
those  that  have  teleological  ones — i.e.,  between  the  separate  points  of  ossifica- 
tion of  a  human  bone  which  typify  vertebral  elements,  often  permanently  dis- 
tinct bones  in  the  lower  animals;  and  the  separate  points  which,  without" such 
signification,  facilitate  the  progress  of  osteogeny,  and  have  for  their  obvious 
final  cause  the  well-being  of  the  growing  animal." 

But  if  there  are  thus  centres  of  ossification  which  have  homo- 
logical  meanings,  and  others  which  have  not,  there  arises  the  ques- 
tion— How  are  they  always  to  be  distinguished  ?  Evidently  in- 
dependent ossification  ceases  to  be  a  homological  test,  if  there  are 
independent  ossifications  that  have  nothing  to  do  with  the  homo- 
logies.  And  this  becomes  the  more  evident  when  we  learn  that 
there  are  cases  where  neither  a  homological  nor  a  teleological 
meaning  can  be  given.  Among  various  modes  of  ossification  of  the 
centrum,  Professor  Owen  points  out  that  "  the  body  of  the  human 
atlas  is  sometimes  ossified  from  two,  rarely  from  three,  distinct 
centres  placed  side  by  side  "  (p.  89) ;  while  at  p.  87  he  says : — "  In 
osseous  fishes  I  find  that  the  centrum  is  usually  ossified  from  six 
points."  It  is  clear  that  this  mode  of  ossification  has  here  no  homo- 
logical  signification ;  and  it  would  be  difficult  to  give  any  teleo- 
logical reason  why  the  small  centrum  of  a  fish  should  have  more 
centres  of  ossification  than  the  large  centrum  of  a  mammal.  The 
truth  is,  that  as  a  criterion  of  the  identity  or  individuality  of  a  bone, 
mode  of  ossification  is  quite  untrustworthy.  Though,  in  his  "  ideal 
typical  vertebra,"  Professor  Owen  delineates  and  classifies  as  sepa- 
rate "  autogenous "  elements,  those  parts  which  are  "  usually 
developed  from  distinct  and  independent  centres ; "  and  though  by- 
doing  so  he  erects  this  characteristic  into  some  sort  of  criterion ; 
yet  his  own  facts  show  it  to  be  no  criterion.  The  parapophyses 
are  classed  among  the  autogenous  elements ;  yet  they  are  auto- 
genous in  fishes  alone,  and  in  these  only  in  the  trunk  vertebrae, 
while  in  all  air-breathing  vertebrates  they  are,  when  present  at  all, 
exogenous.  The  neurapophyses,  again,  "  lose  their  primitive  in- 
dividuality by  various  kinds  and  degrees  of  confluence  : "  in  the 
tails  of  the  higher  Vertebrata  they,  in  common  with  the  neural 
spine,  become  exogenous.  Nay,  even  the  centrum  may  lose  its 
autogenous  character.  Describing  how,  in  some  batrachians, 
"  the  ossification  of  the  centrum  is  completed  by  an  extension  of 
bone  from  the  bases  of  the  neurapophyses,  which  effects  also  the 
coalescence  of  thes-3  with  the  centrum,"  Professor  Owen  adds : — 
"  In  Pelobates  fuscus  and  Pelobates  cultripes,  Miiller  found  the  en- 


558  APPENDIX  B. 

tire  centrum  ossified  from  this  source,  without  any  independent 
points  of  ossification"  (p.  88).  That  is  to  say,  the  centrum  is  in 
these  cases  an  exogenous  process  of  the  neurapophyses.  We  see, 
then,  that  these  so-called  typical  elements  of  vertebrae  have  no 
constant  developmental  character  by  which  they  can  be  identified. 
Not  only  are  they  undistinguishable  by  any  specific  test  from  other 
bones  not  included  as  vertebral  elements ;  not  only  do  they  fail  to 
show  their  typical  characters  by  their  constant  presence ;  but, 
when  present,  they  exhibit  no  persistent  marks  of  individuality. 
The  central  element  may  be  ossified  from  six,  four,  three,  or  two 
points ;  or  it  may  have  no  separate  point  of  ossification  at  all : 
and  similarly  with  various  of  the  peripheral  elements.  The  whole 
group  of  bones  forming  the  "  ideal  typical  vertebra  "  may  severally 
have  their  one  or  more  ossific  centres ;  or  they  may,  as  in  a  mam- 
mal's tail,  lose  their  individualities  in  a  single  bone  ossified  from 
one  or  two  points. 

Another  fact  which  seems  very  difficult  to  reconcile  with  the 
hypothesis  of  an  "  ideal  typical  vertebra,"  is  the  not  infrequent 
presence  of  some  of  the  typical  elements  in  duplicate.  Not  only, 
as  we  have  seen,  may  they  severally  be  absent,  but  they  may  seve- 
rally be  present  in  greater  number  than  they  should  be.  When 
we  see,  in  the  ideal  diagram,  one  centrum,  two  neurapophyses, 
two  pleurapophyses,  two  hsemapophyses,  one  neural  spine,  and  one 
hnemal  spine,  we  naturally  expect  to  find  them  always  bearing  to 
each  other  these  numerical  relations.  Though  we  may  not  be 
greatly  surprised  by  the  absence  of  some  of  them,  we  are  hardly 
prepared  to  find  others  multiplied.  Yet  such  cases  are  common. 
Thus  the  neural  spine  "  is  double  in  the  anterior  vertebra}  of  some 
fishes  "  (p.  98).  Again,  in  the  abdominal  region  of  extinct  saurians, 
and  in  crocodiles,  "  the  freely-suspended  hsemapophyses  are  com- 
pounded of  two  or  more  overlapping  bony  pieces"  (p.  100).  Yet 
again,  at  p.  99,  we  read — "  I  have  observed  some  of  the  expanded 
pleurapophyses  in  the  great  Testudo  elephantopus  ossified  from  two 
centres,  and  the  resulting  divisions  continuing  distinct,  but  united 
by  suture."  Once  more  "  the  neurapophyses,  which  do  not  advance 
beyond  the  cartilaginous  stage  in  the  sturgeon,  consist  in  that  fish 
of  two  distinct  pieces  of  cartilage  ;  and  the  anterior  pleurapophvses 
also  consist  of  two  or  more  cartilages,  set  end  on  end"  (p.  91). 
And  elsewhere  referring  to  this  structure,  he  says : — 

"  Vegetative  repetition  of  perivertebral  parts  not  only  manifests  itself  in 
the  composite  neurapophyses  and  pleurapophyses,  but  in  a  small  accessory 
(interneural)  cartilage,  at  the  fore  and  back  part  of  the  base  of  the  neura- 
pophysis ;  and  by  a  similar  (interhaemal)  one  at  the  fore  and  back  part  of 
most  of  the  parapophyses  "  (p.  87). 

Thus  the  neural  and  haemal  spines,  the  neurapophyses,  the  pleu- 


A  CRITICISM  ON  PROF.  OWEN'S  THEORY.  559 

rapophyses,  the  haemapophyses,  may  severally  consist  of  two  or 
more  pieces.    This  is  not  all :  the  like  is  true  even  of  the  centrums. 

"  In  Hcplanchus  (Squalus  cinereiis)  the  vertebral  centres  are  feebly  and 
vegetatively  marked  out  by  numerous  slender  rings  of  hard  cartilage  in  the 
notochordal  capsule,  the  number  of  vertebrae  being  more  definitely  indicated 
by  the  neurapophyses  and  parapophyses.  ...  In  the  piked  dog-fish 
(Acanthias)  aud  the  spotted  dog-fish  (Sc.yllium)  the  vertebral  centres  coin- 
cide in  number  with  the  neural  arches  "  (p.  87). 

Is  it  not  strange  that  the  pattern  vertebra  should  be  so  little 
adhered  to,  that  each  of  its  single  typical  pieces  may  be  trans- 
formed into  two  or  three  ? 

But  there  are  still  more  startling  departures  from  the  alleged 
type.  The  numerical  relations  of  the  elements  vary  not  only  in 
this  way,  but  in  the  opposite  way.  A  given  part  may  be  present 
not  only  in  greater  number  than  it  should  be,  but  also  in  less.  In 
the  tails  of  homocercal  fishes,  the  centrums  "  are  rendered  by  cen- 
tripetal shortening  and  bony  confluence  fewer  in  number  than  the 
persistent,  neural,  and  haemal  arches  of  that  part " — that  is,  there 
is  only  a  fraction  of  a  centrum  to  each  vertebra.  Nay,  even  this 
is  not  the  most  heteroclite  structure.  Paradoxical  as  it  may  seem, 
there  are  cases  in  which  the  same  vertebral  element  is,  considered 
under  different  aspects,  at  once  in  excess  and  defect.  Speaking 
of  the  haemal  spine,  Professor  Owen  says : — 

"  The  horizontal  extension  of  this  vertebral  element  is  sometimes  accom- 
panied by  a  median  division,  or  in  other  words,  it  is  ossified  from  two 
lateral  centres;  this  is  seen  in  the  development  of  parts  of  the  human 
sternum ;  the  same  vegetative  character  is  constant  in  the  broader  thoracic 
haemal  spines  of  birds;  though,  sometimes,  as  e.g.,  in  the  struthionidee, 
ossification  extends  from  the  same  lateral  centre  lengthwise — i.e.,  forwards  and 
backwards,  calcifying  the  connate  cartilaginous  homoloffues  of  halves  of  four 
or  five  haemal  spines,  before  these  finally  coalesce  with  their  fellows  at  the  median 
line"  (p.  101). 

So  that  the  sternum  of  the  ostrich,  which  according  to  the  hypo- 
thesis, should,  in  its  cartilaginous  stage,  have  consisted  of  four  or 
Jive  transverse  pieces,  answering  to  the  vertebral  segments,  and 
should  have  been  ossified  from  four  or  five  centres,  one  to  each 
cartilaginous  piece,  shows  not  a  trace  of  this  structure ;  but  in- 
stead, consists  of  two  longitudinal  pieces  of  cartilage,  each  ossified 
from  one  centre,  and  finally  coalescing  on  the  median  line.  These 
four  or  five  haemal  spines  have  at  the  same  time  doubled  their  in- 
dividualities transversely,  and  entirely  lost  them  longitudinally  ! 

There  still  remains  to  be  considered  the  test  of  relative  position. 
It  might  be  held  that,  spite  of  all  the  foregoing  anomalies,  if  the 
typical  parts  of  the  vertebrae  always  stood  towards  each  other  in 
the  same  relations — always  preserved  the  same  connexions,  some- 
thing like  a  case  would  be  made  out.  Doubtless,  relative  position 


560  APPENDIX  B. 

is  an  important  point ;  and  it  is  one  on  which  Professor  Owen  mani- 
festly places  great  dependence.  In  his  discussion  of  "  moot  cases 
of  special  homology,"  it  is  the  general  test  to  which  he  appeals. 
The  typical  natures  of  the  alisphenoid,  the  mastoid,  the  orbito- 
sphenoid,  the  prefrontal,  the  malar,  the  squamosal,  &c.,  he  deter- 
mines almost  wholly  by  reference  to  the  adjacent  nerve-perforations 
and  the  articulations  with  neighbouring  bones  (see  pp.  19  to  72)  : 
the  general  form  of  the  argument  being — This  bone  is  to  be  classed 
as  such  or  such,  because  it  is  connected  thus  and  thus  with  these 
others,  which  are  so  and  so.  Moreover,  by  putting  forth  an  "  ideal 
typical  vertebra,"  consisting  of  a  number  of  elements  standing 
towards  each  other  in  certain  definite  arrangement,  this  persistency 
of  relative  position  is  manifestly  alleged.  The  essential  attribute 
of  this  group  of  bones,  considered  as  a  typical  group,  is  the  con- 
stancy in  the  connexions  of  its  parts  :  change  the  connexions,  and 
the  type  is  changed.  But  the  constancy  of  relative  position  thus 
tacitly  asserted,  and  appealed  to  as  a  conclusive  test  in  "  moot 
cases  of  special  homology,"  is  clearly  negatived  by  Professor 
Owen's  own  facts.  For  instance,  in  the  "  ideal  typical  vertebra," 
the  haemal  arch  is  represented  as  formed  by  the  two  haemapophyses 
and  the  haemal  spine ;  but  at  p.  91  we  are  told  that 

"  The  contracted  haemal  arch  in  the  caudal  region  of  the  body  may  be 
formed  by  different  elements  of  the  typical  vertebra:  e.g.,  by  the  para- 
pophyses  (fishes  generally) ;  by  the  pleurapophyses  (lepidosiren) ;  by  both 
parapophyses  and  pleurapophyses  (Sudis,  fapidosteus),  and  by  haemapo- 
physes,  shortened  and  directly  articulated  with  the  centrums  (reptiles  and 
mammals)." 

And  further,  in  the  thorax  of  reptiles,  birds,  and  mammals,  "  the 
haemapophyses  are  removed  from  the  centrum,  and  are  articulated  to 
the  distal  ends  of  the  pleurapophyses ;  the  bony  hoop  being  com- 
pleted by  the  intercalation  of  the  haemal  spine"  (p.  82).  So  that 
there  are  five  different  ways  in  which  the  haemal  arch  may  be  formed 
— four  modes  of  attachment  of  the  parts  different  from  that  shown 
in  the  typical  diagram !  Nor  is  this  all.  The  pleurapophyses  "  may 
be  quite  detached  from  their  proper  segment,  and  suspended  to  the 
haemal  arch  of  another  vertebra ; "  as  we  have  already  seen,  the 
entire  haemal  arch  may  be  detached  and  removed  to  a  distance, 
sometimes  reaching  the  length  of  twenty-seven  vertebrae ;  and,  even 
more  remarkable,  the  ventral  tins  of  some  fishes,  which  theoretically 
belong  to  the  pelvic  arch,  are  so  much  advanced  forward  as  to  be 
articulated  to  the  scapular  arch — "the  ischium  elongating  to  join 
the  coracoid."  With  these  admissions  it  seems  to  us  that  relative 
position  and  connexions  cannot  be  appealed  to  as  tests  of  homology, 
nor  as  evidence  of  any  original  type  of  vertebra. 

In  no  class  of  facts,  then,  do  we  find  a  good  foundation  for  the 
hypothesis  of  an  "  ideal  typical  vertebra."  There  is  no  one  con- 


A  CRITICISM  ON  PROF.   OWEN'S  THEORY.          561 

ceivable  attribute  of  this  archetypal  form  which  is  habitually  realised 
by  actual  vertebrae.  The  alleged  group  of  true  vertebral  elements 
is  not  distinguished  in  any  specified  way  from  bones  not  included  in 
it.  Its  members  have  various  degrees  of  inconstancy  ;  are  rarely 
all  present  together ;  and  no  one  of  them  is  essential.  They  are 
severally  developed  in  no  uniform  way :  each  of  them  may  arise 
either  out  of  a  separate  piece  of  cartilage,  or  out  of  a  piece  con- 
tinuous with  that  of  some  other  element ;  and  each  may  be  ossified 
from  many  independent  points,  from  one,  or  from  none.  Not  only 
may  their  respective  individualities  be  lost  by  absence,  or  by  con- 
fluence with  others ;  but  they  may  be  doubled,  or  tripled,  or  halved, 
or  may  be  multiplied  in  one  direction  and  lost  in  another.  The  en- 
tire group  of  typical  elements  may  coalesce  into  one  simple  bone 
representing  the  whole  vertebra ;  and  even,  as  in  the  terminal  piece 
of  a  bird's  tail,  half-a-dozen  vertebrae,  with  all  their  many  elements, 
may  become  entirely  lost  in  a  single  mass.  Lastly,  the  respective 
elements,  when  present,  have  no  fixity  of  relative  position :  sundry 
of  them  are  found  articulated  to  various  others  than  those  with 
which  they  are  typically  connected ;  they  are  frequently  displaced 
and  attached  to  neighbouring  vertebrae  ;  and  they  are  even  removed 
to  quite  remote  parts  of  the  skeleton.  It  seems  to  us  that  if  this 
want  of  congruity  with  the  facts  does  not  disprove  the  hypothesis, 
no  such  hypothesis  admits  of  disproof. 

Unsatisfactory  as  is  the  evidence  in  the  case  of  the  trunk  and 
tail  vertebrae,  to  which  we  have  hitherto  confined  ourselves,  it  is  far 
worse  in  the  case  of  the  alleged  cranial  vertebrae.  The  mere  fact 
that  those  who  have  contended  for  the  vertebrate  structure  of  the 
skull,  have  differed  so  astonishingly  in  their  special  interpretations 
of  it,  is  enough  to  warrant  great  doubt  as  to  the  general  truth  of 
their  theory.  From  Professor  Owen's  history  of  the  doctrine  of 
general  homology,  we  gather  that  Dumeril  wrote  upon  "  la  tete 
consideree  comme  une  vertebre ; "  that  Kielmeyer,  "  instead  of 
calling  the  skull  a  vertebra,  said  each  vertebra  might  be  called  a 
skull  f"  that  Oken  recognized  in  the  skull  three  vertebrae  and  a 
rudiment ;  that  Professor  Owen  himself  makes  out  four  vertebra?  ; 
that  Goethe's  idea,  adopted  and  developed  by  Carus,  was,  that  the 
skull  is  composed  of  six  vertebrae ;  and  that  Geoffroy  St.  Hilaire 
divided  it  into  seven.  Does  not  the  fact  that  different'comparative 
anatomists  have  arranged  the  same  group  of  bones  into  one,  three, 
four,  six,  and  seven  vertebral  segments,  show  that  the  mode  of  de- 
termination is  arbitrary,  and  the  conclusions  arrived  at  fanciful  1 
May  we  not  properly  entertain  great  doubts  as  to  any  one  scheme 
being  more  valid  than  the  others  1  And  if  out  of  these  conflicting 
schemes  we  are  asked  to  accept  one,  ought  we  not  to  accept  it  only 
on  the  production  of  some  thoroughly  conclusive  proof — some 


562  APPENDIX  B. 

rigorous  test  showing  irrefragably  that  the  others  must  be  wrong 
and  this  alone  right  ?  Evidently  where  such  contradictory  opinions 
have  been  formed  by  so  many  competent  judges,  we  ought,  before 
deciding  in  favour  of  one  of  them,  to  have  a  clearness  of  demon- 
stration much  exceeding  that  required  in  any  ordinary  case.  Let 
us  see  whether  Professor  Owen  supplies  us  with  any  such  clear- 
ness of  demonstration. 

To  bring  the  first  or  occipital  segment  of  the  skull  into  corre- 
spondence with  the  "ideal  typical  vertebra,"  Professor  Owen  argues, 
in  the  case  of  the  fish,  that  the  parapophyses  are  displaced,  and 
wedged  between  the  neurapophyses  and  the  neural  spine  —  removed 
from  the  haemal  arch  and  built  into  the  upper  part  of  the  neural 
arch.  Further,  he  considers  that  the  pleurapophyses  are  teleologi- 
cally  compound.  And  then,  in  all  the  higher  vertebrata,  he  alleges 
that  the  haemal  arch  is  separated  from  its  centrum,  taken  to  a  dis- 
tance, and  transformed  into  the  scapular  arch.  Add  to  which,  he 


confluent  with  it,  are  not  only  removed  to  the  far  ends  of  elements 
placed  above  the  centrum,  but  have  become  exogenous  parts  of  them! 

Conformity  of  the  second  or  parietal  segment  of  the  cranium  with 
the  pattern-vertebra,  is  produced  thus  :  —  The  petrosals  are  excluded 
as  being  partially-ossified  sense-capsules,  not  forming  parts  of  the 
true  vertebral  system,  but  belonging  to  the  "  splanchno-skeleton." 
A  centrum  is  artificially  obtained  by  sawing  in  two  the  bone  which 
serves  in  common  as  centrum  to  this  and  the  preceding  segment;  and 
this  though  it  is  admitted  that  in  fishes,  where  their  individualities 
ought  to  be  best  seen,  these  two  hypothetical  centrums  are  not 
simply  coalescent,  but  connate.  Next,  a  similar  arbitrary  bisection 
is  made  of  certain  elements  of  the  haemal  arches.  And  then,  "  the 
principle  of  vegetative  repetition  is  still  more  manifest  in  this  arch 
than  in  the  occipital  one  :"  each  pleurapophysis  is  double  ;  each 
haemapophysis  is  double  ;  and  the  haemal  spine  consists  of  six  pieces  ! 

The  interpretation  of  the  third  and  fourth  segments  being  of 
the  same  general  character,  need  not  be  detailed.  The  only  point 
calling  for  remark  being,  that  in  addition  to  the  above  various 
modes  of  getting  over  anomalies,  we  find  certain  bones  referred  to 
the  dermo-skeleton. 

Now  it  seems  to  us,  that  even  supposing  no  antagonist  interpre- 
tations had  been  given,  an  hypothesis  reconcilable  with  the  facts 
only  by  the  aid  of  so  many  questionable  devices,  could  not  be  con- 
sidered satisfactory  ;  and  that  when,  as  in  this  case,  various  com- 
parative anatomists  have  contended  for  other  interpretations,  the 
character  of  this  one  is  certainly  not  of  a  kind  to  warrant  the  re- 
jection of  the  others  in  its  favour  ;  but  rather  of  a  kind  to  make 


A  CRITICISM  ON  PROF.   OWEN'S  THEORY.          563 

us  doubt  the  possibility  of  all  such  interpretations.  The  question 
which  naturally  arises  is,  whether  by  proceeding  after  this  fashion, 
groups  of  bones  might  not  be  arranged  into  endless  typical  forms. 
If,  when  a  given  element  was  not  in  its  place,  we  were  at  liberty  to 
consider  it  as  suppressed,  or  connate  with  some  neighbouring  element, 
or  removed  to  some  more  or  less  distant  position  ; — if,  on  finding  a 
bone  in  excess,  we  might  consider  it,  now  as  part  of  the  dermo- 
skeleton,  now  as  part  of  the  splanchno-skeleton,  now  as  transplanted 
from  its  typical  position,  now  as  resulting  from  vegetative  repetition, 
and  now  as  a  bone  Ideologically  compound  (for  these  last  two  are 
intrinsically  different,  though  often  used  by  Professor  Owen  as 
equivalents) ; — if,  in  other  cases,  a  bone  might  be  regarded  as 
spurious  (p.  91),  or  again  as  having  usurped  the  place  of  another; 
— if,  we  say,  these  various  liberties  were  allowed  us,  we  should 
not  despair  of  reconciling  the  facts  with  various  diagrammatic 
types  besides  that  adopted  by  Professor  Owen. 

When,  in  1851,  we  attended  a  course  of  Professor  Owen's  lec- 
tures on  Comparative  Osteology,  beginning  though  we  did  in  the  at- 
titude of  discipleship,  our  scepticism  grew  as  we  listened,  and  reached 
its  climax  when  we  came  to  the  skull ;  the  reduction  of  which  to  the 
vertebrate  structure,  reminded  us  very  much  of  the  interpretation 
of  prophecy.  The  delivery,  at  the  Royal  Society,  of  the  Croonian 
Lecture  for  1858,  in  which  Prof essor  Huxley,  confirming  the  state- 
ments of  several  German  anatomists,  has  shown  that  the  facts  of 
embryology  do  not  countenance  Professor  Owen's  views  respecting 
the  formation  of  the  cranium,  has  induced  us  to  reconsider  the  verte- 
bral theory  as  a  whole.  Closer  examination  of  Professor  Owen's 
doctrines,  as  set  forth  in  his  works,  has  certainly  not  removed  the 
scepticism  generated  years  ago  by  his  lectures.  On  the  contrary, 
that  scepticism  has  deepened  into  disbelief.  Andweventure  to  think 
that  the  evidence  above  cited  shows  this  disbelief  to  be  warranted. 

There  remains  the  question — What  general  views  are  we  to 
take  respecting  the  vertebrate  structure  ?  If  the  hypothesis  of  an 
"  ideal  typical  vertebra  "  is  not  justified  by  the  facts,  how  are  we 
to  understand  that  degree  of  similarity  which  vertebrae  display  ? 

We  believe  the  explanation  is  not  far  to  seek.  All  that  our 
space  will  here  allow,  is  a  brief  indication  of  what  seems  to  us  the 
natural  view  of  the  matter. 

Professor  Owen,  in  common  with  other  comparative  anatomists, 
regards  the  divergences  of  individual  vertebrae  from  the  average 
form,  as  due  to  adaptive  modifications.  If  here  one  vertebral  ele- 
ment is  largely  developed,  while  elsewhere  it  is  small — if  now  the 
form,  now  the  position,  now  the  degree  of  coalescence,  of  a  given 
part  varies ;  it  is  that  the  local  requirements  have  involved  this 
change.  The  entire  teaching  of  comparative  osteology  implies  that 


564  APPENDIX  B. 

differences  in  the  conditions  of  the  respective  vertebrae  necessitate 
differences  in  their  structures. 

Now,  it  seems  to  us  that  the  first  step  towards  a  right  concep- 
tion of  the  phenomena,  is  to  recognize  this  general  law  in  its  converse 
application.  If  vertebra?  are  unlike  in  proportion  to  the  unlike- 
ness  of  their  circumstances,  then,  by  implication,  they  will  be  like  in 
proportion  to  the  likeness  of  their  circumstances.  While  successive 
segments  of  the  same  skeleton,  and  of  different  skeletons,  are  all  in 
some  respects  more  or  less  differently  acted  on  by  incident  forces, 
and  are  therefore  required  to  be  more  or  less  different ;  they  are 
all,  in  other  respects,  similarly  acted  on  by  incident  forces,  and  are 
therefore  required  to  be  more  or  less  similar.  It  is  impossible  to 
deny  that  if  differences  in  the  mechanical  functions  of  the  vertebras 
involve  differences  in  their  forms ;  then,  community  in  their  mechani- 
cal functions,  must  involve  community  in  their  forms.  And  as  we 
knowthat  throughout  the  Vertebrata  generally,  and  in  each  vertebrate 
animal,  the  vertebrae,  amid  all  their  varying  circumstances,  have  a 
certain  community  of  function,  it  follows  necessarily  that  they  will 
have  a  certain  general  resemblance — there  will  recur  that  average 
shape  which  has  suggested  the  notion  of  a  pattern  vertebra. 

A  glance  at  the  facts  at  once  shows  their  harmony  with  this 
conclusion.  In  an  eel  or  a  snake,  where  the  bodily  actions  are  such 
as  to  involve  great  homogeneity  in  the  mechanical  conditions  of  the 
vertebrae,  the  series  of  them  is  comparatively  homogeneous.  On  the 
contrary,  in  a  mammal  or  a  bird,  where  there  is  considerable  hetero- 
geneity in  their  circumstances,  their  similarity  is  no  longer  so  great. 
And  if,  instead  of  comparing  the  vertebral  columns  of  different 
animals,  we  compare  the  successive  vertebrae  of  any  one  animal,  we 
recognize  the  same  law.  In  the  segments  of  an  individual  spine, 
where  is  there  the  greatest  divergence  from  the  common  mechanical 
conditions  ?  and  where  may  we  therefore  expect  to  find  the  widest 
departure  from  the  average  form  ?  Obviously  at  the  two  extremities. 
And  accordingly  it  is  at  the  two  extremities  that  the  ordinary 
structure  is  lost. 

Still  clearer  becomes  the  truth  of  this  view,  when  we  consider  the 
genesis  of  the  vertebral  column  as  displayed  throughout  the  ascend- 
ing grades  of  the  Vertebrata.  In  its  first  embryonic  stage,  the  spine 
is  an  undivided  column  of  flexible  substance.  In  the  early  fishes, 
while  some  of  the  peripheral  elements  of  the  vertebrae  were  marked 
out,  the  central  axis  was  still  a  continuous  unossified  cord.  And 
thus  we  have  good  reason  for  thinking  that  in  the  primitive  verte- 
brate animal,  as  in  the  existing  Amphiozus,  the  notochord  was  per- 
sistent. The  production  of  a  higher,  more  powerful,  more  active 
creature  of  the  same  type,  by  whatever  method  it  is  conceived  to 
have  taken  place,  involved  a  change  in  the  notochordal  structure. 
Greater  muscular  endowments  presupposed  a  firmer  internal  fulcrum 


A  CRITICISM  ON  PROF.   OWEN'S  THEORY.          565 

— a  less  yielding  central  axis.  On  the  other  hand,  for  the  central 
axis  to  have  become  firmer  while  remaining  continuous,  would  have 
entailed  a  stiffness  incompatible  with  the  creature's  movements. 
Hence,  increasing  density  of  the  central  axis  necessarily  went  hand 
in  hand  with  its  segmentation  :  for  strength,  ossification  was  re- 
quired ;  for  flexibility,  division  into  parts.  The  production  of  ver- 
tebrae resulting  thus,  there  obviously  would  arise  among  them  a 
general  likeness,  due  to  the  similarity  in  their  mechanical  condi- 
tions, and  more  especially  the  muscular  forces  bearing  on  them. 
And  then  observe,  lastly,  that  where,  as  in  the  head,  the  terminal 
position  and  the  less  space  for  development  of  muscles,  entailed 
smaller  lateral  bendings,  the  segmentation  would  naturally  be  less 
decided,  less  regular,  and  would  be  lost  as  we  approached  the 
front  of  the  head. 

But,  it  may  be  replied,  this  hypothesis  does  not  explain  all  the 
facts.  It  does  not  tell  us  why  a  bone  whose  function  in  a  given 
animal  requires  it  to  be  solid,  is  formed  not  of  a  single  piece,  but  by 
the  coalescence  of  several  pieces,  which  in  other  creatures  are  sepa- 
rate ;  it  does  not  account  for  the  frequent  manifestations  of  unity 
of  plan  in  defiance  of  teleological  requirements.  This  is  quite  true. 
But  it  is  not  true,  as  Professor  Owen  argues  respecting  such  cases, 
that "  if  the  principle  of  special  adaptation  fails  to  explain  them,  and 
we  reject  the  idea  that  these  correspondences  are  manifestations  of 
some  archetypal  exemplar,  on  which  it  has  pleased  the  Creator  to 
frame  certain  of  his  living  creatures,  there  remains  only  the  alterna- 
tive that  the  organic  atoms  have  concurred  fortuitously  to  produce 
such  harmony."  This  is  not  the  only  alternative :  there  is  another, 
which  Professor  Owen  has  overlooked.  It  is  a  perfectly  tenable 
supposition  that  all  higher  vertebrate  forms  have  arisen  by  the  su- 
perposing of  adaptations  upon  adaptations.  Either  of  the  two  anta- 
fonist  cosmogonies  consists  with  this  supposition.  If,  on  the  one 
and,  we  conceive  species  to  have  resulted  from  acts  of  special 
creation  ;  then  it  is  quite  a  fair  assumption  that  to  produce  a  higher 
vertebrate  animal,  the  Creator  did  not  begin  afresh,  but  took  a 
lower  vertebrate  animal,  and  so  far  modified  its  pre-existing  parts 
as  to  fit  them  for  the  new  requirements ;  in  which  case  the  original 
structure  would  show  itself  through  the  superposed  modifications. 
If,  on  the  other  hand,  we  conceive  species  to  have  resulted  by 
gradual  differentiations  under  the  influence  of  changed  conditions  ; 
then,  it  would  manifestly  follow  that  the  higher,  heterogeneous 
forms,  would  bear  traces  of  the  lower  and  more  homogeneous  forms 
from  which  they  were  evolved. 

Thus,  besides  finding  that  the  hypothesis  of  an  "  ideal  typical 
vertebra  "  is  irreconcilable  with  the  facts,  we  find  that  the  facts  are 
interpretable  without  gratuitous  assumptions.  The  average  com- 
munity of  form  which  vertebrae  display,  is  explicable  as  resulting 


566  APPENDIX  B. 

from  natural  causes.  And  those  typical  similarities  which  are 
traceable  under  adaptive  modifications,  must  obviously  exist  if, 
throughout  creation  in  general,  there  has  gone  on  that  continuous 
superposing  of  modifications  upon  modifications  which  goes  on  in 
every  unfolding  organism. 


[I  might  with  propriety  have  added  to  the  foregoing  criticisms, 
the  remark  that  Professor  Owen  has  indirectly  conferred  a  great 
benefit  by  the  elaborate  investigations  he  has  made  with  the  view  of 
establishing  his  hypothesis.  He  has  himself  very  conclusively 
proved  that  the  teleological  interpretation  is  quite  irreconcilable 
with  the  facts.  In  gathering  together  evidence  in  support  of  his 
own  conception  of  archetypal  forms,  he  has  disclosed  adverse 
evidence  which  I  think  shows  his  conception  to  be  untenable. 
The  result  is  that  the  field  is  left  clear  for  the  hypothesis  of  Evo- 
lution as  the  only  tenable  one.] 


APPENDIX    C. 


[From  the  TRANSACTIONS  OF  THE  LINNEAN  SOCIETY,  VOL.  xxv.] 

XV.  On  Circulation  and  the  formation  of  Wood  in  Plants.  By 
HERBERT  SPENCER,  Esq.  Communicated  by  GEORGE  BUSK, 
£*q.,  F.M.S.,  Sec.  L.S. 

Read  March  1st,  1866. 

OPINIONS  respecting  the  functions  of  the  vascular  tissues  in  plants 
appear  to  make  but  little  progress  towards  agreement.  The  suppo- 
sition that  these  vessels  and  strings  of  partially-united  cells,  lined 
with  spiral,  annular,  reticulated,  or  other  frameworks,  are  carriers 
of  the  plant- juices,  is  objected  to  on  the  ground  that  they  often 
contain  air :  as  the  presence  of  air  arrests  the  movement  of  blood 
through  arteries  and  veins,  its  presence  in  the  ducts  of  stems  and 
petioles  is  assumed  to  unfit  them  as  channels  for  sap.  On  the 
other  hand,  that  these  structures  have  a  respiratory  office,  as  some 
have  thought,  is  certainly  not  more  tenable,  since,  if  the  presence 
of  air  in  them  negatives  the  belief  that  their  function  is  to  dis- 
tribute liquid,  the  presence  of  liquid  in  them  equally  negatives  the 
belief  that  their  function  is  to  distribute  air.  Nor  can  any  better 
defence  be  made  for  the  hypothesis  which  I  find  propounded,  that 
these  parts  serve  "  to  give  strength  to  the  parenchyma."  Tubes 
with  fenestrated  and  reticulated  internal  skeletons  have,  indeed, 
some  power  of  supporting  the  tissue  through  which  they  pass ;  but 
tubes  lined  with  spiral  threads  can  yield  extremely  little  support, 
while  tubes  lined  with  annuli,  or  spirals  alternating  with  annuli,  can 
yield  no  support  whatever.  Though  all  these  types  of  internal 
framework  are  more  or  less  efficient  for  preventing  closure  by 
lateral  pressure,  they  are  some  of  them  quite  useless  for  holding 
up  the  mass  through  which  the  vessels  pass  ;  and  the  best  of  them 
are  for  this  purpose  mechanically  inferior  to  the  simple  cylinder. 
The  same  quantity  of  matter  made  into  a  continuous  tube  would  be 
more  effective  in  giving  stiffness  to  the  cellular  tissue  around  it. 

In  the  absence  of  any  feasible  alternative,  the  hypothesis  that 
these  vessels  are  distributors  of  sap  claims  reconsideration.  The 
objections  are  not,  I  think,  so  serious  as  they  seem.  The  habitual 

567 


568  APPENDIX  C. 

presence  of  air  in  the  ducts  that  traverse  wood,  can  scarcely  be 
held  anomalous  if  when  the  wood  is  formed  their  function  ceases. 
The  canals  which  ramify  through  a  Stag's  horn,  contain  air  after 
the  Stag's  horn  is  fully  developed ;  but  it  is  not  thereby  rendered 
doubtful  whether  it  is  the  function  of  arteries  to  convey  blood. 
Again,  that  air  should  frequently  be  found  even  in  the  vessels  of 
petioles  and  leaves,  will  not  appear  remarkable  when  we  call  to 
mind  the  conditions  to  which  a  leaf  is  subject.  Evaporation  is 
going  on  from  it.  The  thinner  liquids  pass  by  osmose  out  of  the 
vessels  into  the  tissues  containing  the  liquids  thickened  by  evapora- 
tion. And  as  the  vessels  are  thus  continually  drained,  a  draught  is 
made  upon  the  liquid  contained  in  the  stem  and  roots.  Suppose 
that  this  draught  is  unusually  great,  or  suppose  that  around  the 
roots  there  exists  no  adequate  supply  of  moisture.  A  state  of 
capillary  tension  must  result — a  tendency  of  the  liquid  to  pass  into 
the  leaves  resisted  below  by  liquid  cohesion.  Now,  had  the  vessels 
impermeable  coats,  only  their  upper  extremities  would  under  these 
conditions  be  slowly  emptied.  But  their  coats,  in  common  with  all 
the  surrounding  tissues,  are  permeable  by  air.  Hence,  under  this 
state  of  capillary  tension,  air  will  enter ;  and  as  the  upper  ends  of 
the  tubes,  being  both  smaller  in  diameter  and  less  porous  than  the 
lower,  will  retain  the  liquids  with  greater  tenacity,  the  air  will 
enter  the  wider  and  more  porous  tubes  below — the  ducts  of  the 
stem  and  branches.  Thus  the  entrance  of  air  no  more  proves  that 
these  ducts  are  not  sap-carriers,  than  does  the  emptiness  of  tropical 
river-beds  in  the  dry  season  prove  that  they  are  not  channels  for 
water.  There  is,  however,  a  difficulty  which  seems  more  serious. 
It  is  said  that  air,  when  present  in  these  minute  canals,  must  be  a 
great  obstacle  to  the  movement  of  sap  through  them.  The  investi- 
gations of  Jamin  have  shown  that  bubbles  in  a  capillary  tube  resist 
the  passage  of  liquid,  and  that  their  resistance  becomes  very  great 
when  the  bubbles  are  numerous — reaching,  in  some  experiments,  as 
much  as  three  atmospheres.  Nevertheless  the  inference  that  any 
such  resistance  is  offered  by  the  air-bubbles  in  the  vessels  of  a 
plant,  is,  I  think,  an  erroneous  one.  What  happens  in  a  capillary 
tube  having  impervious  sides,  with  which  these  experiments  were 
made,  will  by  no  means  happen  in  a  capillary  tube  having  pervious 
sides.  Any  pressure  brought  to  bear  on  the  column  of  liquid  con- 
tained in  the  porous  duct  of  a  plant,  must  quickly  cause  the  expul- 
sion of  a  contained  air-bubble  through  the  minute  openings  in  the 
coats  of  the  duct.  The  greater  molecular  mobility  of  gases  than 
liquids,  implies  that  air  will  pass  out  far  more  readily  than  sap. 
Whilst,  therefore,  a  slight  tension  on  the  column  of  sap  will  cause 
it  to  part  and  the  air  to  enter,  a  slight  pressure  upon  it  will  force 
out  the  air  and  reunite  the  divided  parts  of  the  column. 

To  obtain  data  for  an  opinion  on  this  vexed  question,  I  have 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  569 

lately  been  experimenting  on  the  absorption  of  dyes  by  plants.  So 
far  as  I  can  learn,  experiments  of  this  kind  have  most,  if  not  all  of 
them,  been  made  on  stems,  and,  as  it  would  seem  from  the  results, 
on  stems  so  far  developed  as  to  contain  all  their  characteristic 
structures.  The  first  experiments  I  made  myself  were  on  such 
parts,  and  yielded  evidence  that  served  but  little  to  elucidate 
matters.  It  was  only  after  trying  like  experiments  with  leaves  of 
different  ages  and  different  characters,  and  with  undeveloped  axes, 
as  well  as  with  axes  of  special  kinds,  that  comprehensible  results 
were  reached ;  and  it  then  became  manifest  that  the  appearances 
presented  by  ordinary  stems  when  thus  tested,  are  in  a  great  degree 
misleading.  Let  me  briefly  indicate  the  differences. 

If  an  adult  shoot  of  a  tree  or  shrub  be  cut  off,  and  have  its  lower 
end  placed  in  an  alumed  decoction  of  logwood  or  a  dilute  solution 
of  magenta,*  the  dye  will,  in  the  course  of  a  few  hours,  ascend  to  a 
distance  varying  according  to  the  rate  of  evaporation  from  the 
leaves.  On  making  longitudinal  sections  of  the  part  traversed  by 
it,  the  dye  is  found  to  have  penetrated  extensive  tracts  of  the 
woody  tissue  ;  and  on  making  transverse  sections,  the  openings  of 
the  ducts  appear  as  empty  spaces  in  the  midst  of  a  deeply-coloured 
prosenchyma.  It  would  thus  seem  that  the  liquid  is  carried  up  the 
denser  parts  of  the  vascular  bundles  ;  neglecting  the  cambium  layer, 
neglecting  the  central  pith,  and  neglecting  the  spiral  vessels  of  the 
medullary  sheath.  Apparently  the  substance  of  the  wood  has 
afforded  the  readiest  channel.  When,  however,  we  examine  these 
appearances  critically,  we  find  reasons  for  doubting  this  conclusion. 
If  a  transverse  section  of  the  lower  part,  into  which  the  dye  passed 
first  and  has  remained  longest,  be  compared  with  a  transverse  sec- 
tion of  the  part  which  the  dye  has  but  just  reached,  a  marked 
difference  is  visible.  In  the  one  case  the  whole  of  the  dense  tissue 
is  stained ;  in  the  other  case  it  is  not.  This  uneven  distribution  of 
stain  in  the  part  which  the  dye  has  incompletely  permeated  is  not 
at  random ;  it  admits  of  definite  description.  A  tolerably  regular 
continuous  ring  of  colour  distinguishes  the  outer  part  of  the  wood 
from  the  inner  mass,  implying  a  passage  of  liquid  up  the  elongated 
cells  next  the  cambium  layer.  And  the  inner  mass  is  coloured  more 
round  the  mouths  of  the  pitted  ducts  than  elsewhere :  the  dense 
tissue  is  darkest  close  to  the  edges  of  these  ducts ;  the  colour  fades 
away  gradually  on  receding  from  their  edges  ;  there  is  most  colour 
where  there  are  several  ducts  together ;  and  the  dense  tissue  which 

*  These  two  dyes  have  affinities  for  different  components  of  the  tissues, 
and  may  be  advantageously  used  in  different  cases.  Magenta  is  rapidly 
taken  up  by  woody  matter  and  other  secondary  deposits;  while  logwood 
colours  the  cell-membranes,  and  takes  but  reluctantly  to  the  substances  seized 
by  magenta.  By  trying  both  of  them  on  the  same  structure,  we  may  guard 
ourselves  against  any  error  arising  from  selective  combination. 


570  APPENDIX  C. 

is  fully  dyed  for  some  space,  is  that  which  lies  between  two  or  more 
ducts.  These  are  indications  that  while  the  layer  of  pitted  cells 
next  the  cambium  has  served  as  a  channel  for  part  of  the  liquid,  the 
rest  has  ascended  the  pitted  ducts,  and  oozed  out  of  these  into  the 
prosenchyma  around.  And  this  conclusion  is  confirmed  by  the 
contrast  between  the  appearances  of  the  lowest  part  of  a  shoot 
under  different  conditions.  For  if,  instead  of  allowing  the  dye 
time  for  oozing  through  the  prosenchyma,  the  end  of  the  shoot  be 
just  dipped  into  the  dye  and  taken  out  again,  we  find,  on  making 
transverse  sections  of  the  part  into  which  the  dye  has  been  rapidly 
taken  up,  that,  though  it  has  diffused  to  some  distance  round  the 
ducts,  it  has  left  tracts  of  wood  between  the  ducts  uncoloured — a 
difference  which  would  not  exist  had  the  ascent  been  through  the 
substance  of  the  wood.  Even  still  stronger  is  the  confirmation 
obtained  by  using  one  dye  after  another.  If  a  shoot  that  has  ab- 
sorbed magenta  for  an  hour  be  placed  for  five  minutes  in  the  log- 
wood decoction,  transverse  sections  of  it  taken  at  a  short  distance 
from  its  end  show  the  mouths  of  the  ducts  surrounded  by  dark 
stains  in  the  midst  of  the  much  wider  red  stains. 

Based  on  these  comparisons  only,  the  inference  pointed  out  has 
little  weight ;  but  its  weight  is  increased  by  the  results  of  experi- 
ments on  quite  young  shoots,  and  shoots  that  develope  very  little 
wood.  The  behaviour  of  these  corresponds  perfectly  with  the  ex- 
pectation that  a  liquid  will  ascend  capillary  tubes  in  preference  to 
simple  cellular  tissue  or  tissue  not  differentiated  into  continuous 
canals.  The  vascular  bundles  of  the  medullary  sheath  are  here 
the  only  channels  which  the  coloured  liquid  takes.  In  sections  of 
the  parts  up  to  which  the  dye  has  but  just  reached,  the  spiral,  f  enes- 
trated,  scalariform,  or  other  vessels  contained  in  these  bundles  are 
alone  coloured,  and  lower  down  it  is  only  after  some  hours  that 
such  an  exudation  of  dye  takes  place  as  suffices  partially  to  colour 
the  other  substances  of  the  bundle.  Further,  it  is  to  be  noted  that 
at  the  terminations  of  shoots,  where  the  vessels  are  but  incompletely 
formed  out  of  irregularly-joined  fibrous  cells  which  still  retain 
their  original  shapes,  the  dye  runs  up  the  incipient  vessels  and 
does  not  colour  in  the  smallest  degree  the  surrounding  tissue. 

Experiments  with  leaves  bring  out  parallel  facts.  On  placing  in 
a  dye  a  petiole  of  an  adult  leaf  of  a  tree,  and  putting  it  before  the 
fire  to  accelerate  evaporation,  the  dye  will  be  found  to  ascend  the 
midrib  and  veins  at  various  rates,  up  even  to  a  foot  per  hour.  At 
first  it  is  confined  to  the  vessels  ;  but  by  the  time  it  has  reached  the 
point  of  the  leaf,  it  will  commonly  be  seen  that  at  the  lower  part  it 
has  diffused  itself  into  the  sheaths  of  the  vessels.  In  a  quite  young 
leaf  from  the  same  shoot,  we  find  a  much  more  rigorous  restriction 
of  the  dye  to  the  vessels.  On  making  oblique  sections  of  its  petiole, 
midrib,  and  veins,  the  vessels  have  the  appearance  of  groups  of 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  571 

sharply  defined  coloured  rods  imbedded  in  the  green  prosenchyma ; 
and  this  marked  contrast  continues  with  scarcely  an  appreciable 
change  after  plenty  of  time  has  been  allowed  for  exudation. 

The  facts  thus  grouped  and  thus  contrasted  seem,  at  first  sight, 
to  imply  that  while  they  are  young  the  coats  of  these  ramifying 
canals  lined  with  spiral  or  allied  structures  are  not  readily  perme- 
able, but  that,  becoming  porous  as  they  grow  old,  they  allow  the 
liquids  they  carry  to  escape  with  increasing  facility ;  and  hence  a 
possible  interpretation  of  the  fact  that,  in  the  older  parts,  the  stain- 
ing of  the  tissue  around  the  vessels  is  so  rapid  as  to  suggest  that  the 
dye  has  ascended  directly  through  this  tissue,  whereas  in  the  younger 
parts  the  reverse  appearance  necessitates  the  reverse  conclusion. 
But  now,  is  this  difference  determined  by  difference  of  age,  or  is  it 
otherwise  determined  ?  The  evidence  as  presented  in  ordinary  stems 
and  leaves  shows  us  that  the  parts  of  the  vascular  system  at  which 
there  is  a  rapid  escape  of  dye  are  not  simply  older  parts,  but  are 
parts  where  a  deposit  of  woody  matter  is  taking  place.  Is  it,  then, 
that  the  increasing  permeability  of  the  ducts,  instead  of  being 
directly  associated  with  their  increasing  age,  is  directly  associated 
with  the  increasing  deposit  of  dense  substance  around  them  ? 

To  get  proof  that  this  last  connexion  is  the  true  one,  we  have 
but  to  take  a  class  of  cases  in  which  wood  is  formed  only  to  a  small 
extent.  In  such  cases  experiments  show  us  a  far  more  general  and 
continued  limitation  of  the  dye  to  the  vessels.  Ordinary  herbs  and 
vegetables,  when  contrasted  with  shrubs  and  trees,  illustrate  this ; 
as  instance  the  petioles  of  Celery,  or  of  the  common  Dock,  and  the 
leaves  of  Cabbages  or  Turnips.  And  then  in  very  succulent  plants, 
such  as  Bryophyllum  calycinum,  Kalanchoe  rotundifolia,  the  various 
species  of  Crassula,  Cotyledon,  Kleinia,  and  others  of  like  habit,  the 
ducts  of  old  and  young  leaves  alike  retain  the  dye  very  persistently : 
the  concomitant  in  these  cases  being  the  small  amount  of  prosen- 
chyma around  the  ducts,  or  the  small  amount  of  deposit  in  it,  or 
both.  More  conclusive  yet  is  the  evidence  which  meets  us  when  we 
turn  from  very  succulent  leaves  to  very  succulent  axes.  The  tender 
young  shoots  of  Kleinia  ante-euphorbium,  QT  Euphorbia  Mauritanica, 
which  for  many  inches  of  their  lengths  have  scarcely  any  ligneous 
fibres,  show  us  scarcely  any  escape  of  the  coloured  liquid  from  the 
vessels  of  the  medullary  sheath.  So,  too,  is  it  with  Stapelia, 
Buffonia,  a  plant  of  another  order,  having  soft  swollen  axes.  And 
then  we  have  a  repetition  of  the  like  connexion  of  facts  throughout 
the  Cactacece  :  the  most  succulent  showing  us  the  smallest  perme- 
ability of  the  vessels.  In  two  species  of  Rhipsalis,  in  two  species  of 
Cereus,  and  in  two  species  of  Mammillaria,  which  I  have  tried,  I 
have  found  this  so.  Mammillaria  gracilis  may  be  named  as  ex- 
emplifying the  relation  under  its  extreme  form.  Into  one  of  these 
small  spheroidal  masses,  the  dye  ascends  through  the  large  bundles 


572  APPENDIX  C. 

of  spiral  or  annular  ducts,  or  cells  partially  united  into  such  ducts, 
colouring  them  deeply,  and  leaving  the  feebly-marked  sheath  of 
prosenchyma,  together  with  the  surrounding  watery  cellular  tissue, 
perfectly  uncoloured. 

The  most  conclusive  evidence,  however,  is  furnished  by  those 
Cactacece  in  which  the  transition  from  succulent  to  dense  tissue 
takes  place  variably,  according  as  local  circumstances  determine. 
Opuntia  yields  good  examples.  If  a  piece  of  it  including  one  of 
the  joints  at  which  wood  is  beginning  to  form,  be  allowed  to  absorb 
a  coloured  liquid,  the  liquid,  running  up  the  irregular  bundles  of 
vessels  and  into  many  of  their  minute  ramifications,  is  restricted  to 
these  where  they  pass  through  the  parenchyma  forming  the  mass  of 
the  stem  ;  but  near  the  joints  the  hardened  tissue  around  the  vessels 
is  coloured.  In  one  of  these  fleshy  growths  we  get  clear  evidence 
that  the  escape  of  the  dye  has  no  immediate  dependence  on  the  age 
of  the  vessels,  since,  in  parts  of  the  stem  that  are  alike  in  age,  some 
of  the  vessels  retain  their  contents  while  others  do  not.  Nay,  we 
even  find  that  the  younger  vessels  are  more  pervious  than  the  older 
ones,  if  round  the  younger  ones  there  is  a  formation  of  wood. 

Thus,  then,  is  confirmed  the  inference  before  drawn,  that  in  ordi- 
nary stems  the  staining  of  the  wood  by  an  ascending  coloured  liquid 
is  due,  not  to  the  passage  of  the  coloured  liquid  up  the  substance  of 
the  wood,  but  to  the  permeability  of  its  ducts  and  such  of  its  pitted 
cells  as  are  united  into  irregular  canals.  And  the  facts  showing 
this,  at  the  same  time  indicate  with  tolerable  clearness  the  process 
by  which  wood  is  formed.  What  in  these  cases  is  seen  to  take  place 
with  a  dye,  may  be  fairly  presumed  to  take  place  with  sap.  Where 
the  dye  exudes  but  slowly,  we  may  infer  that  the  sap  exudes  but 
slowly ;  and  it  is  a  fair  inference  that  where  the  dye  leaks  rapidly  out 
of  the  vessels,  the  sap  does  the  same.  Inferring,  thus,  that  where- 
ever  there  is  a  considerable  formation  of  wood  there  is  a  considerable 
escape  of  the  sap,  we  see  in  the  one  the  result  of  the  other.  The 
thickening  of  the  prosenchyma  is  proportionate  to  the  quantity  of 
nutritive  liquid  passing  into  it ;  and  this  nutritive  liquid  passes 
into  it  from  the  vessels,  ducts,  and  irregular  canals  it  surrounds. 

But  an  objection  is  made  to  such  experiments  as  the  foregoing, 
and  to  all  the  inferences  drawn  from  them.  It  is  said  that  portions 
of  plants  cut  off  and  thus  treated,  have  their  physiological  actions 
arrested,  or  so  changed  as  may  render  the  results  misleading ;  and  it 
is  said  that  when  detached  shoots  and  leaves  have  their  cut  ends 
placed  in  solutions,  the  open  mouths  of  their  vessels  and  ducts  are 
directly  presented  with  the  liquids  to  be  absorbed,  which  does  not 
happen  in  their  natural  states.  Further,  making  these  objections 
look  serious,  it  is  alleged  that  when  solutions  are  absorbed  through 
the  roots,  quite  different  results  are  obtained  :  the  absorbed  matters 
are  found  in  the  tissues  and  not  in  the  vessels.  Clearly,  were  the  ex- 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  573 

periments  yielding  these  adverse  results  conducted  in  unobjection- 
able ways,  the  conclusion  implied  by  them  would  negative  the  con- 
clusions above  drawn.  But  these  experiments  are  no  less  objection- 
able than  those  to  which  they  are  opposed.  Such  mineral  matters 
as  salts  of  iron,  solutions  of  which  have  in  some  cases  been  supplied 
to  the  roots  for  their  absorption,  are  obviously  so  unlike  the  mat- 
ters ordinarily  absorbed,  that  they  are  likely  to  interfere  fatally 
with  the  physiological  actions.  If  experiments  of  this  kind  are 
made  by  immersing  the  roots  in  a  dye,  there  is,  besides  the  dif- 
ficulty that  the  mineral  mordant  contained  by  the  dye  is  injurious 
to  the  plant,  the  further  difficulty  that  the  colouring  matter,  being 
seized  by  the  substances  for  which  it  has  an  affinity,  is  left  behind 
in  the  first  layers  of  root  tissues  passed  through,  and  that  the 
decolorized  water  passing  up  into  the  plant  is  not  traceable.  To 
be  conclusive,  then,  an  experiment  on  absorption  through  roots 
must  be  made  with  some  solution  which  will  not  seriously  inter- 
fere with  the  plant's  vital  processes,  and  which  will  not  have  its 
distinctive  element  left  behind.  To  fulfil  these  requirements  I 
adopted  the  following  method.  Having  imbedded  a  well-soaked 
broad-bean  in  moist  sand,  contained  in  an  inverted  cone  of  card- 
board with  its  apex  cut  off  for  the  radicle  to  come  through  —  having 
placed  this  in  a  wide-mouthed  dwarf  bottle,  partly  filled  with  water, 
so  that  the  protruding  radicle  dipped  into  the  water  —  and  having 
waited  until  the  young  bean  had  a  shoot  some  three  or  more  inches 
high,  and  a  cluster  of  secondary  rootlets  from  an  inch  to  an  inch 
and  a-half  long  —  I  supplied  for  its  absorption  a  simple  decoction  of 
logwood,  which,  being  a  vegetal  matter,  was  not  likely  to  do  it  much 
harm,  and  which,  being  without  a  mordant,  would  not  leave  its  sus- 
pended colour  in  the  first  tissues  passed  through.  To  avoid  any 
possible  injury,  I  did  not  remove  the  plant  from  the  bottle,  but 
slightly  raising  the  cone  out  of  its  neck,  I  poured  away  the  water 
through  the  crevice  and  then  poured  in  the  logwood  decoction  ;  so 
that  there  could  have  been  no  broken  end  or  abraded  surface  of  a 
rootlet  through  which  the  decoction  might  enter.  Being  prepared 
with  some  chloride  of  tin  as  a  mordant,  I  cut  off,  after  some  three 
hours,  one  of  the  lowest  leaves,  expecting  that  the  application  of  the 
mordant  to  the  cut  surface  would  bring  out  the  characteristic  colour 
if  the  logwood  decoction  had  risen  to  that  height.  I  got  no  re- 
action, however.  But  after  eight  hours  I  found,  on  cutting  off 
another  leaf,  that  the  vessels  of  its  petiole  were  made  visible  as  dark 
streaks  by  the  colour  with  which  they  were  charged  —  a  colour  differ- 
ing, as  was  to  be  expected,  from  that  of  the  logwood  decoction, 
which  spontaneously  changes  even  by  simple  exposure.  It  was  then 
too  late  in  the  day  to  pursue  the  observations  ;  but  next  morning 
the  vessels  of  the  whole  plant,  as  far  as  the  petioles  of  its  highest 


unfolded  leaves,  were  full  of  the  colouring-matter  ;  and  on  applying 
,  the  vessels  assumed  that  purplish 


chloride  of  tin  to  the  cut  surfaces, 


574  APPENDIX  C. 

red  which  this  mordant  produces  when  directlj 
wood  decoction.  Subsequently,  when  one  of  the  cotyledons  was  cut 
open  by  Prof.  Oliver,  to  whom,  in  company  with  Dr.  Hooker,  I 
showed  the  specimen,  we  found  that  the  whole  of  its  vascular  system 
was  filled  with  thedecoction,  which  everywhere  gavethe  characteristic 
reaction.  And  it  became  manifest  that  the  liquid  absorbed  through 
the  rootlets,  in  the  central  vessels  of  which  it  was  similarly  traceable, 
had  part  of  it  passed  directly  up  the  vessels  of  the  axis,  while  part  of 
it  had  passed  through  other  vessels  into  the  cotyledon,  out  of  which, 
no  doubt,  the  liquid  ordinarily  so  carried  returns  charged  with  a 
supply  of  the  stored  nutriment.  I  have  since  obtained  a  verification 
by  varying  the  method.  Digging  up  some  young  plants  (Marigolds 
happened  to  afford  the  best  choice)  with  large  masses  of  soil  round 
them,  placing  them  in  water,  so  as  gradually  to  detach  the  soil  with- 
out injuring  the  rootlets,  planting  them  afresh  in  a  flower-pot  full 
of  washed  sand,  and  then,  after  a  few  days,  watering  them  with  a 
logwood  decoction,  I  found,  as  before,  that  in  less  than  twenty- 
four  hours  the  colouring-matter  had  run  up  into  the  vessels  of  the 
leaves.  Though  the  reaction  produced  by  the  mordant  was  not  so 
strong  as  before,  it  was  marked  enough  to  be  quite  unquestionable. 

As  these  experiments  were  so  conducted  that  there  was  no  ac- 
cess to  the  vessels  except  through  the  natural  channels,  and  as  the 
vital  actions  of  the  plants  were  so  little  interfered  with  that  at  the 
end  of  twenty-four  hours  they  showed  no  traces  of  disturbance,  I 
think  the  results  must  be  held  conclusive. 

Taking  it,  then,  as  a  fact  that  in  plants  possessing  them  the  vessels 
and  ducts  are  the  channels  through  which  sap  is  distributed,  we  come 
now  to  the  further  question — What  determines  the  varying  permea- 
bility of  the  walls  of  the  vessels  and  ducts,  and  the  consequent  vary- 
ing formation  of  wood  ?  To  this  question  I  believe  the  true  reply  is 
— Theexposureof  the  parts  to  intermittent  mechanical  strains,  actual 
or  potential,  or  both.  By  actual  strains  I  of  course  mean  those 
which  the  plant  experiences  in  the  course  of  its  individual  life.  Bv 
potential  strains  T  mean  those  which  the  form,  attitude,  and  circum- 
stances common  to  its  kind  involve,  and  which  its  inherited  struc- 
ture is  adapted  to  meet.  In  plants  with  stems,  petioles,  and  leaves, 
having  tolerably  constant  attitudes,  the  increasing  porosity  of  the 
tubes  and  consequent  deposit  of  dense  tissue  takes  place  in  anticipa- 
tion of  the  strains  to  which  the  parts  of  the  individual  are  liable,  but 
takes  place  at  parts  which  have  been  habitually  subject  to  such 
strains  in  ancestral  individuals.  But  though  in  such  plants  the 
tendency  to  repeat  that  distribution  of  dense  tissue  caused  by 
mechanical  actions  on  past  generations,  goes  on  irrespective  of  the 
mechanical  actions  to  which  the  developing  individual  is  subject, 
these  direct  actions,  while  they  greatly  aid  the  assumption  of  the 
typical  structure,  are  the  sole  causes  of  those  deviations  in  the  rela- 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  575 


of  its  kind.  And  then,  in  certain  irregularly  growing  plants,  such  as 
Cactuses  and  Euphorbias,  where  the  strains  fall  on  parts  that  do 
not  correspond  in  successive  individuals,  we  distinctly  trace  a  direct 
relation  between  the  degrees  of  strain  and  the  rates  of  these  changes 
which  result  in  dense  tissue.  I  will  not  occupy  space  in  detailing 
the  evidence  of  this  relation,  which  is  conspicuous  in  the  orders 
named,  but  will  pass  to  thequestion — What  are  the  physical  processes 
by  which  intermittent  mechanical  strains  produce  this  deposit  of 
resistant  substance  at  places  where  it  is  needed  to  meet  the  strains? 
We  have  not  to  seek  far  for  an  answer.  If  a  trunk,  a  bough,  a 
shoot,  or  a  petiole,  is  bent  by  a  gust  of  wind,  the  substance  of  its 
convex  side  is  subject  to  longitudinal  tension:  the  substance  of  its 
concave  side  being  at  the  same  time  compressed.  This  is  the  pri- 
mary mechanical  effect.  There  is,  however,  a  secondary  mechani- 
cal effect,  which  here  chiefly  concerns  us.  That  bend  by  which  the 
tissues  of  the  convex  side  are  stretched,  also  produces  lateral  com- 
pression of  them.  Buttoning  on  a  tight  glove  and  then  closing  the 
hand,  will  make  this  necessity  clear :  the  leather,  while  it  is  strained 
along  the  backs  of  the  fingers,  presses  with  considerable  force  on  the 
knuckles.  It  is  demonstrable  that  the  tensions  of  the  outer  layer 
of  a  mass  made  convex  by  bending,  must,  by  composition  of  forces, 
produce  at  every  point  a  resultant  at  right  angles  to  the  layer  be- 
neath it ;  that,  similarly,  the  joint  tensions  of  these  two  layers  must 
throw  a  pressure  on  the  next  deeper  layer ;  and  so  on.  Hence,  if 
at  some  little  distance  beneath  the  surface  of  a  stem,  twig,  or  leaf- 
stalk, there  exist  longitudinal  tubes,  these  tubes  must  be  squeezed 
each  time  the  side  of  the  branch  they  are  placed  on  becomes  convex. 
Modifying  the  illustration  just  drawn  from  the  clenched  hand  will 
make  this  clear.  When,  on  forcibly  grasping  something,  the  skin  is 
drawn  tightly  over  the  back  of  the  hand,  the  whitening  of  the 
knuckles  shows  how  the  blood  is  expelled  from  the  vessels  below 
the  surface  by  the  pressure  of  the  tightened  skin.  If,  then,  the  sap- 
vessels  must  be  thus  compressed,  what  will  happen  to  the  liquid  they 
contain  ?  It  will  move  away  along  the  lines  of  least  resistance. 
Part,  and  probably  the  greater  part,  will  escape  lengthways  from  the 
place  of  greatest  pressure :  some  of  it  being  expelled  downwards, 
and  some  of  it  upwards.  But,  at  the  same  time,  part  of  it  will  be 
likely  to  ooze  through  the  walls  of  the  tubes.  If  these  walls  are  so 
perfect  as  to  permit  the  passage  of  liquid  only  by  osmose,  it  may 
still  be  inferred  that  the  osmose  will  increase  under  pressure ;  and 
probably,  under  recurrent  pressure,  the  places  at  which  the  osmotic 
current  passes  most  readily  will  become  more  and  more  permeable, 
until  they  eventually  form  pores.  At  any  rate  it  is  manifest  that 
where  pores  and  slits  exist,  whether  thus  formed  or  formed  in  any 
other  way,  the  escape  of  sap  into  the  adjacent  tissue  at  each  bend 


576  APPENDIX  0. 

will  become  easy  and  rapid.  What  further  must  happen  ?  When 
the  branch  or  shoot  recoils,  the  vessels  on  the  side  that  was  convex, 
being  relieved  from  pressure,  will  tend  to  resume  their  previous 
diameters ;  and  will  be  helped  to  do  this  by  the  elasticity  of  the 
surrounding  tissue,  as  well  as  by  those  spiral,  annular,  and  allied 
structures  which  they  contain.  But  this  resumption  of  their  previ- 
ous diameters  must  cause  an  immediate  rush  of  sap  back  into  them. 
Whence  will  it  come  ?  Not  to  any  considerable  extent  from  the  sur- 
rounding tissues  into  which  part  of  it  has  been  squeezed,  seeing  that 
the  resistance  to  the  return  of  liquid  through  small  pores  will  be 
greater  than  the  resistance  to  its  return  along  the  vessels  themselves. 
Manifestly  the  sap  which  was  thrust  up  and  down  the  vessels  from 
the  place  of  compression  will  return — the  quantities  returning  from 
above  and  from  below  varying,  as  we  shall  hereafter  see,  according 
to  circumstances.  But  this  is  not  all.  From  some  side  a  greater 
quantity  must  come  back  than  was  sent  away  ;  for  the  amount  that 
has  escaped  out  of  the  tube  into  the  prosenchyma  has  to  be  replaced. 
Thus  during  the  time  when  the  side  of  the  branch  or  twig  becomes 
concave,  more  sap  returns  from  above  or  below  than  was  expelled 
upwards  or  downwards  during  the  previous  compression.  The 
refilled  vessels,  when  the  next  bend  renders  their  side  convex,  again 
have  part  of  their  contents  forced  through  their  parietes,  and  are 
again  refilled  in  the  same  way.  There  is  thus  set  up  a  draught  of  sap 
to  the  place  where  these  intermittent  strains  are  going  on,  an  exuda- 
tion proportionate  to  the  frequency  and  intensity  of  the  strains, 
and  a  proportionate  nutrition  or  thickening  of  the  wood- cells, 
fitting  them  to  resist  the  strains.  A  rude  idea  of  this  action  may 
be  obtained  by  grasping  in  one  hand  a  damp  sponge,  having  its 
lower  end  in  water,  while  holding  a  piece  of  blotting-paper  in 
contact  with  its  upper  end,  and  then  giving  the  sponge  repeated 
squeezes.  At  each  squeeze  some  of  the  water  will  be  sent  into 
the  blotting-paper ;  at  each  relaxation  the  sponge  will  refill  from 
below,  to  give  another  portion  of  its  contents  to  the  blotting- 
paper  when  again  squeezed. 

But  how  does  this  explanation  apply  to  roots  ?  If  the  formation 
of  wood  is  due  to  intermittent  transverse  strains,  such  as  are  pro- 
duced in  the  aerial  parts  of  upright  plants  by  the  wind,  how  does  it 
happen  that  woody  matter  is  deposited  in  roots,  where  there  are  no 
lateral  oscillations,  no  transverse  strains  ?  The  answer  is,  that 
longitudinal  strains  also  are  capable  of  causing  the  effects  described. 
It  is  true  that  perfectly  straight  fibres  united  into  a  bundle  and  pulled 
lengthways  would  not  exert  on  one  another  any  lateral  pressure,  and 
would  not  laterally  compress  any  similarly-straight  canals  running 
along  with  them.  But  if  the  fibres  united  into  a  bundle  are  variously 
bent  or  twisted,  they  cannot  be  longitudinally  strained  without  com- 
pressing one  another  and  structures  imbedded  in  them.  It  needs 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  577 

but  to  watch  a  wet  rope  drawn  tight  by  a  capstan,  to  see  that  an 
action  like  that  which  squeezes  the  water  out  of  its  strands,  will 
squeeze  the  sap  out  of  the  vessels  of  a  root  into  the  surrounding 
tissue,  as  often  as  the  root  is  pulled  by  the  swaying  of  the  plant  it 
belongs  to.  Here,  too,  as  before,  the  vessels  will  refill  when  the 
pull  intermits ;  and  so,  in  the  roots  as  in  the  branches,  this  rude 
pumping  process  will  produce  a  growth  of  hard  tissue  proportion- 
ate to  the  stress  to  be  borne. 

These  conclusions  are  supported  by  the  evidence  which  excep- 
tional cases  supply.  If  intermittent  mechanical  strains  thus  cause 
the  formation  of  wood  where  wood  is  found,  then  where  it  is  not 
found,  there  should  be  an  absence  of  intermittent  mechanical  strains. 
There  is  such  an  absence.  Vascular  plants  characterized  by  little 
or  no  deposit  of  dense  substance,  are  those  having  vessels  so  con- 
ditioned that  no  considerable  pressures  are  borne  by  them.  The 
more  succulent  a  petiole  or  leaf  becomes,  the  more  do  the  effects  of 
transverse  strains  fall  on  its  outer  layers  of  cells.  Its  mechanical  sup- 
port is  chiefly  derived  from  the  ability  of  these  minute  vesicles,  full 
of  liquid,  to  resist  bursting  and  tearing  under  the  compressions  and 
tensions  they  are  exposed  to.  And  just  as  fast  as  this  change  from 
a  thin  leaf  or  foot-stalk  to  a  thick  one  entails  increasing  stress  on  the 
superficial  tissue,  so  fast  does  it  diminish  the  stress  on  the  internally- 
seated  vascular  tissue.  The  succulent  leaf  cannot  be  swayed  about 
by  the  wind  as  much  as  an  ordinary  leaf ;  and  such  small  bends  as 
can  be  given  to  it  and  its  foot-stalk  are  prevented  from  affecting 
in  any  considerable  degree  the  tubes  running  through  its  interior. 
Hence  the  retentiveness  of  the  vessels  in  these  fleshy  leaves,  as  shown 
by  the  small  exudation  of  dye ;  and  hence  the  small  thickening  of 
their  surrounding  prosenchyma  by  woody  deposit.  Still  more  con- 
spicuously is  this  connexion  of  facts  shown  when,  from  the  soft  thick 
leaves  before  named  and  such  others  as  those  of  JEcheveria,  Rochea, 
Pereskia,  we  turn  to  the  thick  leaves  that  have  strong  exo-skeletons. 
Gasteria  serves  as  an  illustration.  The  leathery  or  horny  skin  here 
evidently  bears  the  entire  weight  of  the  leaf,  and  is  so  stiff  as  to  pre- 
vent any  oscillation.  Here,  then,  the  vessels  running  inside  are  pro- 
tected from  all  mechanical  stress ;  and  accordingly  we  find  that  the 
cells  surrounding  them  are  not  appreciably  thickened. 

Equally  clear,  and  more  striking  because  more  obviously  excep- 
tional, is  the  evidence  given  by  succulent  stems  which  are  leafless. 
Stapelia  Bu/onia,  having  soft  procumbent  axes  not  liable  to  be 
bent  backwards  and  forwards  in  any  considerable  degree  by  the 
wind,  has,  ramifying  through  its  tissue,  vessels  that  allow  but  an  ex- 
tremely slow  escape  of  dye  and  have  unthickened  sheaths.  Such  of 
the  Euphorbias  as  have  acquired  the  fleshy  character  while  retaining 
the  arborescentgrowth,like Euphorbia  Canaricnsis,tea,ch  us  the  same 
truth  in  another  way.  In  them  the  formation  of  wood  around  the 
83 


578  APPENDIX  C. 

vessels  is  inconspicuous  where  the  intermittent  strains  are  but  slight; 
but  it  is  conspicuous  at  those  joints  on  which  lateral  oscillations  of 
the  attached  branches  throw  great  extensions  and  compressions  of 
tissue.  Throughout  the  Cactacece  we  find  varied  examples  of  the 
alleged  relation.  Mammillaria  furnishes  a  very  marked  one.  The 
substance  of  one  of  these  globular  masses,  resting  on  the  ground, 
admits  of  no  bending  from  side  to  side ;  and  accordingly  its  large 
bundles  of  spiral  and  annular  vessels,  or  partially-united  cells,  have 
very  feebly-marked  sheaths  not  at  all  thickened.  In  such  types  as 
Cereus  and  Opuntia  we  see,  as  in  the  Euphorbias,  that  where  little 
stress  falls  on  the  vessels,  little  deposit  takes  place  around  them ; 
while  there  is  much  deposit  where  there  is  much  stress.  Here  let  me 
add  a  confirmation  obtained  since  writing  the  above.  After  observ- 
ing among  the  Cactuses  the  very  manifest  relation  between  strain 
and  the  formation  of  wood,  I  inquired  of  Mr.  Croucher,  the  intelli- 
gent foreman  of  the  Cactus-house  at  Kew,  whether  he  found  this 
relation  a  constant  one.  He  replied  that  he  did,  and  that  he  had 
frequently  tested  it  by  artificially  subjecting  parts  of  them  to  strains. 
Neglecting  at  the  time  to  inquire  how  he  had  done  this,  it  afterwards 
occurred  to  me  that  if  he  had  so  done  it  as  to  cause  constant  strains, 
the  observed  result  would  not  tell  in  favour  of  the  foregoing  inter- 
pretation. Subsequently,  however,  I  learned  that  he  had  produced 
the  strains  by  placing  the  plants  in  inclined  attitudes — a  method 
which,  by  permitting  oscillations  of  the  strained  joints,  allowed  the 
strains  to  intermit.  And  then,  making  the  proof  conclusive,  Mr. 
Croucher  volunteered  the  statement  that  where  he  had  produced 
constant  strains  by  tying,  no  formation  of  wood  took  place. 

Aberrant  growths  of  another  class  display  the  same  relations 
of  phenomena.  Take  first  the  underground  stems,  such  as  the 
Potato  and  the  Artichoke.  The  vessels  which  run  through  these, 
slowly  take  up  the  dye  without  letting  it  pass  to  any  considerable 
extent  into  the  surrounding  tissues.*  Only  after  an  interval  of 
many  hours  does  the  prose'nchyma  become  stained  in  some  places. 
Here,  as  before,  an  absence  of  rapid  exudation  accompanies  an 
absence  of  woody  deposit ;  and  both  these  go  along  with  the  ab- 
sence of  intermittent  strains.  Take  again  the  fleshy  roots.  The 
Turnip,  the  Carrot,  and  the  Beetroot,  have  vessels  that  retain  very 
persistently  the  coloured  liquids  they  take  up.  And  differing  in  this, 
as  these  roots  do,  from  ordinary  roots,  we  see  that  they  also  differ 
from  them  in  not  being  woody,  and  in  not  being  appreciably  sub- 

*  Those  who  repeat  these  experiments  must  be  prepared  for  great  irresru- 
larities  in  the  rates  of  absorption.  Succulent  structures  in  general  absorb 
much  more  slowly  than  others,  and  sometimes  will  scarcely  take  up  the  dye 
at  all.  The  differences  between  different  structures,  and  the  same  structure 
at  different  times,  probably  depend  on  the  degrees  in  which  the  tissues  are 
charged  with  liquid  and  the  rates  at  which  they  are  losing  it  by  evaporation. 


CIRCULATION  AND  FORMATION  OP  WOOD  IN  PLANTS.  579 

ject  to  the  usual  mechanical  actions.  In  these  cases,  as  in  the 
others,  parts  that  ordinarily  become  dense,  deviate  from  this  typ- 
ical character  when  they  are  not  exposed  to  those  forces  which 
produce  dense  tissue  by  increasing  the  extravasation  of  sap. 

To  complete  the  proof  that  such  a  relation  exists,  let  me  add  the 
results  of  some  experiments  on  equal  and  similarly-developed  parts, 
kept  respectively  at  rest  and  in  motion.  I  have  tested  the  effects  on 
large  petioles,  on  herbaceous  shoots,  and  on  woody  shoots.  If  two 
such  petioles  as  those  of  Rhubarb,  with  their  leaves  attached,  have 
their  cut  ends  inserted  in  bottles  of  dye,  and  the  one  be  bent  back- 
wards and  forwards  while  the  other  remains  motionless,  there  arises, 
after  the  lapse  of  an  hour,  scarcely  any  difference  in  the  states  of 
their  vessels:  a  certain  proportion  of  these  are  in  both  cases  charged 
with  the  dye,  and  little  exudation  has  been  produced  by  the  motion. 
Here,  however,  it  is  to  be  observed  that  the  causes  of  exudation  are 
scarcely  operative ;  the  vascular  bundles  are  distributed  all  through 
the  mass  of  the  petiole,  which  is  formed  of  soft  watery  tissue ;  and 
they  are,  therefore,  not  so  circumstanced  as  to  be  effectually  com- 
pressed by  the  bends.  In  herbaceous  stems,  such  as  those  of  the 
Jerusalem  Artichoke  and  of  the  Foxglove,  an  effect  scarcely  more 
decided  is  produced ;  and  here,  too,  when  we  seek  a  reason,  we  find 
it  in  the  non-fulfilmentof  the  mechanical  conditions;  for  thevascular 
bundles  are  not  so  seated  between  a  tough  layer  of  bark  and  a  solid 
core  as  to  be  compressed  at  each  bend.  When,  however,  we  come 
to  experiment  upon  woody  shoots,  we  meet  with  conspicuous  effects, 
though  by  no  means  uniformly.  In  some  cases  oscillations  produce 
immense  amounts  of  exudation — parallel  transverse  sections  of  the 
compared  shoots  showing  that  where,  in  the  one  that  has  been  at 
rest,  there  are  spots  of  colour  round  but  a  few  pitted  ducts,  in  the 
one  that  has  been  kept  in  motion  the  substance  of  the  wood  is  soaked 
almost  uniformly  through  with  dye.  In  other  cases,  especially  where 
there  is  much  undifferentiated  tissue  remaining,  the  exudation  is  not 
very  marked.  The  difference  appears  to  depend  on  the  quantity  of 
'liquid  contained  in  the  shoot.  If  its  substance  is  relatively  dry,  the 
exudation  is  great ;  but  it  is  comparatively  small  if  all  the  tissues 
are  fully  charged  with  sap.  This  contrast  of  results  is  one  which 
contemplation  of  the  mechanical  actions  will  lead  us  to  expect. 

And  now,  with  these  facts  to  aid  our  interpretation,  let  us  re- 
turn to  ordinary  stems.  If  the  upper  end  of  a  growing  shoot,  the 
prosenchyma  of  which  is  but  little  thickened,  be  allowed  to  imbibe 
the  dye,  the  vessels  of  its  medullary  sheath  alone  become  charged  ; 
and  from  them  there  takes  place  but  a  slow  oozing.  If  a  like  ex- 
periment be  tried  with  a  lower  part  of  the  shoot,  where  the  wood  in 
course  of  formation  has  its  inner  boundary  marked  but  not  its  outer 
boundary,  we  find  that  the  pitted  ducts,  and  more  especially  the 
inner  ones,  come  into  play.  And  then  lower  still,  where  the  wood 


580  APPENDIX  C. 

has  its  periphery  defined  and  its  histological  characters  decided, 
the  appearances  show  that  the  tissue  forming  its  outer  surface 
begins  to  take  a  leading  part  in  the  transmission  of  liquid.  What 
now  is  the  explanation  of  these  changes,  mechanically  considered  ? 
In  the  young  soft  part  of  the  shoot,  as  in  all  normal  and  abnormal 
growths  that  have  not  formed  wood,  the  channels  for  the  passage  of 
sap  are  the  spiral,  annular,  f  enestrated,  or  reticulated  vessels.  These 
vessels,  here  included  in  the  bundles  of  the  medullary  sheath,  are, 
in  common  with  the  tissues  around  them,  subject,  by  the  bendings 
of  the  shoot,  to  slight  intermittent  compressions,  and,  especially 
the  outermost  of  them,  are  thus  forced  to  give  the  prosenchyma 
an  extra  supply  of  nutritive  liquid.  The  thickening  of  the  pro- 
senchyma, spreading  laterally  as  well  as  outwards  from  each  bundle 
of  the  medullary  sheath,  goes  on  until  it  meets  the  thickenings  that 
spread  from  the  other  bundles ;  and  there  is  so  formed  an  irregular 
cylinder  of  hardened  tissue,  surrounding  the'  medulla  and  the  vas- 
cular bundles  of  its  sheath.  As  soon  as  this  happens,  these  vascular 
bundles  become,  to  a  considerable  extent,  shielded  from  the  effects 
of  transverse  strains,  since  the  tensions  and  compressions  chiefly 
fall  on  the  developing  wood  outside  of  them.  Clearly,  too,  the 
greatest  stress  must  be  felt  by  the  outer  layer  of  the  developing 
wood :  being  further  removed  from  the  neutral  axis,  it  must  be 
subject  to  severer  strains  at  each  bend ;  and  lying  between  the 
bark  and  the  layer  of  wood  first  formed,  it  must  be  most  exposed 
to  lateral  compressions.  Among  the  elongated  cells  of  this  outer 
layer,  some  unite  to  form  the  pitted  ducts.  Being,  as  we  see, 
better  circumstanced  mechanically,  they  become  greater  carriers 
of  sap  than  the  original  vessels,  and,  in  consequence  of  this,  as  well 
as  in  consequence  of  their  relative  proximity,  become  the  sources 
of  nutrition  to  the  still  more  external  layers  of  wood-cells.  The 
same  causes  and  the  same  effects  hold  with  each  new  indurated 
coat  deposited  round  the  previously  indurated  coats. 

This  description  may  be  thought  to  go  far  towards  justifying  the 
current  views  respecting  the  course  taken  by  the  sap.  But  the 
justification  is  more  apparent  than  real.  In  the  first  place,  the  im- 
plication here  is  that  the  sap-carrying  function  is  at  first  discharged 
entirely  by  the  vessels  of  the  medullary  sheath,  and  that  they  cease 
to  discharge  this  function  only  as  fast  as  they  are  relatively  incapaci- 
tated by  their  mechanical  circumstances.  And  the  second  implica- 
tion is,  that  it  is  not  the  wood  itself,  but  the  more  or  less  continuous 
canals  formed  in  it,  which  are  the  subsequent  sap-distributors.  This, 
though  readily  made  clear  by  microscopic  examination  of  the  large 
pitted  ducts  in  a  partial lylignified  shoot  that  has  absorbed  the  dye, 
is  less  manifestly  true  of  the  peripheral  layer  of  sap-carrying  tissue 
finally  formed.  But  it  is  really  true  here.  For  this  layer,  though 
nominally  a  layer  of  wood,  is  practically  a  layer  of  inosculating 


CIRCULATION  AND  FORMATION  OP  WOOD  IN  PLANTS.  581 

vessels.  It  is  formed  out  of  irregular  lines  and  networks  of  elon- 
gated pitted  cells,  obliquely  united  by  their  ends.  Examination  of 
them  after  absorption  of  a  dye,  shows  that  it  is  only  along  the  con- 
tinuous channels  they  unite  to  form  that  the  current  has  passed. 
But  the  essentially  vascular  character  of  this  outer  and  latest-formed 
layer  of  the  alburnum  is  best  seen  in  the  fact  that  the  vascular  sys- 
tems of  new  axes  take  their  rise  from  it,  and  form  with  it  continuous 
canals.  If  a  shoot  of  last  year  in  which  growth  is  recommencing,  be 
cut  lengthways  after  it  has  imbibed  a  dye,  clear  proof  is  obtained 
that  the  passage  of  the  dye  into  a  lateral  bud  takes  place  from  this 
outermost  layer  of  pitted  cells,  and  that  the  channels  taken  by  the 
dye  through  the  new  tissue  are  composed  of  cells  that  pass  through 
modified  forms  into  the  spiral  vessels  of  the  new  medullary  sheath. 
This  transition  may  be  still  more  clearly  traced  in  a  terminal  bud 
that  continues  the  line  of  last  year's  shoot.  A  longitudinal  section 
of  this  shows  that  the  vessels  of  the  new  medullary  sheath  do  not 
obtain  their  sap  from  the  vessels  of  last  year's  sheath  (which,  as 
shown  by  the  non-absorption  of  dye,  have  become  inactive),  but 
that  their  supplies  are  obtained  from  those  inosculating  canals 
formed  out  of  last  year's  outermost  layer  of  prosenchyma,  and  that 
between  the  component  cells  of  this  and  those  of  the  new  vascular 
system  there  are  all  gradations  of  structure.* 

*  It  may  be  added  here  that,  on  considering  the  mechanical  actions  that 
must  go  on,  we  are  enabled  in  some  measure  to  understand  both  how  such  inos- 
culating channels  are  initiated,  and  how  the  structures  of  their  component 
cells  are  explicable.  What  must  happen  to  one  of  these  elongated  prosen- 
chyma-cells  if,  in  the  course  of  its  development,  it  is  subject  to  intermittent 
compressions  ?  Its  squeezed-out  liquid  while  partially  escaping  laterally, 
will  more  largely  escape  upwards  and  downwards;  and  while  repeated 
lateral  escape  will  tend  to  form  lateral  channels  communicating  with 
laterally-adjacent  cells,  repeated  longitudinal  escape  will  tend  to  form 
channels  communicating  with  longitudinally-adjacent  cells— so  producing 
continuous  though  irregular  longitudinal  canals.  Meanwhile  each  cell  into 
and  out  of  which  the  nutritive  liquid  is  from  time  to  time  squeezed 
through  small  openings  in  its  walls,  cannot  thicken  internally  in  an 
even  manner:  deposition  will  be  interfered  with  by  the  passage  of  the 
currents  through  the  pores.  The  rush  to  or  from  each  pore  will  tcnu 
to  maintain  a  funnel-shaped  depression  in  the  deposit  around;  and  the 
opening  from  cell  to  cell  will  so  acquire  just  that  shape  which  the  microscope 
shows  up — two  hollow  cones  with  their  apices  meeting  at  the  point  where 
the  cell-membranes  are  in  contact.  Moreover,  as  confirming  this  inter- 
pretation, it  may  be  remarked  that  we  are  thus  supplied  with  a  reason 
for  the  differences  of  shape  between  these  passages  from  one  pitted  cell 
to  another,  and  the  analogous  passages  that  exist  between  cells  other- 
wise formed  and  otherwise  conditioned.  In  the  cells  of  the  medulla,  and 
others  which  are  but  little  exposed  to  compression,  the  passages  are  seve- 
rally formed  more  like  a  tube  with  two  trumpet-mouths,  one  in  each  cell. 
This  is  just  the  form  which  might  be  expected  where  the  nutritive  fluid 
passes  from  cell  to  cell  in  moderate  currents,  and  not  by  the  violent  rushes 
caused  by  intermittent  pressures.  Of  course  it  is  not  meant  that  in  each 


582  APPENDIX  C. 

It  is  not  the  aim  of  the  foregoing  reasoning  to  show  that  mechani- 
cal actions  are  the  sole  causes  of  the  formation  of  dense  tissue  in 
plants.  Dense  tissue  is  in  many  cases  formed  where  no  such  causes 
have  come  into  play — as,  for  example,  in  thorns  and  in  the  shells  of 
nuts.  Here  the  natural  selection  of  variations  can  alone  have  ope- 
rated. It  is  manifest,  too,  that  even  those  supporting  structures  the 
building  up  of  which  is  above  ascribed  to  intermittent  strains,  may, 
in  the  individual  plant  of  a  species  that  ordinarily  has  them,  be  de- 
veloped to  a  great  extent  when  intermittent  strains  are  prevented. 
We  see  this  in  trees  that  are  artificially  supported  by  nailing  to 
walls ;  and  we  also  see  a  kindred  fact  in  natural  climbers.  Though 
in  these  cases  the  formation  of  wood  is  obviously  less  than  it  would 
be  were  the  stem  and  branches  habitually  moved  about  by  the  wind,  it 
nevertheless  goes  on.  Clearly  the  tendency  of  the  plant  to  repeat  the 
structure  of  its  type  (in  the  one  case  the  structure  of  its  species,-  and 
in  the  other  case  that  of  the  order  from  which  it  has  diverged  in  be- 
coming a  climber)  is  here  almost  the  sole  cause  of  wood-formation. 
But  though  in  plants  so  circumstanced  intermittent  mechanical 
strains  have  little  or  no  direct  share,  it  may  still  be  true,  and  I  believe 
is  true,  that  intermittent  mechanical  strains  are  the  original  cause ; 
for,  as  before  hinted,  the  typical  structure  which  the  individual  thus 
repeats  irrespective  of  its  own  conditions,  is  interpretable  as  a  typical 
structure  that  is  itself  the  product  of  these  actions  and  reactions  be- 
tween the  plant  and  its  environment.  Grant  the  inheritance  of  func- 
tionally-produced modifications ;  grant  that  natural  selection  will 
always  co-operate  in  such  way  as  to  favour  those  individuals  and 
families  in  which  functionally-produced  modifications  have  pro- 
gressed most  advantageously ;  and  it  will  follow  that  this  mechani- 
cally-caused formation  of  dense  substance,  accumulating  from  gen- 
eration to  generation  by  the  survival  of  the  fittest,  will  result  in  an 
organic  habit  of  forming  dense  tissue  at  the  required  places.  The 
deposit  arising  from  exudation  at  the  places  of  greatest  strain,  re- 
curring from  generation  to  generation  at  the  same  places,  will 
come  to  be  reproduced  in  anticipation  of  strain,  and  will  continue 
to  be  reproduced  for  a  long  time  after  a  changed  habit  of  the 
species  prevents  the  strain — eventually,  however,  decreasing,  both 
through  functional  inactivity  and  natural  selection,  to  the  point 
at  which  it  is  in  equilibrium  with  the  requirement. 

individual  cell  these  structures  are  determined  by  these  mechanical  actions. 
The  facts  clearly  negative  any  such  conclusion,  showing  us,  as  they  in  many 
cases  do,  that  these  structures  are  assumed  in  advance  of  these  mechanical 
actions.  The  implication  is,  that  such  mechanical  actions  initiated  modifi- 
cations that  have,  with  the  aid  of  natural  selection,  been  accumulated  from 
generation  to  generation;  until,  in  conformity  with  ordinary  embryological 
laws,  the  cells  of  the  parts  exposed  to  such  actions  assume  these  special 
structures  irrespective  of  the  actions — the  actions,  however,  still  serving  to 
aid  and  complete  the  assumption  of  the  inherited  type. 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  583 

Anotherside  of  the  general  question  may  now  be  considered.  We 
have  seen  how,  by  intermittent  pressures  on  capillary  vessels  and 
ducts  and  inosculating  canals,  there  must  be  produced  a  draught  of 
sap  towards  the  point  of  compression  to  replace  the  sap  squeezed  out. 
But  we  have  still  to  inquire  what  will  be  the  effect  on  the  distribu- 
tion of  sap  throughout  the  plant  as  a  whole.  It  was  concluded  that 
out  of  the  compressed  vessels  the  greater  part  of  the  liquid  would 
escape  longitudinally — the  longitudinal  resistance  to  movement 
being  least.  In  every  case  the  probabilities  are  infinity  to  one  against 
the  resistances  being  equal  upwards  and  downwards.  Always,  then, 
more  sap  will  be  expelled  in  one  direction  than  in  the  other.  But  in 
whichever  direction  least  sap  is  expelled,  from  that  same  direction 
most  sap  will  return  when  the  vessels  are  relieved  from  pressure — the 
force  which  is  powerful  in  arrestingthe  back  current  in  that  direction 
being  the  same  force  which  is  powerful  in  producing  a  forward  cur- 
rent. Ordinarily,  the  more  abundant  supply  of  liquid  being  from  be- 
low, there  will  result  an  upward  current.  At  each  bend  a  portion  of 
the  contents  will  be  squeezed  out  through  the  sides  of  the  vessels — a 
portion  will  be  squeezed  downwards,  reversing  the  current  ascending 
from  the  roots,  but  soon  stopped  by  its  resistance ;  while  a  larger  por- 
tion will  be  squeezed  upwards  towards  the  extremities  of  the  vessels, 
where  consumption  and  loss  are  most  rapid.  At  each  recoil  the  ves- 
sels will  be  replenished,  chiefly  by  the  repressed  upward  current ;  and 
at  the  next  bend  more  of  it  will  be  thrust  onwards  than  backwards. 
Hence  we  have  everywhere  in  action  a  kind  of  rude  force-pump, 
worked  by  the  wind ;  and  we  see  how  sap  may  thus  be  raised  to  a 
height  far  beyond  that  to  which  it  could  be  raised  by  capillary  ac- 
tion, aided  by  osmose  and  evaporation. 

Thus  far,  however,  the  argument  proceeds  on  the  assumption  that 
there  is  liquid  enough  to  replenish  every  time  the  vessels  subject 
to  this  process.  But  suppose  the  supply  fails — suppose  the  roots 
have  exhausted  the  surrounding  stock  of  moisture.  Evidently  the 
vessels  thus  repeatedly  having  their  contents  squeezed  out  into  the 
surrounding  tissue,  cannot  go  on  refilling  themselves  from  other 
vessels  without  tending  to  empty  the  vascular  system.  On  the  one 
hand,  evaporation  from  the  leaves  causing  a  draught  on  the  capillary 
tubes  that  end  in  them,  continually  generates  a  capillary  tension  up- 
wards ;  while,  on  the  other  hand,  the  vessels  below,  expanding  after 
their  sap  has  been  squeezed  out,  produce  a  tension  both  upwards 
and  downwards  towards  the  point  of  loss.  Were  the  limiting  mem- 
branes of  the  vessels  impermeable,  the  movement  of  sap  would,  under 
these  conditions,  soon  be  arrested.  But  these  membranes  are  perme- 
able ;  and  the  surrounding  tissues  readily  permit  the  passage  of  air. 
This  state  of  tension,  then,  will  cause  an  entrance  of  air  into  the  tubes; 
the  columns  of  liquid  they  contain  will  be  interrupted  by  bubbles. 
It  seems,  indeed,  not  improbable  that  this  entrance  of  air  may  take 


584  APPENDIX  C. 

place  even  when  there  is  a  good  supply  of  liquid,  if  the  mechanical 
strains  are  so  violent  and  the  exudation  so  rapid  that  the  currents 
cannot  refill  the  half -emptied  vessels  with  sufficient  rapidity.  And 
in  this  case  the  intruding  air  may  possibly  play  the  same  part  as 
that  contained  in  the  air-chamber  of  a  force-pump — tending,  by 
moderating  the  violence  of  the  jets,  and  by  equalizing  the  strains, 
to  prevent  rupture  of  the  apparatus.  Of  course  when  the  supply 
of  liquid  becomes  adequate,  and  the  strains  not  too  violent,  these 
bubbles  will  be  expelled  as  readily  as  they  entered. 

Here,  as  before,  let  me  add  the  conclusive  proof  furnished  by 
a  direct  experiment.  To  ascertain  the  amount  of  this  propulsive 
action,  I  took  from  the  same  tree,  a  Laurel,  two  equal  shoots,  and 
placing  them  in  the  same  dye,  subjected  them  to  conditions  that 
were  alike  in  all  respects  save  that  of  motion  :  while  one  remained 
at  rest,  the  other  was  bent  backwards  and  forwards,  now  by  switch- 
ing and  now  by  straining  with  the  fingers.  After  the  lapse  of  an 
hour,  I  found  that  the  dye  had  ascended  the  oscillating  shoot  three 
times  as  far  as  it  had  ascended  the  stationary  shoot — this  result 
being  an  average  from  several  trials.  Similar  trials  brought  out 
similar  effects  in  other  structures.  The  various  petioles  and  herba- 
ceous shoots  experimented  upon  for  the  purpose  of  ascertaining 
the  amount  of  exudation  produced  by  transverse  strains,  showed 
also  the  amount  of  longitudinal  movement.  It  was  observable 
that  the  height  ascended  by  the  dye  was  in  all  cases  greater  where 
there  had  been  oscillation  than  where  there  had  been  rest — the 
difference,  however,  being  much  less  marked  in  succulent  struc- 
tures than  in  woody  ones. 

It  need  scarcely  be  said  that  this  mechanical  action  is  not  here 
assigned  as  the  sole  cause  of  circulation,  but  as  a  cause  co-operating 
with  others,  and  helping  others  to  produce  effects  that  could  not 
otherwise  be  produced.  Trees  growing  in  conservatories  afford  us 
abundant  proof  that  sap  is  raised  to  considerable  heights  by  other 
forces.  Though  it  is  notorious  that  trees  so  circumstanced  do  not 
thrive  unless,  through  open  sashes,  they  are  frequently  subject  to 
breezes  sufficient  to  make  their  parts  oscillate,  yet  there  is  evidently 
a  circulation  that  goes  on  without  mechanical  aid.  The  causes  of 
circulation  are  those  actions  only  which  disturbtheliquidequilibrium 
in  a  plant,  by  permanently  abstracting  water  or  sap  from  some  part 
of  it;  and  of  these  the  first  is  the  absorption  of  materials  for  the  for- 
mation of  new  tissue  in  growing  parts ;  the  second  is  the  loss  by 
evaporation,  mainly  through  adult  leaves;  and  the  third  is  the  loss  by 
extravasation,  through  compressed  vessels.  Only  so  far  as  it  pro- 
duces this  last,  can  mechanical  strain  be  regarded  as  truly  a  cause  of 
circulation.  All  the  other  actions  concerned  must  be  classed  as  aids 
to  circulation — as  facilitating  that  redistribution  of  liquid  that  con- 
tinually restores  the  equilibrium  continually  disturbed  ;  and  of  these 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  585 

capillary  action  may  be  named  as  the  first,  osmose  as  the  second,  and 
the  propulsive  effect  of  mechanical  strains  as  the  third.  The  first 
two  of  these  aids  are  doubtless  capable  by  themselves  of  producing 
a  large  part  of  the  observed  result — more  of  the  observed  result  than 
is  at  first  sight  manifest ;  for  there  is  an  important  indirect  effect 
of  osmotic  action  which  appears  to  be  overlooked.  Osmose  does  not 
aid  circulation  only  by  setting  up,  within  the  plant,  exchange,  currents 
between  the  more  dense  and  the  less  dense  solutions  in  different  parts 
of  it ;  but  it  aids  circulation  much  more  by  producing  distention 
of  the  plant  as  a  whole.  In  consequence  of  the  average  contrast  in 
density  between  the  water  outside  of  the  plant  and  the  sap  inside  of 
it,  the  constant  tendency  is  for  the  plant  to  absorb  a  quantity  in  excess 
of  its  capacity,  and  so  to  produce  distention  and  erection  of  its 
tissues.  It  is  because  of  this  that  the  drooping  plant  raises  itself 
when  watered ;  for  capillary  action  alone  could  only  refill  its  tissues 
without  changing  their  attitudes.  And  it  is  because  of  this  that 
juicy  plants  with  collapsible  structures  bleed  so  rapidly  when  cut, not 
only  from  the  cut  surface  of  the  rooted  part,  but  from  the  cut  sur- 
face of  the  detached  part — the  elastic  tissues  tending  to  press  out  the 
liquid  which  distends  them.  And  manifestly  if  osmose  serves  thus 
to  maintain  a  state  of  distention  throughout  a  plant,  it  indirectly  fur- 
thers circulation  ;  since  immediately  evaporation  or  growth  at  any 
part,  by  abstracting  liquid  from  the  neighbouring  tissues, begins  to 
diminish  the  liquid  pressure  within  such  tissues,the  distended  struc- 
tures throughout  the  rest  of  the  plant  thrust  their  liquid  contents  to- 
wards the  place  of  diminished  pressure.  This,  indeed,  may  very  pos- 
sibly be  the  most  efficient  of  the  agencies  at  work.  Remembering 
how  great  is  the  distention  producible  by  osmotic  absorption — great 
enough  to  burst  a  bladder — it  is  clear  that  the  force  with  which  the 
distended  tissues  of  a  plant  urge  forward  the  sap  to  places  of  con- 
sumption, is  probably  very  great.  We  must  therefore  regard  the  aid 
which  mechanical  strains  give  as  being  one  of  several.  Oscillations 


lary  action  and  the  process  just 

oscillations  the  equilibrium  may  still  be  restored,  though  less  rapidly 

and  within  narrower  limits  of  distance. 

One  half  of  the  problem  of  the  circulation,  however,  has  been  left 
out  of  sight.  Thus  far  our  inquiry  has  been,  how  the  ascending  cur- 
rent of  sap  is  produced.  There  remains  the  rationale  of  the  descend- 
ing current.  What  forces  cause  it, and  through  what  tissues  it  takes 
place,  are  questions  to  which  no  satisfactory  answers  have  been 
given.  That  the  descent  is  due  to  gravitation,  as  some  allege, 
is  difficult  to  conceive,  since,  as  gravitation  acts  equally  on  all 
liquid  columns  contained  in  the  stem,  it  is  not  easy  to  see  why 
it  should  produce  downward  movements  in  some  while  permitting 


586  APPENDIX  C. 

upward  movements  in  others — unless,  indeed,  there  existed  descend- 
ing tubes  too  wide  to  admit  of  much  capillary  action,  which  there 
do  not.  Moreover,  gravitation  is  clearly  inadequate  to  cause  cur- 
rents towards  the  roots  out  of  branches  that  droop  to  the  ground. 
Here  the  gravitation  of  the  contained  liquid  columns  must  nearly 
balance  that  of  the  connected  columns  in  the  stem,  leaving  no 
appreciable  force  to  cause  motion.  Nor  does  there  seem  much 
probability  in  the  assumption  that  the  route  of  the  descending  sap 
is  through  the  cambium  layer,  since  experiments  on  the  absorption 
of  dyes  prove  that  simple  cellular  tissue  is  a  very  bad  conductor 
of  liquids:  their  movement  through  it  does  not  take  place  with  one- 
fiftieth  of  the  rapidity  with  which  it  takes  place  through  vessels.* 
Of  course  the  defence  for  these  hypotheses  is,  that  there  must  be 
a  downward  current,  which  must  have  a  course  and  a  cause ;  and  the 
very  natural  assumption  has  been  that  the  course  and  the  cause  must 
be  other  than  those  which  produce  the  ascending  current.  Never- 
theless there  is  an  alternative  supposition  to  which  the  foregoing 
considerations  introduce  us.  It  is  quite  possible  for  the  same  vascular 
system  to  serve  as  a  channel  for  movement  in  opposite  directions 
at  different  times.  We  have  among  animals  well-known  cases  in 
which  the  blood-vessels  carry  a  current  first  in  one  direction  and 
then,  after  a  brief  pause,  in  the  reverse  direction.  And  there  seems 
an  a  priori  probability  that,  lowly  organized  as  they  are,  plants  are 
more  likely  to  have  distributing  appliances  of  this  imperfect  kind 
than  to  have  two  sets  of  channels  for  two  simultaneous  currents.  If, 
led  by  this  suspicion,  we  inquire  whether  among  the  forces  which 
unite  to  produce  movements  of  sap,  there  are  any  variations  or  inter- 
missions capable  of  determining  the  currents  in  different  directions, 
we  quickly  discover  that  there  are  such,  and  that  the  hypothesis  of 
an  alternating  motion  of  the  sap,  now  centrifugal  and  now  centri- 
petal, through  the  same  vessels,  has  good  warrant.  What  are  the 
several  forces  at  work  ?  First  may  be  set  down  that  tendency 
existing  in  every  part  of  a  plant  to  expand  into  its  typical  form,  and 
to  absorb  nutritive  liquids  in  doing  this.  The  resulting  competition 

*  Some  exceptions  to  this  occur  in  plants  that  have  retrograded  in  the 
character  of  their  tissues  towards  the  simpler  vegetal  types.  Certain  very 
succulent  leaves,  such  as  those  of  Scmpervimtm,  in  which  the  cellular  tissue 
is  immensely  developed  in  comparison  with  the  vascular  tissue,  seem  to 
have  resumed  to  a  considerable  extent  what  we  must  regard  as  the  primitive 
form  of  vegetal  circulation — simple  absorption  from  cell  to  cell.  These, 
when  they  have  lost  much  of  their  water,  will  take  up  the  dye  to  some  dis- 
tance through  their  general  substance,  or  rather  through  its  interstices,  even 
neglecting  the  vessels.  At  other  times,  in  the  same  leaves,  the  vessels  will 
become  charged  while  comparatively  little  absorption  takes  place  through 
the  cellular  tissue.  Even  in  these  exceptional  cases,  however,  the  movement 
through  cellular  tissue  is  nothing  like  as  fast  as  the  movement  through 
vessels. 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  587 

for  sap  will,  other  things  being  equal,  cause  currents  towards  the 
most  rapidly -growing  parts — towards  unfolding  shoots  and  leaves, 
but  not  towards  adult  leaves.  Next  we  have  evaporation,  acting 
more  on  the  adult  leaves  than  on  those  which  are  in  the  bud, 
or  but  partially  developed.  This  evaporation  is  both  regularly 
and  irregularly  intermittent.  Depending  chiefly  on  the  action 
of  the  sun,  it  is,  in  fine  weather,  greatly  checked  or  wholly 
arrested  every  evening ;  and  in  cloudy  weather  must  be  much 
retarded  during  the  day.  Further,  every  hygrometric  variation, 
as  well  as  every  variation  in  the  movement  of  the  air,  must 
vary  the  evaporation.  This  chief  action,  therefore,  which,  by  con- 
tinually emptying  the  ends  of  the  capillary  tubes,  makes  upward 
currents  possible,  is  one  which  intermits  every  night,  and  every  day 
is  strong  or  feeble  as  circumstances  determine.  Then,  in  the  third 
place,  we  have  this  rude  pumping  process  above  described,  going 
on  with  greater  vigour  when  the  wind  is  violent,  and  with  less 
vigour  when  it  is  gentle  —  drawing  liquid  towards  different 
parts  according  to  their  degrees  of  oscillation,  and  from  diffe- 
rent parts  according  as  they  can  most  readily  furnish  it.  And 
now  let  us  ask  what  must  result  under  changing  conditions  from 
these  variously-conflicting  and  conspiring  forces.  When  a  warm 
sunshine,  causing  rapid  evaporation,  is  emptying  the  vessels  of  the 
leaves,  the  osmotic  and  capillary  actions  that  refill  them  will  be 
continually  aided  by  the  pumping  action  of  the  swaying  petioles, 
twigs,  andbranches,  provided  their  oscillations  are  moderate.  Under 
these  conditions  the  current  of  sap,  moving  in  the  direction  of  least 
resistance,  will  set  towards  the  leaves.  But  what  will  happen  when 
the  sun  sets  ?  There  is  now  nothing  to  determine  currents  either 
upwards  or  downwards,  except  the  relative  rates  of  growth  in  the 
parts  and  the  relative  demands  set  up  by  the  oscillations ;  and  the 
oscillations  acting  alone,  will  draw  sap  to  the  oscillating  parts  as 
much  from  above  as  from  below.  If  the  resistance  to  be  overcome 
bv  a  current  setting  back  from  the  leaves  is  less  than  the  resistance 
to  be  overcome  by  a  current  setting  up  from  the  roots,  then  a 
current  will  set  back  from  the  leaves.  Now  it  is,  I  think,  tolerably 
manifest  that  in  the  swaying  twigs  and  minor  branches,  less  force 
will  be  required  to  overcome  the  inertia  of  the  short  columns  of 
liquid  between  them  and  the  leaves  than  to  overcome  the  inertia  of 
the  long  columns  between  them  and  the  roots.  Hence  during  the 
night,  as  also  at  other  times  when  evaporation  is  not  going  on,  the 
sap  will  be  drawn  out  of  the  leaves  into  the  adjacent  supporting 
parts  ;  and  their  nutrition  will  be  increased.  If  the  wind  is  strong 
enough  to  produce  a  swaying  of  the  thicker  branches,  the  back 
current  will  extend  to  thein  also ;  and  a  further  strengthening  will 
result  from  their  absorption  of  the  elaborated  sap.  And  when  the 
great  branches  and  the  stem  are  bent  backwards  and  forwards  by  a 


588  APPENDIX  C. 

gale,  they  too  will  share  in  the  nutrition.  It  may  at  first  sight  seem 
that  these  parts,  being  nearer  to  the  roots  than  to  the  leaves,  will 
draw  their  supplies  from  the  roots  only.  But  the  quantity  which  the 
roots  can  furnish  is  insufficient  to  meet  so  great  a  demand.  Under 
the  conditions  described,  the  exudation  of  sap  from  the  vessels  will 
be  very  great,  and  the  draught  of  liquid  required  to  refill  them,  not 
satisfied  by  that  which  the  root-fibres  can  take  in,  will  extend  to  the 
leaves.  Thus  sap  will  flow  to  the  several  parts  according  to  their 
respective  degrees  of  activity — to  the  leaves  while  light  and  heat 
enable  them  to  discharge  their  functions,  and  back  to  the  twigs, 
branches,  stem,  and  roots  when  these  become  active  and  the  leaves 
inactive,  or  when  their  activity  dominates  over  that  of  the  leaves. 
And  this  distribution  of  nutriment,  varying  with  the  varying 
activities  of  the  parts,  is  just  such  a  distribution  as  we  know  must 
be  required  to  keep  up  the  organic  balance. 

To  this  explanation  it  may  be  objected  that  it  does  not  account 
for  the  downward  current  of  sap  in  plants  that  are  sheltered.  The 
stem  and  roots  of  a  drawing-room  Geranium  display  a  thickening 
which  implies  that  nutritive  matters  have  descended  from  the  leaves, 

to  be  drawn  downwards  as  well  as  upwards.  The  reply  is,  that  the 
stem  and  roots  tend  to  repeat  their  typical  structures,  and  that  the 
absorption  of  sap  for  the  formation  of  their  respective  dense  tissues, 
is  here  the  force  which  determines  the  descent.  Indeed  it  must  be 
borne  in  mind  that  the  mechanical  strains  and  the  pumping  process 
which  they  keep  up,  as  well  as  the  distention  caused  by  osmose,  do 
not  in  themselves  produce  a  current  either  upwards  or  downwards : 
they  simply  help  to  move  the  sap  towards  that  place  where  there  is 
the  most  rapid  abstraction  of  it — the  place  towards  which  its  motion 
is  least  resisted.  Whether  there  is  oscillation  or  whether  there  is 
not,  the  physiological  demands  of  the  different  parts  of  the  plant 
determine  the  direction  of  the  current ;  and  all  which  the  oscillations 
and  the  distention  do  is  to  facilitate  the  supply  of  these  demands. 
Just  as  much,  therefore,  in  a  plant  at  rest  as  in  a  plant  in  motion, 
the  current  will  set  downwards  when  the  function  of  the  leaves  is 
arrested,  and  when  there  is  nothing  to  resist  that  abstraction  of  sap 
caused  by  the  tendency  of  the  stem-  and  root-tissues  to  assume  their 
typical  structures.  To  which  admission,  however,  it  must  be  added 
that  since  this  typical  structure  assumed,  though  imperfectly  as- 
sumed, by  the  hot-house  plant,  is  itself  interpretable  as  the  in- 
herited effect  of  external  mechanical  actions  on  its  ancestors,  we 
may  still  consider  the  current  set  up  by  the  assumption  of  the 
typical  structure  to  be  indirectly  due  to  such  actions. 

Interesting  evidence  of  another  order  here  demands  notice.  In 
the  course  of  experiments  on  the  absorption  of  dyes  by  leaves,  it 
happened  that  in  making  sections  parallel  to  the  plane  of  a  leaf,  with 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  589 

the  view  of  separating  its  middle  layer  containing  the  vessels,  I  came 
upon  some  structures  that  were  new  to  me.  These  structures,  where 
they  are  present,  form  the  terminations  of  the  vascular  system.  They 
are  masses  of  irregular  and  imperfectly  united  fibrous  cells,  such  as 
those  out  of  which  vessels  are  developed ;  and  they  are  sometimes 
slender,  sometimes  bulky — usually, however,  being  more  or  less  club- 
shaped.  In  transverse  sections  of  leaves  their  distinctive  characters 
are  not  shown :  they  are  taken  for  the  smaller  veins.  It  is  only  by 
carefully  slicing  away  the  surface  of  a  leaf  until  we  come  down 
to  that  part  which  contains  them,  that  we  get  any  idea  of  their 
nature.  Fig.  1  represents  a  specimen  taken  from  a  leaf  of  Eu- 
phorbia neriifolia.  Occupying  one  of  the  interspaces  of  the  ulti- 
mate venous,  network,  it  consists  of  a  spirally-lined  duct  or  set  of 
ducts,  which  connects  with  the  neighbouring  vein  a  cluster  of  half- 
reticulated,  half-scalariform  cells.  Thesecellshave  projections,many 
of  them  tapering,  that  insert  themselves  into  the  adjacent  intercell- 
ular spaces,  thus  producing  an  extensive  surface  of  contact  between 
the  organ  and  the  imbedding  tissues.  A  further  trait  is,  that  the  en- 
sheathing  prosenchyma  is  either  but  little  developed  or  wholly  ab- 
sent ;  and  consequently  this  expanded  vascular  structure,  especially 
at  its  end,  comes  immediately  in  contact  with  the  tissues  concerned 
in  assimilation.  The  leaf  of  Euphorbia  neriifolia  is  a  very  fleshy 
one ;  and  in  it  these  organs  are  distributed  through  a  compact, 
though  watery,  cellular  mass.  But  in  any  leaf  of  the  ordinary  type 
which  possesses  them, they  lie  in  the  networkparenchyma  composing 
its  lower  layer ;  and  wherever  they  occur  in  this  layer  its  cells  unite 
to  enclose  them.  This  arrangement  is  shown  in  fig.  2,  representing 
a  sample  from  the  Caoutchouc-leaf,  as  seen  with  the  upper  part  of 
its  envelope  removed ;  and  it  is  shown  still  more  clearly  in  a  sample 
from  the  leaf  of  Panax  Z/essonii,  fig.  3.  Figures  4  and  5  represent, 
without  their  sheaths,  other  such  organs  from  the  leaves  of  Panax 
Lessonii  and  Clusia  flava.  Some  relation  seems  to  exist  between 
their  forms  and  the  thicknesses  of  the  layers  in  which  they  lie. 
Certain  very  thick  leaves,  such  as  those  of  Clusia  flava,  have  them 
less  abundantly  distributed  than  is  usual,  but  more  massive.  Where 
the  parenchyma  is  developed  not  to  so  great  an  extreme,  though 
still  largely,  as  in  the  leaves  of  Holly,  Aucuba,  Camellia,  they  are 
not  so  bulky ;  and  in  thinner  leaves,  like  those  of  Privet,  Elder, 
&c.,  they  become  longer  and  less  conspicuously  club-shaped.  Some 
adaptations  to  their  respective  positions  seem  implied  by  these  modi- 
fications ;  and  we  may  naturally  expect  that  in  many  thin  leaves 
these  free  ends,  becoming  still  narrower,  lose  the  distinctive  and 
suggestive  characters  possessed  by  those  shown  in  the  diagrams. 
Relations  of  this  kind  are  not  regular,  however.  In  various  other 
genera,  members  of  which  I  have  examined,  as  Rhus,  Viburnum, 
Griselinia,  Brexia,  JBotryodendron,  Pereskia,  the  variations  in  the 


590  APPENDIX  C. 

bulk  and  form  of  these  structures  are  not  directly  determined  by 
the  spaces  which  the  leaves  allow :  obviously  there  are  other  modi- 
fying causes.  It  should  be  added  that  while  these  expanded  free 
extremities  graduate  into  tapering  free  extremities,  not  differing 
from  ordinary  vessels,  they  also  pass  insensibly  into  the  ordinary 
inosculations.  Occasionally,  along  with  numerous  free  endings, 
there  occur  loops ;  and  from  such  loops  there  are  transitions  to 
the  ultimate  meshes  of  the  veins. 

These  organs  are  by  no  means  common  to  all  leaves.  In  many 
that  afford  ample  spaces  for  them  they  are  not  to  be  found.  So  far 
as  I  have  observed,  they  are  absent  from  the  thick  leaves  of  plants 
which  form  very  little  wood.  In  Semper  viwum,  in  Echeveria,  in 
Bryophyllum,  they  do  not  appear  to  exist ;  and  I  have  been  unable 
to  discover  them  in  Kalanchoe  rotundifolia,  in  Kleinia  ante-euphor- 
bium  and  ficoides,  in  the  several  species  of  Crassula,  and  in  other 
succulent  plants.  It  may  be  added  that  they  are  not  absolutely 
confined  to  leaves,  but  occur  in  stems  that  have  assumed  the  func- 
tions of  leaves.  At  least  I  have  found,  in  the  green  parenchyma 
of  Opuntia,  organs  that  are  analogous  though  much  more  rudely 
and  irregularly  formed.  In  other  parts,  too,  that  have  usurped 
the  leaf -function,  they  occur,  as  in  the  phyllodes  of  the  Australian 
Acacias.  These  have  them  abundantly  developed ;  and  it  is  interest- 
ing to  observe  that  here,  where  the  two  vertically-placed  surfaces 
of  the  flattened-out  petiole  are  equally  adapted  to  the  assimilative 
function,  there  exist  two  layers  of  these  expanded  vascular  termina- 
tions, one  applied  to  the  inner  surface  of  each  layer  of  parenchyma. 

Considering  the  structures  and  positions  of  these  organs,  as  well 
as  the  natures  of  the  plants  possessing  them,  may  we  not  form  a 
shrewd  suspicion  respecting  their  function  ?  Is  it  not  probable  that 
they  facilitate  absorption  of  the  juices  carried  back  from  the  leaf  for 
the  nutrition  of  the  stem  and  roots  ?  They  are  admirably  adapted 
for  performing  this  office.  Their  component  fibrous  cells,  having 
angles  insinuated  between  the  cells  of  the  parenchyma,  are  shaped 
just  as  they  should  be  for  taking  up  its  contents ;  and  the  absence 
of  sheathing  tissue  between  them  and  the  parenchyma  facilitates  the 
passage  of  the  elaborated  liquids.  Moreover  there  is  the  fact  that 
they  are  allied  to  organs  which  obviously  have  absorbent  functions. 
I  am  indebted  to  Dr.  Hooker  for  pointing  out  the  figures  of  two 
such  organs  in  the  "  Icones  Anatomicae  "  of  Link.  One  of  them  is 
from  the  end  of  a  dicotyledonous  root-fibre,  and  the  other  is  from 
the  prothallus  of  a  young  Fern.  In  each  case  a  cluster  of  fibrous 
cells,  seated  at  a  place  from  which  liquid  has  to  be  drawn,  is  con- 
nected by  vessels  with  the  parts  to  which  liquid  has  to  be  carried. 
There  can  scarcely  be  a  doubt,  then,  that  in  both  cases  absorption 
is  effected  through  them.  I  have  met  with  another  such  organ, 
more  elaborately  constructed,  but  evidently  adapted  to  the  same 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  591 

office,  in  the  common  Turnip-root.  As  shown  by  the  end  view 
and  longitudinal  section  in  figs.  6  and  7,  this  organ  consists  of 
rings  of  fenestrated  cells,  arranged  with  varying  degrees  of  regu- 
larity into  a  funnel,  ordinarily  having  its  apex  directed  towards  the 
central  mass  of  the  Turnip,  with  which  it  has,  in  some  cases  at  least, 
a  traceable  connexion  by  a  canal.  Presenting  as  it  does  an  external 
porous  surface  terminating  one  of  the  branches  of  the  vascular  sys- 
tem, each  of  these  organs  is  well  fitted  for  taking  up  with  rapidity 
the  nutriment  laid  by  in  the  Turnip-root,  and  used  by  the  plant 
when  it  sends  up  its  flower-stalk.  Nor  does  even  this  exhaust  the 
analogies.  The  cotyledons  of  the  young  bean,  experimented  upon 
as  before  described,  furnished  other  examples  of  such  structures, 
exactly  in  the  places  where,  if  they  are  absorbents,  we  might  ex- 
pect to  find  them.  Amid  the  branchings  and  inosculations  of  the 
vascular  layer  running  through  the  mass  of  nutriment  deposited  in 
each  cotyledon,  there  are  conspicuous  free  terminations  that  are  club- 
shaped,  and  prove  to  be  composed,  like  those  in  leaves,  of  irregularly 
formed  and  clustered  fibrous  cells ;  and  some  of  them,  diverging 
from  the  plane  of  the  vascular  layer,  dip  down  into  the  mass  of 
starch  and  albumen  which  the  young  plant  has  to  utilize,  and 
which  these  structures  can  have  no  other  function  but  to  take  up. 
Besides  being  so  well  fitted  for  absorption,  and  besides  being 
similar  to  organs  which  we  cannot  doubt  are  absorbents,  these  vas- 
cular terminations  in  leaves  afford  us  vet  another  evidence  of  their 
functions.  They  are  seated  in  a  tissue  so  arranged  as  specially  to 
facilitate  the  abstraction  of  liquid.  The  centripetal  movement  of 
the  sap  must  be  set  up  by  a  force  that  is  comparatively  feeble,  since, 
the  parietes  of  the  ducts  being  porous,  air  will  enter  if  the  tension 
on  the  contained  columns  becomes  considerable.  Hence  it  is  needful 
that  the  exit  of  sap  from  the  leaves  should  meet  with  very  little  resist- 
ance. Now  were  it  not  for  an  adjustment  presently  to  be  described, 
it  would  meet  with  great  resistance,  notwithstanding  the  peculiar 
fitness  of  these  organs  to  take  it  in.  Liquid  cannot  be  drawn  out 
of  any  closed  cavity  without  producing  a  collapse  of  the  cavity's 
sides;"  and  if  its  sides  are  not  readily  collapsible,  there  must  be  a  cor- 
responding resistance  to  the  abstraction  of  liquid  from  it.  Clearly 
the  like  must  happen  if  the  liquid  is  to  be  drawn  out  of  a  tissue 
which  cannot  either  diminish  in  bulk  bodily  or  allow  its  components 
individually  to  diminish  in  bulk.  In  an  ordinary  leaf,  the  upper 
layer  of  parenchyma,  formed  as  it  is  of  closely-packed  cells  that  are 
without  interspaces,  and  are  everywhere  held  fast  within  their  frame- 
work of  veins,  can  neither  contract  easily  as  a  mass,  nor  allow  its  sep- 
arate cells  to  do  so.  Quite  otherwise  is  it  with  the  network-paren- 
chyma below.  The  long  cells  of  this,  united  merely  by  their  ends 
and  having  their  flexible  sides  surrounded  by  air,  may  severally  have 
their  contents  considerably  increased  and  decreased  without  offering 


592  APPENDIX  C. 

appreciable  resistances;  and  the  network- tissue  which  they  form  will, 
at  the  same  time,  be  capable  of  undergoing  slightexpansions  and  con- 
tractions of  its  thickness.  In  this  layer  occur  these  organs  that  are  so 
obviously  fitted  for  absorption.  Here  we  find  them  in  direct  commu- 
nication with  its  system  of  collapsible  cells.  The  probability  appears 
to  be,  that  when  the  current  sets  into  the  leaf,  it  passes  through  the 
vessels  and  their  sheaths  chiefly  into  the  upper  layer  of  cells  (this 
upper  layer  having  a  larger  surface  of  contact  with  the  veins  than  the 
lower  layer,  and  being  the  seat  of  more  active  processes) ;  and  that 
the  juices  of  the  upper  layer,  enriched  by  the  assimilated  matters, 
pass  into  the  network-parenchyma,  which  serves  as  a  reservoir  from 
which  they  are  from  time  to  time  drawn  for  the  nutrition  of  the  rest 
of  the  plant,  when  the  actions  determine  the  downward  current. 
Should  it  be  asked  what  happens  where  the  absorbents,  instead  of 
being  inserted  in  a  network-parenchyma,  are,  as  in  the  leaves  of 
Euphorbia  neriifolia,  inserted  in  a  solid  parenchyma,  the  reply  is, 
that  such  a  parenchyma,  though  not,  furnished  with  systematically 
arranged  air-chambers,  nevertheless  contains  air  in  its  intercellular 
spaces ;  and  that  when  there  occurs  a  draught  upon  its  contents, 
the  expansion  of  this  air  and  the  entrance  of  more  from  without, 
quickly  supply  the  place  of  the  abstracted  liquid. 

If  then,  returning  to  the  general  argument,  we  conclude  that 
these  expanded  terminations  of  the  vascular  system  in  leaves  are  ab- 
sorbent organs,  we  find  a  further  confirmation  of  the  views  set  forth 
respecting  the  alternating  movement  of  the  sap  along  the  same  chan- 
nels. These  spongioles  of  the  leaves,  like  the  spongioles  of  the  roots, 
being  appliances  by  which  liquid  is  taken  up  to  be  carried  into  the 
mass  of  the  plant,  we  are  obliged  to  regard  the  vessels  that  end  in 
these  spongioles  of  the  leaves  as  being  the  channels  of  the  down 
current  whenever  it  is  produced.  If  the  elaborated  sap  is  abstracted 
from  the  leaves  by  these  absorbents,  then  we  have  no  alternative 
but  to  suppose  that,  having  entered  the  vascular  system,  the  elab- 
orated sap  descends  through  it.  And  seeing  how,  by  the  help  of 
these  special  terminations,  it  becomes  possible  for  the  same  vessels 
to  carry  back  a  quality  of  sap  unlike  that  which  they  bring  up,  we 
are  enabled  to  understand  tolerably  well  how  this  rhythmical 
movement  produces  a  downward  transfer  of  materials  for  growth. 

The  several  lines  of  argument  may  now  be  brought  together ; 
and  along  with  them  may  be  woven  up  such  evidences  as  remain. 
Let  me  first  point  out  the  variety  of  questions  to  which  the 
hypothesis  supplies  answers. 

It  is  required  to  account  for  the  ascent  of  sap  to  a  height  beyond 
that  to  which  capillary  action  can  raise  it.  This  ascent  is  accounted 
for  by  the  propulsive  action  of  transverse  strains,  joined  with  that  of 
osmotic  distention.  A  cause  has  to  be  assigned  for  that  rise  of  sap 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  593 

which,  in  the  spring,  while  yet  there  is  no  considerable  evaporation 
to  aid  it,  goes  on  with  a  power  which  capillarity  does  not  explain. 
The  co-operation  of  the  same  two  agencies  is  assignable  for  this  result 
also.*  The  circumstance  that  vessels  and  ducts  here  contain  sap  and 
there  contain  air,  and  at  the  same  place  contain  at  different  seasons 
now  air  and  now  sap  is  a  fact  calling  for  explanation.  An  explana- 
tion is  furnished  by  these  mechanical  actions  which  involve  the  en- 
trance or  expulsion  of  air  according  to  the  supply  of  liquid.  That 
vessels  and  ducts  which  were  originally  active  sap-carriers  go  com- 
pletely out  of  use,  and  have  their  function  discharged  by  other 
vessels  or  ducts,  is  an  anomaly  that  has  to  be  solved.  Again,  we 
are  supplied  with  a  solution  :  these  deserted  vessels  and  ducts  are 
those  which,  by  the  formation  of  dense  tissue  outside  of  them, 
become  so  circumstanced  that  they  cannot  be  compressed  as  they 
originally  were.  A  channel  has  to  be  found  for  the  downward 
current  of  sap,  which,  on  any  other  hypothesis  than  the  foregoing, 
must  be  a  channel  separate  from  that  taken  by  the  upward 
current ;  and  yet  no  good  evidence  of  a  separate  channel  has  been 
pointed  out.  Here,  however,  the  difficulty  disappears,  since  one 
channel  suffices  for  the  current  alternating  upwards  and  downwards 
according  to  the  conditions.  Moreover  there  has  to  be  found  a 
force  producing  or  facilitating  the  downward  current,  capable  even 
of  drawing  sap  out  of  drooping  branches ;  and  no  such  force  is 
forthcoming.  The  hypothesis  set  forth  dispenses  with  this  necessity ; 
under  the  recurring  change  of  conditions,  the  same  distention  and 
oscillation  which  before  raised  the  sap  to  the  places  of  consumption, 
now  bring  it  down  to  the  places  of  consumption.  A  physical 
process  has  to  be  pointed  out  by  which  the  material  that  forms 
dense  tissue  is  deposited  at  the  places  where  it  is  wanted,  rather 
than  at  other  places.  This  physical  process  the  hypothesis  in- 
dicates. It  is  requisite  to  find  an  explanation  of  the  fact  that, 
when  plants  ordinarily  swayed  about  by  the  wind  are  grown  indoors, 
the  formation  of  wood  is  so  much  diminished  that  they  become  ab- 
normally slender.  Of  this  an  explanation  is  supplied.  Yet  a  further 

*  It  seems  probable,  however,  that  osmotic  distention  is  here,  especially, 
the  more  important  of  the  two  factors.  The  rising  of  the  sap  in  spring  may 
indirectly  result,  like  the  sprouting  of  the  seed,  from  the  transformation  of 
starch  into  sugar.  During  germination,  this  change  of  an  oxy-hydro-carbon 
from  an  insoluble  into  a  soluble  form,  leads  to  rapid  endosmose;  con- 
sequently to  great  distention  of  the  seed ;  and  therefore  to  a  force  which 
thrusts  the  contained  liquids  into  the  plumule  and  radicle,  and  gives  them 
power  to  displace  the  soil  in  their  way :  it  sets  up  an  active  internal  move- 
ment when  neither  evaporation  nor  the  change  which  light  produces  can  be 
operative.  And  similarly,  if,  in  the  spring,  the  starch  stored  up  in  the  roots 
of  a  tree  passes  into  the  form  of  sugar,  the  unusual  osmotic  absorption  that 


arises  will  cause  an  unusual  distention — a  distention  which,  being  resisted  by 
the  tough  bark  of  the  roots  and  stem,  will  result  in  a  powerful  upward  thrust 
of  the  contained  liquid. 
84 


594:  APPENDIX  C. 

fact  to  be  interpreted  is,  that  in  the  same  individual  plant  homol- 
ogous parts,  which,  according  to  the  type  of  the  plant,  should  be 
equally  woody,  become  much  thicker  one  than  another  if  subject 
to  greater  mechanical  stress.  And  of  this  too  an  interpretation  is 
similarly  afforded. 

Now  the  sufficiency  of  the  assigned  actions  to  account  for  so  many 
phenomena  not  otherwise  explained,  would  be  strong  evidence  that 
the  rationale  is  the  true  one,  even  were  it  of  a  purely  hypothetical 
kind.  How  strong,  then,  becomes  the  reason  for  believing  it  the 
true  one  when  we  remember  that  the  actions  alleged  demonstrably 
go  on  in  the  way  asserted.  They  are  ever  operating  before  our 
eyes ;  and  that  they  produce  the  effects  in  question  is  a  conclusion 
deducible  from  mechanical  principles,  a  conclusion  established  by 
induction,  and  a  conclusion  verified  by  experiment.  These  three 
orders  of  proof  may  be  briefly  summed  up  as  follows. 

That  plants  which  have  to  raise  themselves  above  the  earth's  sur- 
face, and  to  withstand  the  actions  of  the  wind,  must  have  a  power  of 
developing  supporting  structure,  is  an  a  priori  conclusion  which  may 
be  safely  drawn.  It  is  an  equally  safe  a  priori  conclusion,  that  if 
the  supporting  structure,  either  as  a  whole  or  in  any  of  its  parts,  has 
to  adapt  itself  to  the  particular  strains  which  the  individual  plant 
is  subject  to  by  its  particular  circumstances,  there  must  be  at  work 
some  process  by  which  the  strength  of  the  supporting  structure  is 
everywhere  brought  into  equilibrium  with  the  forces  it  has  to  bear. 
Though  the  typical  distribution  of  supporting  structure  in  each  kind 
of  plant  may  be  explained  ideologically  by  those  whom  teleological 
explanations  satisfy ;  and  though  otherwise  this  typical  distribution 
may  be  ascribed  to  natural  selection  acting  apart  from  any  directly 
adaptive  process ;  yet  it  is  manifest  that  those  departures  from  the 
typical  distribution  which  fit  the  parts  of  each  plant  to  their  special 
conditions  are  explicable  neither  ideologically  nor  by  natural  selec- 
tion. We  are,  therefore,  compelled  to  admit  that,  if  in  each  plant 
there  goes  on  a  balancing  of  the  particular  strains  by  the  particular 
strengths,  there  must  be  a  physical  or  physico-chemical  process  by 
which  the  adjustments  of  the  two  are  effected.  Meanwhile  we  are 
equally  compelled  to  admit,  a  priori,  that  'the  mechanical  actions  to 
be  resisted,  themselves  affect  the  internal  tissues  in  such  ways  as  to 
further  the  increase  of  that  dense  substance  by  which  they  are  re- 
sisted. It  is  demonstrable  that  bending  the  petioles,  shoots,  and 
stems  must  compress  the  vessels  beneath  their  surfaces,  and  increase 
the  exudation  of  nutritive  matters  from  them,  and  must  do  this  act- 
ively in  proportion  as  the  bends  are  great  and  frequent ;  so  that 
while,  on  the  one  hand,  it  is  a  necessary  deduction  that,  if  the  parts  of 
each  plant  are  to  be  severally  strengthened  according  to  the  several 
strains,  there  must  be  some  direct  connexion  between  strains  and 
strengths,  it  is,  on  the  other  hand,  a  necessary  deduction  from 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  595 

mechanical  principles  that  the  strains  do  act  in  such  ways  as  to  aid 
the  increase  of  the  strengths.  How  a  like  correspondence  between 
two  a  priori  arguments  holds  in  the  case  of  the  circulation,  needs 
not  to  be  shown  in  detail.  It  will  suffice  to  remind  the  reader 
that  while  the  raising  of  sap  to  heights  beyond  the  limit  of  cap- 
illarity implies  some  force  to  effect  it,  we  have  in  the  osmotic 
distention  and  the  intermittent  compressions  caused  by  transverse 
strains,  forces  which,  under  the  conditions,  cannot  but  tend  to 
effect  it ;  and  similarly  with  the  requirement  for  a  downward 
current,  and  the  production  of  a  downward  current. 

Among  the  inductive  proofs  we  find  a  kindred  agreement.  Diffe- 
rent individuals  of  the  same  species,  and  different  parts  of  the  same 
individual,  do  strengthen  in  different  degrees ;  and  there  is  a  clearly 
traceable  connexion  between  their  strengthenings  and  the  intermit- 
tentstrainsthey  are  exposed  to.  This  evidence,  derived  from  contrasts 
between  growths  on  the  same  plant  or  on  plants  of  the  same  type,  is 
enforced  by  evidence  derived  from  contrasts  between  plants  of  diffe- 
rent types.  The  deficiency  of  woody  tissue  which  we  see  in  plants 
called  succulent,  is  accompanied  by  a  bulkiness  of  the  parts  which 
prevents  any  considerable  oscillations;  and  this  character  is  also 
habitually  accompanied  by  a  dwarfed  growth.  When,  leaving  these 
relations  as  displayed  externally,  we  examine  them  internally,  we 
find  the  facts  uniting  to  show,  by  their  agreements  and  differences, 
that  between  the  compression  of  the  sap-canals  and  the  production 
of  wood  there  is  a  direct  relation.  We  have  the  facts,  that  in  each 
plant,  and  in  every  new  part  of  each  plant,  the  formation  of  sap- 
canals  precedes  the  formation  of  wood ;  that  the  deposit  of  woody 
matter,  when  it  begins,  takes  place  around  these  sap-canals,  and 
afterwards  around  the  new  sap-canals  successively  developed ;  that 
this  formation  of  wood  around  the  sap-canals  takes  place  where  the 
coats  of  the  canals  are  demonstrably  permeable,  and  that  the  amount 
of  wood-formation  is  proportionate  to  the  permeability.  And  then 
that  the  permeability  and  extravasation  of  sap  occur  wherever,  in 
the  individual  or  in  "the  type,  there  are  intermittent  compressions, 
is  proved  alike  by  ordinary  cases  and  by  exceptional  cases.  In 
the  one  class  of  cases  we  see  that  the  deposit  of  wood  round  the 
vessels  begins  to  take  place  when  they  come  into  positions  that 
subject  them  to  intermittent  compressions,  while  it  ceases  when 
they  become  shielded  from  compressions.  And  in  the  other  class 
of  cases,  where,  from  the  beginning,  the  vessels  are  shielded  from 
compression  by  surrounding  fleshy  tissue,  there  is  a  permanent 
absence  of  wood-formation. 

To  which  complete  agreement  between  the  deductive  and  induc- 
tive inferences  has  to  be  added  the  direct  proof  supplied  by  experi- 
ments. It  is  put  beyond  doubt  by  experiment  that  the  liquids  ab- 
sorbed by  plants  are  distributed  to  their  different  parts  through  their 


596  APPENDIX  C. 

vessels — at  first  by  the  spiral  or  allied  vessels  originally  developed, 
and  then  by  the  better-placed  ducts  formed  later.  By  experiment 
it  is  demonstrated  that  the  intermittent  compressions  caused  by  os- 
cillations urge  the  sap  along  the  vessels  and  ducts.  And  it  is  also  ex- 
perimentallyprovedthat  the  same  intermittentcompressionsproduce 
exudation  of  sap  from  vessels  and  ducts  into  the  surrounding  tissue. 
That  the  processes  here  described,  acting  through  all  past  time, 
have  sufficed  of  themselves  to  develope  the  supporting  and  distribut- 
ing structures  of  plants,  is  not  alleged.  What  share  the  natural 
selection  of  variationsdistinguishedas  spontaneous,  has  had  in  estab- 
lishing them,  is  a  question  which  remains  to  be  discussed.  Whether 
acting  alone  natural  selection  would  have  sufficed  to  evolve  these 
vascular  and  resisting  tissues,  I  do  not  profess  to  say.  That  it  has 
been  a  co-operating  cause,  I  take  to  be  self-evident :  it  must  all  along 
have  furthered  the  action  of  any  other  cause,  by  preserving  the  in- 
dividuals on  which  such  other  cause  had  acted  most  favourably. 
Seeing,  however,  the  conclusive  proof  which  we  have  that  another 
cause  has  been  in  action — certainly  on  individuals,  and,  in  all  proba- 

the  genesis  of  these  internal  structures  to  this  cause,  and  regard 
natural  selection  as  having  here  played  the  part  of  an  accelerator. 

EXPLANATION  OF   PLATE. 

Fig.  1.  Absorbent  organ  from  the  leaf  of  Euphorbia  neriifolia. 
The  cluster  of  fibrous  cells  forming  one  of  the  terminations  of  the 
vascular  system  is  here  imbedded  in  a  solid  parenchyma. 

Fig.  2.  A  structure  of  analogous  kind  from  the  leaf  of  Ficus 
elastica.  Here  the  expanded  terminations  of  the  vessels  are  im- 
bedded in  the  network  parenchyma,  the  cells  of  which  unite  to 
form  envelopes  for  them. 

Fig.  3.  Shows  on  a  larger  scale  one  of  these  absorbents  from 
the  leaf  of  Panax  Lessonii.  In  this  figure  is  clearly  seen  the  way 
in  which  the  cells  of  the  network  parenchyma  unite  into  a  closely- 
fitting  case  for  the  spiral  cells. 

Fig.  4.  Represents  a  much  more  massive  absorbent  from  the 
same  leaf,  the  surrounding  tissues  being  omitted. 

Fig.  5.  Similarly  represents,  without  its  sheath,  an  absorbent 
from  the  leaf  of  Clusia  fava. 

Fig.  6.  End  view  of  an  absorbent  organ  from  the  root  of  a 
Turnip.  It  is  taken  from  the  outermost  layer  of  vessels.  Its 
funnel-shaped  interior  is  drawn  as  it  presents  itself  when  looked 
at  from  the  outside  of  this  layer,  its  narrow  end  being  directed 
towards  the  centre  of  the  Turnip. 

Fig.  7.  A  longitudinal  section  through  the  axis  of  another  such 
organ,  showing  its  annuli  of  reticulated  cells  when  cut  through. 
The  cellular  tissue  which  fills  the  interior  is  supposed  to  be  removed. 


CIRCULATION  AND  FORMATION  OF  WOOD  IN  PLANTS.  597 

Fig.  8.  A  less-developed  absorbent,  showing  its  approximate 
connexion  with  a  duct.  In  their  simplest  forms,  these  structures 
consist  of  only  two  fenestrated  cells,  with  their  ends  bent  round 
so  as  to  meet.  Such  types  occur  in  the  central  mass  of  the  Turnip, 


where  the  vascular  system  is  relatively  imperfect.  Besides  the 
comparatively  regular  forms  of  these  absorbents,  there  are  forms 
composed  of  amorphous  masses  of  fenestrated  cells.  It  should 
be  added  that  both  the  regular  and  irregular  kinds  are  very  vari- 
able in  their  numbers :  in  some  turnips  they  are  abundant,  and  in 
others  scarcely  to  be  found.  Possibly  their  presence  depends  on 


598  APPENDIX  C. 

the  age  of  the  Turnip.  Judging  from  the  period  during  which 
my  investigations  were  made,  namely  winter  and  early  spring,  I 
suspect  that  they  are  developed  only  in  preparation  for  sending 
up  the  flower-stalk. 

Let  me  add  that  experiments  on  circulation  in  plants  made 
during  the  state  of  inactivity,  when  it  is  to  be  presumed  that  the 
vessels  and  tissues  contain  but  little  sap,  are  much  more  suc- 
cessful than  those  made  in  the  summer.  It  would  seem  that 
when  the  tissues  are  fully  charged  with  sap  the  taking  up  of 
dyes  is  comparatively  slow  and  the  above-described  effects  are 
not  so  easily  demonstrable. 

[An  expert  writes  concerning  this  essay : — "  I  have  not 
attempted  to  annotate  critically  this  paper.  There  is  no  doubt 
that  many  of  your  conclusions  are  perfectly  sound,  particularly 
those  relating  to  the  passage  of  crude  sap  through  the  cavities  of 
the  elements  of  the  wood,  though  the  opinion  that  the  actual 
passage  was  through  the  walls  very  generally  held  till  about  12 
years  ago."] 


APPENDIX    D. 


ON  THE   ORIGIN   OF  THE  VERTEBRATE   TYPE. 

[  When  studying  the  development  of  the  vertebrate  skeleton,  there 
occurred  to  me  the  following  idea  respecting  the  possible  origin  of  the 
notochord.  I  was  eventually  led  to  omit  the  few  pages  of  Appendix  in 
which  I  had  expressed  this  idea,  because  it  was  unsupported  by  develop- 
mental evidence.  The  developmental  evidence  recently  discovered,  how- 
ever, has  led  Professor  Haeckeland  others  to  analogous  views  respecting 
the  affiliation  of  the  Vertebrata  on  the  Molluscoida.  Having  fortu- 
nately preserved  a  proof  of  the  suppressed  pages,  I  am  able  now  to 
add  them.  With  the  omission  of  a  superfluous  paragraph,  they  are 
reprinted  verbatim  from  this  proof,  which  dates  back  to  the  autumn 
of  1865,  at  which  time  the  chapter  on  "  The  Shapes  of  Vertebrate 
Skeletons"  was  written. — December,  1869.] 

The  general  argument  contained  in  Chap.  XVI.  of  Part  IV.,  I 
have  thought  it  undesirable  to  implicate  with  any  conception  more 
speculative  than  those  essential  to  it ;  and  to  avoid  so  implicating 
it,  I  transfer  to  this  place  an  hypothesis  respecting  the  derivation 
of  the  rudimentary  vertebrate  structure,  which  appears  to  me 
worth  considering. 

Among  those  molluscoid  animals  with  which  the  lowest  verte- 
brate animal  has  sundry  traits  in  common,  it  very  generally  happens 
that  while  the  adult  is  stationary  the  larva  is  locomotive.  The 
locomotion  of  the  larva  is  effected  by  the  undulations  of  a  tail.  In 
shape  and  movement  one  of  these  young  Ascidians  is  not  altogether 
unlike  a  Tadpole.  And  as  the  tail  of  the  Tadpole  disappears 
when  its  function  comes  to  be  fulfilled  by  limbs ;  so  the  Ascidian 
larva's  tail  disappears  when  fixation  of  the  larva  renders  it  useless. 
This  disappearance  of  the  tail,  however,  is  not  without  exception. 
The  Appendicularia  is  an  Ascidian  which  retains  its  tail  through- 
out life ;  and  by  its  aid  continues  throughout  life  to  swim  about. 
Now  this  tail  of  the  Appendicularia  has  a  very  suggestive  structure. 
It  is  long,  tapering  to  a  point,  and  flattened.  From  end  to  end 
there  runs  a  mid-rib,  which  appears  to  be  an  imbedded  gelatinous 
rod,  not  unlike  a  notochord.  Extending  along  the  two  sides  of 


600  APPENDIX  D. 

this  mid-rib,  are  bundles  of  muscular  fibres ;  and  its  top  bears  a 
gangliated  nervous  thread,  giving  off,  at  intervals,  branches  to  the 
muscular  fibres.  In  the  Appendicularia  this  tail,  which  is  inserted 
at  the  lower  part  of  the  back,  is  bent  forwards,  so  as  not  to  be 
adapted  for  propelling  the  body  of  the  animal  head  foremost ;  but 
the  homologous  tails  of  the  larval  Ascidians  are  directed  backwards, 
so  as  to  produce  forward  movement.  If  we  suppose  a  type  like  the 
Appendicularia  in  the  structure  and  insertion  of  its  permanent  tail, 
but  resembling  the  larval  forms  in  the  direction  of  its  tail,  it  is,  I 
think,  not  difficult  to  see  that  functional  adaptation  joined  with 
natural  selection,  might  readily  produce  a  type  approximating  to 
that  whose  origin  we  are  considering.  It  is  a  fair  assumption 
that  an  habitually  -  locomotive  creature  would  profit  by  in- 
creased power  of  locomotion.  This  granted,  it  follows  that 
such  further  development  of  the  tail-structures  as  might  arise 
from  enhanced  function,  and  such  better  distribution  of  them 
as  spontaneous  variation  might  from  time  to  time  initiate, 
would  be  perpetuated.  What  must  be  the  accompanying  changes  ? 
The  more  vigorous  action  of  such  an  appendage  implies  a  firmer 
insertion  into  the  body ;  and  this  would  be  effected  by  the  pro- 
longation forwards  of  the  central  axis  of  the  tail  into  the  creature's 
back.  As  fast  as  there  progressed  this  fusion  of  the  increasingly- 
powerful  tail  with  the  body,  the  body  would  begin  to  partake  of  its 
oscillations ;  and  at  the  same  time  that  the  resistant  axis  of  the  tail 
advanced  along  the  dorsal  region,  its  accompanying  muscular  fibres 
would  spread  over  the  sides  of  the  body :  gradually  taking  such 
modified  directions  and  insertions  as  their  new  conditions  rendered 
most  advantageous.  Without  further  explanation,  those  who 
examine  drawings  of  the  structures  described,  will,  I  think, 
see  that  in  such  a  way  a  tail  homologous  with  that  of  the 
Appendicularia,  would  be  likely,  in  the  course  of  that  de- 
velopment required  for  its  greater  efficiency,  gradually  to 
encroach  on  the  body,  until  its  mid-rib  became  the  dorsal 
axis,  its  gangliated  nerve-thread  the  spinal  chord,  and  its 
muscular  fibres  the  myocommata.  Such  a  development  of  an 
appendage  into  a  dominant  part  of  the  organism,  though  at  first 
sight  a  startling  supposition,  is  not  without  plenty  of  parallels : 
instance  the  way  in  which  the  cerebral  ganglia,  originally  mere 
adjuncts  of  the  spinal  chord,  eventually  become  the  great  centres  of 
the  nervous  system  to  which  the  spinal  chord  is  quite  subordinate ; 
or  instance  the  way  in  which  the  limbs,  small  and  inconspicuous  in 
fishes,  become,  in  Man,  masses  which,  taken  together,  outweigh  the 
trunk.  It  may  be  added  that  these  familiar  cases  have  a  further 
appropriateness ;  for  they  exhibit  higher  degrees  of  that  same 
increasing  dominance  of  the  organs  of  external  relation,  which  the 
hypothesis  itself  implies. 


ON  THE  ORIGIN  OF  THE  VERTEBRATE  TYPE.     601 

Of  course,  if  the  rudimentary  vertebrate  apparatus  thus  grew 
into,  and  spread  over,  a  raolluscoid  visceral  system,  the  formation 
of  the  notochord  under  the  action  of  alternating  transverse  strains, 
did  not  take  place  as  suggested  in  §  255  ;  but  it  does  not  therefore 
follow  that  its  ditferentiation  from  surrounding  tissues  was  not 
mechanically  initiated  in  the  way  described.  For  what  was  said  in 
that  section  respecting  the  effects  of  lateral  bendings  of  the  body, 
equally  applies  to  lateral  bendings  of  the  tail ;  and  as  fast  as  the 
developing  tail  encroached  on  the  body,  the  body  would  become 
implicated  in  the  transverse  strains,  and  the  differentiation  would 
advance  forwards  under  the  influences  originally  alleged.  Obviously, 
too,  though  the  lateral  muscular  masses  would  in  this  case  have  a 
different  history ;  yet  the  segmentation  of  them  would  be  eventually 
determined  by  the  assigned  causes.  For  as  fast  as  the  strata  of 
contractile  fibres,  developing  somewhat  in  advance  of  the  dorsal 
axis,  spread  along  the  sides,  they  would  come  under  the  influence 
of  the  alternate  flexions ;  and  while,  by  survival  of  the  fittest,  their 
parts  became  adjusted  in  direction,  their  segmentation  would,  as 
before,  accompany  their  increasing  massiveness.  The  actions  and 
reactions  due  to  lateral  undulations  would  still,  therefore,  be  the 
causes  of  differentiation,  with  which  natural  selection  would  co- 
operate. 


APPENDIX  D2. 


THE  ANNULOSE  TYPE. 

THE  production  of  a  segmental  structure  by  undulatory  move- 
ments, suggested  in  Appendix  D,  as  also  in  B  (first  published  in 
1858)  as  explaining  the  vertebral  column,  has  been  recently 
suggested  by  Prof.  Korschelt  as  the  cause  of  that  segmentation  of 
the  annulose  type  which  gives  the  name  to  it.  He  espouses  a — 

"  view  which  is  based  upon  the  assumption  that  at  first  an  unsegmented, 
elongated  ancestral  form  was  produced  by  terminal  growth,  whereupon  the 
entire  body  became  separated  at  once  into  a  large  number  of  segments  by  a 
rearrangement  of  the  individual  organs.  This  assumption  is  supported  by 
the  consideration  that  with  the  lateral  sinuous  movement  of  the  body,  and 
with  the  rigidity  of  the  tissues  caused  by  increasing  differentiation,  the 
formation  of  alternating  regions  of  greater  and  less  motility  was  of  con- 
siderable advantage  to  the  individual,  and  rendered  possible  a  further  elon- 
gation of  the  body.  The  first  cause  for  the  appearance  of  metamcric 
segmentation  would  then  be  sought  in  the  manner  of  locomotion  and  in 
mechanical  conditions.  However,  this  latter  view  is  not  supported  in  any 
way  by  embryology."  (Embryology  of  Invertebrates,  Part  I,  pp.  349-50.) 

I  venture  to  think  the  confession  that  this  view  "  is  not  sup- 
ported in  any  way  by  embryology "  should  be  joined  with  the 
confession  that  it  is  at  variance  with  that  abstract  embryology 
which  comprehends  the  process  of  development  in  general.  The 
assumption  that  there  took  place  "a  rearrangement  of  the 
individual  organs "  of  "  an  unsegmented,  elongated  ancestral 
form,"  in  such  wise  that  the  organs,  previously  single,  presently 
became  multiple,  so  that  instead  of  one  organ  of  each  kind  there 
were  substituted  many  organs  of  each  kind,  is  inconsistent  with 
the  general  law  of  evolution,  organic  and  other — implies  not 
integration  but  disintegration.  Everywhere  the  advance  is  from 
many  like  parts  performing  like  functions  to  relatively  few  unlike 
parts  performing  unlike  functions.  The  higher  forms  of  the  annu- 
lose type  itself  show  this.  Compare  a  myriapod  and  a  crab.  In  the 
one  we  have  not  only  a  great  number  of  similar  segments  bearing 
similar  limbs,  but  we  have  in  each  segment  a  dilatation  of  the  main 
blood-vessel — a  rudimentary  heart — a  swollen  portion  of  the 
nerve  cord — a  small  ganglion — and  so  on ;  whereas  in  the  other, 
602 


THE  ANNULOSE  TYPE.  603 

besides  relatively  few  segments  and  few  limbs  (sundry  of  them 
extremely  unlike  the  rest)  we  have  a  vascular  system  concentrated 
into  a  central  heart  with  arteries  and  a  concentrated  nervous 
system,  such  that  the  great  ganglia  in  the  integrated  carapace 
immensely  subordinate  the  ganglia  of  the  remaining  segments; 
and  similarly  with  the  other  organs.  Now  unless  it  be  denied 
that  these  highest  decapods  have  been  evolved  from  low  types 
akin  to  myriapods  in  composition,  it  must  be  admitted  that  the 
progress  has  been  from  a  string  of  many  like  segments  with 
similar  sets  of  organs  to  a  group  of  relatively-few  unlike  segments 
with  dissimilar  sets  of  organs.  If  so  we  cannot  rationally  deny 
that  the  progress  has  been  of  this  nature  up  from  the  lowest 
annelid,  instead  of  having  been,  as  Prof.  Korschelt's  hypothesis 
implies,  of  opposite  nature'  at  the  beginning. 

In  a  preceding  passage  a  clear  recognition  of  the  normal 
course  of  development  occurs.  In  opposing  the  view  set  forth  in 
§§  205-7  of  this  work,  Prof.  Korschelt  says : — 

"  It  seems  scarcely  favourable  to  this  theory  that  the  degree  of  inde- 
pendence which  the  individual  segments  present  is  comparatively  slight. 
The  most  important  organs  (nervous  system,  body  musculature,  blood-vascu- 
lar system)  show  themselves  to  be  single  fundaments  of  the  entire  body, 
and  are  also  developed  as  such  even  though  they  also  exhibit  evidences 
of  metamerism.  Even  the  excretory  canals  may  give  up  their  segmental 
isolation  and  become  united  to  one  another  by  means  of  longitudinal  canals." 
(Ib.  p.  348.) 

On  turning  back  to  §  206,  the  reader  will,  I  think,  demur  to  the 
assertion  that  the  independence  is  "  comparatively  slight "  ;  seeing 
that,  as  in  Ctenodrilus,  a  single  segment  sometimes  becomes  sepa- 
rate and  reproduces  other  segments  to  form  a  new  series.  Instead 
of  admitting  that  "  the  most  important  organs "  "  show  them- 
selves to  be  single  fundaments  of  the  entire  body,"  it  may  be 
held,  contrariwise,  that  their  original  independence  in  each  seg- 
ment is  masked  only  to  the  degree  involved  by  their  cooperation 
as  parts  of  a  compound  organism.  But  chiefly  I  remark  that 
when  it  is  said  that  "  the  excretory  canals  may  give  up  their  seg- 
mental isolation  and  become  united "  by  "  longitudinal  canals," 
there  is  a  clear  confession  that  the  isolation  of  these  organs  was 
original  and  their  union  superinduced — an  implication  that  the 
course  of  evolution  is  as  I  have  described  it,  and  at  variance  with 
the  course  of  evolution  assumed  by  Prof.  Korschelt. 

Yet  another  incongruity  is  involved  in  his  interpretation.  He 
writes : — 

"  Just  as  in  the  consideration  of  the  tape-worm  chain  we  were  induced  by 
the  comparison  with  unsegmented  forms  to  refer  the  entire  chain  to  an 
unsegmented  individual,  and,  on  the  other  hand,  to  sec  in  the  proglottis, 
not  a  complete  individual,  but  only  the  abstncted  hinder  portion  of  the 
body  of  the  Cestode,  in  the  same  manner,  and  with  much  more  reason,  we 
aukere  to  the  individuality  of  the  Annelid  body."  (P.  349.) 


604  APPENDIX  D2. 

And  then  on  the  preceding  page,  referring  to  the  composition  of 
the  Annelid  body,  he  says : — "  The  most  natural  comparisons  are 
those  with  the  tapeworm  chain  and  with  the  strobila  of  the 
Scyphomedusse."  Now  since  it  is  here  assumed  that  the  tape- 
worm and  the  strobila  are  analogous  in  composition,  it  is  implied 
that  the  detached  proglottis  and  the  detached  medusa  are  analo- 
gous ;  and  hence  if  we  are  to  regard  the  proglottis  as  "  not  a  com- 
plete individual  but  only  the  abstricted  hinder  portion  of  the  body 
of  the  Cestode,"  then  we  must  similarly  regard  the  medusa  as  not 
a  complete  individual,  but  only  the  abstricted  hinder  portion  of 
the  strobila.  This  commits  us  to  the  strange  conclusion  that 
whereas  individuality  is  ascribed  to  the  original  simple  polyp, 
and  by-and-by  to  the  partially-segmented  strobila,  though  these 
are  without  special  senses  and  with  only  rudiments  of  muscular 
and  nervous  systems,  individuality  is  denied  to  the  detached 
medusa,  which  has  organs  of  sense,  a  distinct  nervo-muscular 
system  and  a  considerable  power  of  locomotion,  as  well  as  a 
generative  system :  traits  which  in  other  cases  characterize 
developed  individuals.  Here  also,  then,  there  seems  to  be  an 
inversion  of  the  ordinary  conception. 

This  conception  of  the  proglottis  and  the  medusa  is,  I  see, 
accepted  by  some  as  tenable.  But  if  we  accept  it  we  must  accept 
also  an  analogous  conception,  which  will  I  think  be  regarded  as 
untenable.  It  is  that  supplied  by  the  Aphides.  From  an  egg 
proceeds  a  series  of  sexless  and  wingless  females,  and  at  the  end 
of  the  series  there  come  winged  males  and  females  with  resulting 
gamic  reproduction.  If  instead  of  forming  a  discrete  series  the 
imperfect  females  formed  a  concrete  series,  the  members  of  which 
could  individually  feed  without  being  detached  from  one  another, 
as  the  segments  of  a  tapeworm  can,  the  parallelism  would  be 
complete ;  and  then,  according  to  the  view  in  question,  we  should 
have  to  regard  the  perfect  males  and  females  eventually  arising, 
not  as  individuals  but  as  terminal  portions  of  the  series,  contain- 
ing generative  products  and  having  wings  for  the  dispersion  of 
them — locomotive  egg-bearing  segments  of  the  chain.  Whoever 
espouses  this  view  must  hold  either  that  the  first  imperfect  female 
of  the  series  was  the  individual  or  that  the  entire  string  of  them 
constituted  the  individual  (in  conformity  with  a  view  once  pro- 
pounded by  Prof.  Huxley).  But  he  must  do  more  than  this. 
Since  the  Aphides  have  descended  from  some  winged  species  of 
the  order  Ifemiptera,  he  must  hold  that  among  those  remote 
ancestors  each  particular  fly,  male  or  female,  was  an  individual ; 
but  that  when  abundant  food  and  inert  life  led  to  the  partheno- 
genetic  habit,  and  to  chains  of  sexless  forms,  the  males  and 
females  eventually  produced  at  the  end  of  each  chain,  though, 
like  their  remote  ancestors,  possessed  of  procreative  organs  and 
wings,  are  not  individuals. 


THE  ANNULOSE  TYPE.  605 

[Some  memoranda  bearing  on  the  question  here  discussed, 
mislaid  at  the  time  when  the  chapter  dealing  with  it  was  revised, 
have  been  discovered  in  time  for  utilization  in  this  appendix.] 

One  of  my  critics  says : — 

"  You  have  overstated  the  case  in  your  favour :  the  alimentary  canaf  does 
not,  as  you  suggest,  show  a  segmentation  corresponding  to  that  of  the  other 
organs  in  Annelids.  Either  it  is  a  simple  uniform  tube,  or  else  its  differ- 
entiations (pharynx,  oesophagus,  crop,  intestine)  are  quite  independent  of 
the  repetition  of  the  somites." 

In  presence  of  statements  made  in  works  of  authority,  this  objec- 
tion greatly  surprises  me.  I  meet  with  the  descriptive  word 
"  moniliform "  applied  to  the  intestine  in  some  Annelids,  and 
then  in  the  Text-book  of  Glaus,  translated  and  edited  by 
Sedgwick,  it  is  said,  concerning  the  alimentary  canal  in  the 
Annelida : — 

"This  is  followed  by  the  gastric  region  of  the  gut,  which  occupies  the 
greatest  portion  of  the  length  of  the  body,  and  is  either  regularly  con- 
stricted in  correspondence  with  the  segments,  or  possesses  lateral  diverti- 
cula."  (P.  365.) 

And  again  on  p.  369  it  is  said : — 

"The  intestine  usually  preserves  the  same  structure  in  its  entire  length 
and  is  divided  by  regular  constrictions  into  a  number  of  divisions  or 
chambers,  which  correspond  to  the  segments  and  dilate  again  into  lateral 
diverticula  and  caeca." 

The  alimentary  canal  thus  presents  the  segmental  character  as 
clearly  as  consists  with  fulfilment  of  its  function.  If  the  suc- 
cessive segments  are  cooperating  units  of  a  compound  animal 
having  but  one  mouth,  then,  necessarily,  the  gut  cannot  be  com- 
pletely cut  into  parts,  each  answering  to  a  segment,  for  there 
could  be,  in  that  case,  no  passage  for  the  food.  If  the  portion  of 
the  intestine  belonging  to  each  segment  has  a  conspicuous  dila- 
tation, or  has  a  caecum  on  each  side,  it  exhibits  the  segmental 
character  as  much  as  the  physical  requirements  permit.  So  far 
from  being  at  variance  with  the  hypothesis,  its  structure  exhibits 
a  verification  of  it. 

The  next  objection  runs  as  follows : — 

"  Then,  again,  the  ovaries  and  testes  do  not  exhibit  a  corresponding  seg- 
mentation. When  it  is  allowable  to  speak  of  ovary  or  testis  at  all  as  in 
Lumbricus,  we  find  that  in  the  case  of  both  organs  we  have  at  most  two 
pairs." 

It  seems  to  me  that  the  distribution  of  the  generative  organs  in 
a  comparatively-developed  member  of  the  Annelid  type,  is  not  the 
question.  We  have  to  ask  what  it  is  in  undeveloped  members  of 
that  type.  Among  them  the  repetition  of  generative  parts  is  in 
some  cases  just  what  the  theory  implies.  Thus  in  Glaus  I  read : — 
"  In  the  marine  Chcetopoda,  the  ova  or  spermatozoa  originate  on 
the  body-wall  from  cells  of  the  peritoneal  membrane,  either  in 


606  APPENDIX  D  2. 

the  anterior  segments  alone  or  along  the  whole  length  of  the 
body."  So  that  in  these  last  cases  there  are,  in  all  the  segments, 
parts  from  which  arise  generative  products.  The  fact  that  these 
parts  are  not  definite  ovaries  and  testes  is  irrelevant.  Ovaries 
and  testes  are  developed  generative  structures,  and  in  the  order 
of  evolution  are  preceded  by  undeveloped  ones ;  and  the  fact  that 
these  undeveloped  ones  are  found  in  little-developed  members  of 
the  type  conforms  perfectly  to  the  hypothesis.  [I  may  remark  in 
passing  that  here  is  a  good  illustration  of  that  process  of  evolu- 
tion which,  in  the  above  speculation  of  Prof.  Korschelt,  is  sup- 
posed to  be  inverted :  many  dispersed,  similar,  and  indefinite 
parts,  are  integrated  into  a  few  localized  and  definite  parts.] 

In  continuation  the  critic  above  quoted  says : — "  My  position 
is  that  the  repetition  of  segments  in  an  Annelid  is  a  phenomenon 
of  the  same  nature  as  the  repetition  of  hairs  in  a  Mammal  or  of 
scutes  in  a  Reptile  ",  and  he  proceeds  to  give  instances  of  repe- 
titions of  organs  in  other  types,  as  of  the  reproductive  struc- 
tures and  excretory  system  in  the  young  Dog-fish  or  of  the 
ovaries  in  Amphioxus.  These  examples  do  not  seem  to  me 
relevant.  No  parallelism  exists  between  the  repetition  of  a  par- 
ticular organ  in  an  animal,  and  the  repetition  of  an  entire  cluster 
of  organs  constituting  a  physiological  whole.  The  repetitions  of 
the  ovaries  in  Amphioxus  and  of  the  excretory  system  in  a  young 
Dog-fish,  occur  without  threatening  to  divide  into  similar  parts 
the  entire  organism.  But  the  segmental  repetitions  in  an  annu- 
lose  creature  implicate  the  structures  at  large,  and  would,  if 
pushed  a  little  further,  result  in  separate  creatures.  The  segment 
of  a  low  Annelid  contains  alimentary,  vascular,  nervous,  excre- 
tory, reproductive,  sensory  and  locomotive  organs — all  the  organs 
required  for  carrying  on  life,  save  certain  organs  of  external 
relation  which  its  position  excludes.  When  there  is  shown  some 
vertebrate  animal,  or  proto-vertebrate  animal,  that  is  divisible 
into  parts  each  of  which  is  in  great  measure  physiologically  inde- 
pendent, I  shall  feel  obliged  to  abandon  my  position. 

While  this  appendix  is  in  hand  I  have  received  from  another 
expert,  whose  view  is  in  general  agreement  with  my  own,  a  letter 
containing  the  following  passage : — 

"  You  will  see  that  Dohrn's  theory  was  the  antithesis  of  your  own  view 
of  vertebrate  structure,  namely  that  the  vertebrae  were  formed  by  the  peg- 
mentation,  from  mechanical  causes  of  a  body  originally  simple.  This  view 
of  yours  has  been  confirmed  by  later  researches,  which  have  shown  that 
the  most  primitive  forms  allied  to  the  Vertebrates,  possessing  the  essential 
organs,  viz.,  gill-slits,  notochord,  and  dorsal  nerve  cord,  are  not  segmented 
animals,  like  Annelids  and  Crustacea,  but  simple  animals,  having  at  most 
three  regions,  not  exactly  corresponding  to  segments.  These  primitive 
unsegmented  forms  are  Ascidian  tadpoles,  Balaiwglossus,  and  certain  other 
primitive  forms.  The  embryology ' :  Vertebrates  also  proves  that  they  are 


THE  ANNULOSE  TYPE.  607 

originally  simple  and  not  segmented  animals,  especially  the  fact  that  there 
ia  originally  one  pronepliric  uuct  or  primitive  kidney." 

Nevertheless  there  survives  a  leaning  towards  the  notion  of  a 
segmental  origin  of  the  Vertebrata.  But  the  repetitions  of  organs 
named  in  support  of  this  notion  have,  1  think,  no  more  relation 
to  the  genesis  of  the  vertebrate  type  than  the  multiplication  of 
vertebrae  in  a  snake  has  relation  to  the  genesis  of  the  vertebral 
column. 


APPENDIX  E. 


THE  SHAPES  AND  ARRANGEMENTS  OF  FLOWERS. 

IKT  Part  IV.,  Chapter  X.,  under  the  title  of  "  The  Shapes  of 
Flowers,"  I  have,  after  describing  their  several  kinds  of  symmetry, 
as  habitually  related  to  their  positions,  made  some  remarks  by  way 
of  interpretation.  The  truth  that  flowers  exhibit  a  radial  symmetry 
when  they  are  so  placed  as  to  be  equally  affected  all  round  by 
incident  forces,  having  been  exemplified,  and  also  the  truth  that 
they  assume  a  bilateral  symmetry  when  they  are  so  placed  that 
their  two  sides  are  conditioned  in  ways  different  from  the  ways  in 
which  their  upper  and  lower  parts  are  conditioned ;  I  have  gone 
on  to  inquire  (in  §  234)  by  what  causes  such  modifications  of 
form  are  produced.  I  have  stated  that,  originally,  I  inclined  to 
ascribe  them  entirely  to  differences  in  the  relations  of  the  parts 
to  physical  forces — light,  heat,  gravitation,  etc. ;  but  that  I  found 
sundry  facts  stood  in  the  way  of  this  interpretation.  And  1  have 
said  that  "  Mr.  Darwin's  investigations  into  the  fertilization  of 
Orchids  led  me  to  take  into  account  an  unnoticed  agency."  Con- 
tinuing to  recognize  the  physical  forces  as  factors  having  some 
influence,  I  have  concluded  that  the  most  important  factor  is  the 
action  of  insects ;  which,  aiding  most  the  fertilization  of  those 
flowers  which  most  facilitate  their  entrance,  produce,  in  course  of 
generations,  a  form  of  flower  specially  adapted  to  the  special 
position. 

Though  still  adhering  to  this  interpretation,  I  have  since  found 
reason  to  think  that  the  original  interpretation  contains  a  larger 
portion  of  truth  than  I  supposed  at  the  time  when  I  was  led  thus  to 
revise  it.  While  staying  at  Miirren,  in  Switzerland,  in  1872, 1  ob- 
served some  modifications  in  a  species  of  Gentian,  which  proved  to  me 
that  the  action  of  incident  physical  forces  on  flowers  is,  in  some  cases, 
very  rapid  and  decided.  The  species  furnishing  this  evidence  was  the 
Gentiana  Asclepiadea  ;  which  I  found  in  a  copse  formed  of  bushes  that 
were  here  wide  apart  and  there  close  together.  In  some  places  not 
near  to  the  bushes,  the  individuals  of  the  species  grew  vertically  ;  in 
other  places,  partially  shaded,  their  inclined  shoots  curved  in  such 


THE  SHAPES  AND   ARRANGEMENTS  OF  FLOWERS.  609 

directions  as  to  get  the  most  light ;  and  in  other  cases  their  shoots 
were  led  to  take  directions  almost  or  quite  horizontal.  That,  along 
with  these  modifications  in  the  directions  of  their  shoots,  there  went 
adjustments  in  the  attitudes  of  their  leaves,  was  a  fact  not  specially 
worthy  of  remark ;  for  plants  placed  inside  the  windows  of  houses 
habitually  show  us  that  leaves  quickly  bend  themselves  into  atti- 
tudes giving  them  the  greatest  amounts  of  light.  But  the  fact 
which  attracted  my  attention  was,  that  the  flowers  changed  their 
attitudes  in  an  equally-marked  manner.  The  radial  distribution 
passed  into  a  bilateral  distribution  with  the  greatest  readiness. 
Comparison  of  the  annexed  figures  will  show  the  character  of  this 
change. 

Figure  I.  represents  part  of  a  vertically  growing  shoot.  This 
belonged  to  an  individual  growing  unimpeded  by  bushes,  and  getting 
light  on  all  sides.  Here  it  is  observable  that  the  pairs  of  leaves, 
placed  alternately  in  directions  transverse  to  one  another — one  pair 
pointing,  say,  north  and  south,  and  the  next  pair  pointing  east  and 
west — maintain,  taking  them  in  the  aggregate,  a  radial  distribution  ; 
and  it  is  also  observable  that  the  alternate  pairs  of  flowers  are 
similarly  arranged. 

Figure  II.  is  a  sketch  from  a  shoot  which  leaned  towards  one 
side,  and  of  which  the  higher  part,  as  it  bent  more  and  more, 
got  its  upper  side  more  and  more  differently  conditioned  from  its 
lower  side.  Here  we  find  that  not  only  the  leaves,  but  also  the 
flowers,  have  adjusted  themselves  to  the  changed  conditions.  The 
leaves  of  the  lowest  pair  hang  out  in  the  normal  way,  on  the  op- 
posite sides  of  the  axis,  so  that  a  plane  passing  through  their  sur- 
faces will  cut  the  axis  transversely  ;  and  their  two  axillary  flower- 
buds,  c  and  of,  are  similarly  placed  on  opposite  sides  of  the  axis. 
But  at  the  other  part  of  the  shoot,  we  see  both  that  the  leaves  have 
adjusted  themselves  so  that  their  planes,  no  longer  cutting  the  axis 
transversely,  keep  a  fit  adjustment  with  respect  to  the  light ;  and 
also  that  the  flowers,  no  longer  on  opposite  sides  of  the  axis,  have 
bent  round  to  the  upper  side,  as  at  a  and  b. 

Figure  III.  shows  us  this  re-arrangement  carried  still  further. 
The  shoot  it  represents  was  growing  in  a  direction  nearly  horizontal, 
and  therefore  receiving  the  light  only  on  one  side.  And  here, 


all  lie  in  approximately  the  same  plane,  which  is  parallel  to  the  axis 
instead  of  transverse  to  it,  we  see  that  the  two  pairs  of  flower-buds 
have  both  come  round  to  the  upper  side  of  the  axis.  So  that 
in  this  shoot,  the  original  radial  symmetry  in  the  arrangement 
of  leaves  and  flowers,  is  completely  changed  into  a  bilateral 
symmetry. 

These  facts  do  not,  it  is  true,  prove  any  modification  in  the  forms 
85 


THE  SHAPES  AND  ARRANGEMENTS  OP  FLOWERS.  611 

of  the  flowers  themselves  :  they  only  prove  modification  in  the 
grouping  of  the  flowers.  But  beyond  showing,  as  they  do  conclu- 
sively, how  readily  a  bilateral  arrangement  of  flowers  is  producible 
out  of  an  arrangement  that  was  not  bilateral,  by  the  action  of  light, 
etc. ;  they  give  increased  probability  to  the  belief  that  changes  in  the 
shapes  of  flowers  are  producible  by  the  same  agencies.  Doubtless 
this  change  in  the  attitudes  of  the  flower-buds  is  due  to  the  action  of 
light  on  their  calyces  and  peduncles  more  than  to  its  action  on  their 
unfolding  corollas.  But  along  with  an  action  so  decided  on  the  growth 
of  these  sheathing  and  supporting  organs  containing  chlorophyl,  it  is 
scarcely  probable  that  there  is  no  action  on  the  growth  of  the  petals, 
containing  other  colouring  matter;  considering  that  in  both  cases 
the  development  of  the  colouring  matter  depends  on  the  action  of 
light,  and  considering  also  the  effect  of  light  on  petals,  familiarly 
shown  by  their  opening  and  closing.  And  if  even  but  a  small 
effect  is  producible  on  the  growth  of  the  corolla,  then  it  is  to  be 
expected  that  light  will  be  an  agent  in  changing  the  form  of  the 
corolla,  when  the  attitude  of  the  flower  causes  its  parts  to  be  dif- 
ferently exposed.  For  a  small  effect  on  the  individual  flower  will 
become  a  great  effect  in  the  flowers  of  remote  descendants ;  pro- 
vided the  changed  attitudes  of  the  flowers  preserve  considerable 
constancy  throughout  the  succession  of  individuals. 

Be  this  as  it  may,  however,  the  facts  I  have  here  described, 
which  I  doubt  not  other  observers  have  seen  paralleled  in  other 
plants,  are  instructive,  as  showing  how  quickly  certain  metamor- 
phoses are  produced,  and  as  implying  the  easy  establishment  of  such 
metamorphoses  as  permanent  characters  in  a  species,  if  the  modify- 
ing conditions  become  permanent.  The  changes  of  arrangement 
I  have  pointed  out,  do  not  become  permanent  in  this  species  because 
its  individuals  are  variously  affected  by  the  modifying  forces :  on 
some  they  do  not  act  at  all,  on  some  a  little,  on  some  much ;  and 
even  on  the  same  individual  the  different  shoots  are  quite  differently 
affected.  But  if  the  habit  of  this  plant  were  greatly  changed — if, 
for  instance,  by  spreading  into  habitats  yielding  abundant  nutri- 
ment, the  plant  became  very  luxuriant,  and,  multiplying  its  branches, 
grew  shrub-like  ;  it  is  clear  that,  being  shaded  by  one  another,  these 
branches  would  be  habitually  circumstanced  in  a  way  like  that  which 
we  here  see  produces  bilateralness  in  the  distribution  of  the  flowers, 
if  not  in  the  flowers  themselves ;  and  being  thus  permanently 
affected,  would  become  permanently  bilateral.  Accumulating  by 
inheritance,  what  is  here  only  an  individual  peculiarity,  would 
become  a  peculiarity  of  the  species — a  specific  character. 


APPENDIX    F. 


PHYSIOLOGICAL   (OR   CONSTITUTIONAL)   UNITS. 

THERE  has  recently  come  before  me  a  fact  which  has  a  signifi- 
cant bearing  on  the  hypothesis  of  Constitutional  units :  serving, 
indeed,  to  give  an  apparently  conclusive  proof  of  its  truth.  Before 
stating  it,  however,  I  may  with  advantage  re-state  the  several 
evidences  already  assigned  in  support  of  it. 

1.  First  comes  the  a  priori  reason.    These  units  in  the  germ  of 
an  organism  which  cause  development  into  a  special  structure, 
cannot  be  chemical  units— cannot  be  simply  molecules  of  proteid 
substance  in  one  or  other  of  its  forms ;  since  these  are  not  special 
to  any  type  of  creature  but  common  to  all  creatures.     Nor  can 
they  be  what  we  may  call  morphological  units — the  cells  or  proto- 
plasts ;  because  in  the  early  stages  of  development  the  cells  of 
one  organism  are  indistinguishable   from  those   of  others,  and 
because  were  cells  the  units  of  composition  there  could  be  no 
interpretation  of  what  are  called  unicellular  organisms — nothing 
to  account  for  the  innumerable  varieties  of  them.      Hence,  of 
necessity,  the  structural  elements  of  which  each  organism  is  built, 
being  neither  proteid   molecules   nor  cells,   must   be  something 
between  them :  probably  some  complex  combination  of  different 
isomeric  forms  of  proteids. 

2.  That  units  of  such  natures  are  the  essential  components  of 
each  species  of  organism,  is  shown  by  the  fact  that  in  low  types  of 
creatures,  little  differentiated  into  special  tissues,  any  considerable 
portion  of  the  body  will,  when   separated,  begin  to  assume  the 
structure  proper  to  the  species — a  truth  recently  shown  afresh  by 
Prof.  T.  H.  Morgan's  experiments  on  the  regeneration  of  Planaria 
maculata  (already  referred  to  in  §  206)  showing  that  various  frag- 
ments cut  out  develop  into  new  individuals,  and  that  when,  being 
too  small  they  die  before  doing  this,  there  is  always  an  abortive 
attempt  to  assume  the  specific  structure. 

3.  This  truth  that  a  portion  of  undifferentiated  tissue,  if  ade- 
quate in  quantity,  assumes  the  structure  of  the  type,  illustrating 

612 


PHYSIOLOGICAL  (OR  CONSTITUTIONAL)  UNITS.     613 

as  it  does  the  proclivity  of  the  constitutional  units  towards  the 
structure  of  the  species,  allies  itself  with  the  phenomena  of  both 
agamogenesis  and  gamogenesis.  The  first  of  these  shows  us  how 
a  fissiparously-detached  portion  of  the  parental  tissue  takes  on 
the  same  form  as  the  parent;  and  the  second  shows  how  those 
small  detached  portions  distinguished  as  sperm-cell  and  germ-cell 
also,  when  united  and  supplied  with  the  needful  materials,  do  the 
same  thing. 

4.  But  the  set  of  phenomena  following  the  union  of  sperm-cell 
and  germ-cell  differ  in  a  certain  way  from  those  which  follow 
when  a  gemma  or  other  unfertilized  portion  of  parental  tissue  is 
detached.  The  incomprehensibleness  of  this  difference  as  other- 
wise contemplated,  and  the  partial  comprehensibleness  of  it  when 
joined  with  the  hypothesis  of  physiological  units,  furnish  a 
further  support  for  the  hypothesis. 

The  familiar  truth  learnt  by  the  tyro  in  algebra  that  an  ap- 
parent solution  which  contains  the  unknown  quantity  is  no 
solution,  is  a  truth  apt  to  be  overlooked  in  other  spheres  than 
the  algebraic.  An  illustration  is  supplied  by  the  answer  once 
given  in  Parliament  to  the  question  "  What  is  an  Archdeacon  ? " 
— "  One  who  discharges  archidiaconal  functions."  But  science  as 
well  as  daily  life  furnishes  examples.  When  it  is  said  by  Engel- 
mann,  Hensen,  Hertwig,  and  Maupas  that  "  the  essential  end  of 
sexuality  is  rejuvenescence,  that  is,  the  restoration  of  growth- 
energy,"  we  have  another  instance  of  an  explanation  which  explains 
nothing.  WThat  is  the  phenomenon  to  be  explained  ?  That  un- 
folding of  an  organism  from  a  germ  which  displays  growth-energy. 
And  what  is  the  explanation  ?  The  giving  of  fresh  growth-energy. 
The  unknown  quantity  "  growth-energy "  is  contained  in  the 
explanation  proposed.  There  exists  no  conception  of  "juven- 
escence  "  save  that  derived  from  observing  developing  plants  and 
animals ;  and  if  "  re "  be  prefixed,  no  interpretation  is  thereby 
given  to  the  unexplained  thing  "  juvenescence." 

Coleridge  somewhere  comments  on  a  source  of  fallacy  which  he 
calls  the  "  hypostasis  of  a  relation  " — the  changing  of  a  relation 
into  a  thing.  The  plumber  who  tells  you  that  water  rises  in  a 
pump  "  by  suction  "  supplies  an  instance.  Having  assumed  suction 
to  be  an  agent,  he  thinks  that  he  understands  how  the  piston  does 
its  work.  Some  of  the  explanations  given  of  fertilization  supply 
further  instances.  When  it  is  said  that  sexual  union  has  for  its 
end  "  to  give  increased  vigour  to  all  the  vital  processes,"  it  is 
tacitly  implied  that  vigour  is  a  something — a  something  which 
can  be  given.  But  now,  in  the  first  place,  it  is  only  by  the  hypo- 
stasis  of  a  relation  that  we  are  led  to  think  of  vigour  as  a  thing. 
Vigour  is  a  state — that  state  of  a  living  body  which  enables  it  to 


614  APPENDIX   P. 

give  out  much  motion.  What  enables  it  to  do  this  ?  The  presence 
in  it  of  abundant  molecules  containing  much  molecular  motion 
which  can  be  transformed  into  molar  motion  :  the  transformation 
being  effected  by  the  falling  of  these  molecules  into  their  simpler 
and  relatively-inert  components,  which  are  thereupon  excreted. 
Energy-containing  matter  is  used  up,  and  more  energy  or  vigour 
can  be  given  only  by  supplying  more  such  matter.  How  then 
can  the  union  of  two  nuclei — those  of  the  sperm-cell  and  germ- 
cell — give  vigour  ?  Only  an  infinitesimal  portion  of  vigour  in  the 
sense  above  explained  exists  in  either,  and  the  union  of  them 
leaves  it  still  infinitesimal.  And  then,  even  supposing  the  vigour 
to  be  an  entity  and  to  be  appreciable  in  quantity,  how  could  it 
go  on  producing  that  immense  combination  of  physiological 
actions  seen  in  the  unfolding  of  the  germ  into  an  organism  ?  and 
how  could  it  go  on  producing  the  physiological  actions  of  an  adult 
organism  during  a  whole  century  ? 

May  we  not  then  say  that  these  proposed  explanations  leave  the 
question  where  it  was — are  nominal  solutions,  not  real  solutions  ? 

5.  But  the  hypothesis  of  constitutional  units  furnishes,  if  not  a 
satisfactory  answer  yet,  something  in  the  nature  of  an  answer — a 
true  cause ;  that  is  to  say,  a  cause  actually  known  to  us  as 
operating  in  other  cases.  In  §  92  it  was  pointed  out  that  in 
proportion  as  units  are  similar,  there  may  be  built  up  from  them 
an  aggregate  which  is  relatively  stable,  and  that  along  with  in- 
creasing dissimilarity  the  stability  of  the  aggregate  decreases.  It 
was  inferred  that  if  a  group  of  constitutional  units  belonging  to 
one  individual  which  have  become  moulded  into  relatively  exact 
congruity  with  the  organism  and  with  one  another  by  long  co- 
operation, are  mingled  with  some  belonging  to  another  individual 
which,  differently  circumstanced,  has  become  somewhat  different 
in  itself  and  in  its  units,  then  the  mass  formed  by  the  union  of 
the  two  groups  will  be  relatively  unstable — relatively  modifiable 
by  incident  forces.  Whereas  in  either  organism,  no  longer  per- 
petually changed  in  the  relations  of  its  parts  by  growth,  there  is 
an  approach  towards  equilibrium  between  the  whole  and  its  com- 
ponents, the  components  contributed  by  the  two  to  form  a 
germ,  being  slightly  unlike  one  another,  will  not  form  a  group 
in  a  state  of  equilibrium.  The  group  they  form  will  be  capable 
of  easy  change  by  incident  forces ;  and  they  will  so  be  rendered 
free  to  follow  their  proclivities  towards  the  typical  form  of  the 
species.  Inferring  this  we  must  also  infer  that  so  long  as  these 
two  sets  of  slightly  different  units  are  not  exposed  to  any  constant 
forces  tending  to  coerce  them  into  the  same  form,  there  will  con- 
tinue to  exist  in  the  nuclei  of  all  descendant  cells  this  same  rela- 
tive instability  and  consequent  plasticity. 


PHYSIOLOGICAL  (OR  CONSTITUTIONAL)  UNITS.      615 

Such  evidence  as  we  have  verifies  this  interpretation.  There  is 
first  the  universal  fact  that  development  of  the  germ  begins 
when  it  is  exposed  to  an  incident  force — heat — the  undulations  of 
which,  increasing  the  oscillations  of  the  mixed  units,  give  them 
greater  freedom  to  arrange  .themselves  in  conformity  with  their 
type.  We  see  this  alike  when  spring  warmth  makes  a  seed 
germinate  and  when  the  warmth  of  a  sitting  hen  sets  up  organiza- 
tion in  her  eggs.  Heat  frees  the  molecules  of  inorganic  matter 
from  local  restraints  and,  as  we  see  in  molten  metal,  lets  them 
yield  to  other  forces ;  and  similarly  in  this  organic  matter,  the 
units  are  made  free  to  follow  their  proclivities.  Then,  secondly, 
there  comes  the  evidence  from  comparisons  between  the  effects  of 
mixing  constitutional  units  differing  in  various  degrees.  Let  the 
cluster  of  mixed  units  be  derived  from  animals  that  are  ordinally 
distinct.  Nothing  happens.  The  units  each  contributes  tend  to 
arrange  themselves  after  the  parental  type.  Hence  a  conflict 
between  the  tendencies  towards  two  markedly  unlike  structures, 
and  no  structure  arises.  Suppose  the  mixed  units  come  from  two 
kindred  species— say  horse  and  ass.  The  structures  which  they 
respectively  tend  to  form,  being  in  their  main  characters  alike, 
there  is  such  cooperation  as  produces  a  working  organism  but  an 
organism  in  certain  respects  imperfect — a  mule.  Suppose,  again, 
the  units  come  from  two  varieties  of  the  same  species.  A  perfect 
organism  results,  and,  as  shown  by  Mr.  Darwin  when  detailing 
the  effects  of  crossing,  an  unusually  vigorous  organism.  The 
units  being  more  unlike  than  those  belonging  to  the  same  variety, 
the  instability  of  the  germ-plasm  is  unusually  great,  and  the 
transformations  which  constitute  development  and  action  become 
unusually  active.  When,  as  in  ordinary  cases,  the  units  are  sup- 
plied by  members  of  the  same  variety  who  have  not  been  made 
very  much  alike  by  their  antecedents,  there  follows  the  usual 
amount  of  organic  vigour.  Coming  now  to  the  results  of  breed- 
ing in-and-in — breeding  between  individuals  whose  constitutions 
(i.e.  constitutional  units)  have  for  generations  been  growing  more 
alike  in  the  absence  of  crossing  with  other  stirps — we  see  that 
diminution  of  organic  vigour  is  displayed :  there  is  a  decrease  in 
the  rate  of  physiological  change.  Finally,  on  coming  to  a  closer 
relationship,  as  in  marriages  between  cousins,  in  whom  the 
constitutional  units  are  more  than  commonly  alike,  we  see  there 
frequently  follows  either  barrenness  or  the  production  of  feeble 
offspring. 

All  these  facts,  then,  are  congruous  with  the  hypothesis  that 
the  use  of  fertilization  is  the  mixing  of  unlike  units,  and  conse- 
quent production  of  plasticity.  Leaving  out  cases  in  which  the 
unlikenesses  are  so  great  as  wholly  to  prevent  cooperation  among 
the  units,  the  degree  of  vigour,  that  is,  the  activity  of  physiologi- 


616  APPENDIX  F. 

cal  change,  is  great  where  the  unlikeness  is  great  and  diminishes 
with  the  approach  towards  likeness. 

6.  The   existence   of    constitutional   units    seems    otherwise 
necessarily  implied.     I  refer  to  the,  fact  that  no  organism  is  a 
homogeneous  mean  between  its  parents  but  consists  of  a  mixture 
of  parts,  some  following  one  parent  and  some  the  other.     Among 
illustrations  of  this  the  most  conspicuous  are  those  yielded  by 
the  variously-mixed  colours  of  hair  or  feathers.     Horses,  cattle, 
dogs,  cats,  hens,  pigeons  display  these  mixtures :  colours  in  one 
place  like  the  mother  and  in  another  place  like  the  father.    As  the 
internal  organs  are  invisible,  and  as  visible  organs  have  indefinite 
shapes  and  graduate  indefinitely  into  adjacent  ones,  the  mixture 
of  traits  is  elsewhere  less  conspicuous ;   but  occasional  marked 
cases  (especially  in  malformations)  leave  no  doubt  that  it  pervades 
the  entire  organism. 

This  peculiarity  of  transmission  seems  necessarily  to  imply  that 
there  are  distinct  units  derived  from  the  two  parents,  and  that  in 
the  course  of  development  there  is  more  or  less  segregation  of 
them — those  of  the  one  origin  predominating  so  far  in  some 
places  as  to  give  special  likeness  to  one  parent,  and  those  derived 
from  the  other  doing  the  like  in  other  places.  All  which  inter- 
pretation is  impossible  unless  the  hypothesis  of  constitutional 
units  be  admitted. 

7.  I  come  at  length  to  the  special  evidence  referred  to  at  the 
outset.     It  is  evidence  of  the  same  nature  as  that  just  assigned, 
but  carried  to  a  higher  stage.     It  is  furnished  not  by  the  segre- 
gation of  traits  derived  from  two  parents  of  the  same  variety, 
but  is  furnished  by  the  segregation  of  traits  derived  from  parents 
of  different  varieties.     In  articles  on  "  Bud  Variations  or  Sports  " 
(Gardener's  Chronicle,  1891)  Dr.  Masters  gives  various  examples 
of  the  separation  or  unmixing  of  ancestral  constitutions.     Mr. 
Noble  formed  a  hybrid  between  Clematis  Jackmani  and  C.  patens. 
One  of  these  varieties  flowers  in  the  autumn  on  new  wood,  while 
the  other  flowers  in  the  spring  on  old  wood ;  and  the  result  is 
that  flowers  of  two  kinds,  quite  unlike,  are  produced  at  different 
parts  of  the  year,  and  that  by  pruning  so  as  to  cut  away  one  or 
other  set  of  shoots,  the  plant  may  be  made  to  produce  exclusively 
for  the  time  being  one  or  other  sort  of  flower. 

"  Another  very  interesting  case  of  unmixing,  or,  if  it  be  preferred,  of  par- 
tial mixture,  is  afforded  by  Neubert's  Berberis.  This  is  a  hybrid  between 
the  evergreen  pinnate-leaved  Mahonia  and  the  deciduous  simple-leaved 
Berberis  vulgaris,  and  it  bears  leaves  some  of  which  are  intermediate  in 
appearance,  while  others  are  much  like  those  of  one  or  other  of  its  parents. 

"  A  not  uncommon  illustration  of  a  similar  kind,  is  the  production  of  a 
Peach  and  a  Nectarine  on  the  same  branch,  and  we  have  just  learnt  from 


PHYSIOLOGICAL  (OR  CONSTITUTIONAL)  UNITS.      617 

Canon  Ellacombe  that  some  of  the  Berlin  Hellebores  show  evidence  of  their 
hybrid  nature  by  occasionally  producing  foliage  [and  flowers  ?]  of  the  two 
parents  separately  from  the  same  root-stock. 

"  In  addition  to  the  cases  given  above,  we  may  here  cite  a  few  more  which 
have  come  under  our  notice,  such  as  a  Chrysanthemum,  half  the  florets  of 
which  are  of  one  colour,  half  of  another.  A  hybrid  Calanthe,  showing  a 
similar  piebald  variation,  is  shown  in  Fig.  14.  A  very  curious  case  was  that 
of  the  Narcissus  received  from  Mr.  Walker,  and  in  which  flowers  of  two 
distinct  varieties  sprang  from  the  same  bulb.  Grapes  not  uncommonly 
show  their  crossed  origin  by  presenting  a  striped  appearance,  one  stripe 
being  of  one  colour,  one  of  another,  as  may  also  be  seen  in  the  Orange, 
Apple,  Lemon,  and  Currant." 

Thus,  however  the  germ- plasm  is  constituted  its  essential  com- 
ponents cannot  be  all  alike.  Before  there  can  be  this  dissociation 
of  ancestral  characters,  there  must  be  in  the  germ-plasm  different 
elements  capable  of  being  dissociated.  This  single  fact  seems  to 
compel  us  to  assume  constitutional  units. 


APPENDIX    G. 


THE   INHERITANCE   OF  FUNCTIONALLY-CAUSED 
MODIFICATIONS. 

IN  Part  II,  Chapter  XA,  I  have  confessed  that  the  process  by 
which  a  structure  changed  by  use  or  disuse  affects  the  sperm- 
cells  or  germ-cells  whence  arise  descendants,  is  unimaginable : 
without,  however,  inferring  that  therefore  such  a  process  does 
not  exist.  With  others  it  seems  different.  Some  three  years 
ago  the  following  expression  of  opinion  came  to  me  from  a 
zoological  expert : — 

"  Many  zoologists — most  of  us  here  at  Cambridge — are  intensely  opposed 
to  the  doctrine  of  the  inheritability  of  acquired  variations.  Even  assuming 
that  the  developmental  power  of  a  germ  is  determined  by  its  molecular 
structure  (and  I  for  one  would  question  this — Driesch  and  his  school  when 
they  find  that  they  can  squeeze  a  developing  egg  into  all  sorts  of  shapes 
without  altering  the  final  result,  that  one  blastomere  in  an  egg  which  has 
divided  into  8  is  still  able  to  reproduce  a  whole  embryo — question  it  also), 
we  still  fail  to  conceive  any  means  by  which,  for  instance,  a  change  in  the 
development  of  a  muscle  or  nerve  can  effect  a  corresponding  change  in  that 
part  of  the  germ  which  is  destined  to  produce  a  corresponding  part  in  the 
descendant." 

Here  it  will  be  observed  that  belief  in  the  inheritance  of 
structural  effects  wrought  by  use  and  disuse,  is  rejected  because 
of  inability  "  to  conceive  any  means  "  by  which  the  modifications 
produced  in  an  organ  can  effect  a  correlated  modification  in  the 
germ  of  a  descendant :  failure  to  conceive  is  the  test.  The  im- 
plication is  that  some  alternative  hypothesis  is  accepted  because 
the  correlating  of  a  variation  in  an  organ  with  a  corresponding 
germ-variation  is  effected  by  a  means  which  is  conceivable.  This 
is  the  hypothesis  of  Weismann.  Concerning  its  conceivability  1 
have,  in  the  chapter  just  named,  already  written  as  follows : — 

"  If  we  follow  Prof.  Weismann  we  are  led  into  an  astounding  supposition. 
He  admits  that  every  variable  part  must  have  a  special  determinant,  and 
that  this  results  in  the  assumption  of  over  two  hundred  thousand  for  the 
four  wings  of  a  butterfly.  Let  us  ask  what  must  happen  in  the  case  of  a 
peacock's  feather.  On  looking  at  the  eye  near  its  end,  we  see  that  the 
minute  processes  on  the  edge  of  each  lateral  thread  must  have  been  in  some 
way  exactly  adjusted,  in  colour  and  position,  so  as  to  fall  into  line  with  the 
processes  on  adjacent  threads :  otherwise  the  symmetrical  arrangement  of 
coloured  rings  would  be  impossible.  Each  of  these  processes,  then,  being  an 
618 


FUNCTIONALLY-CAUSED  MODIFICATIONS.  619 

independent  variable,  must  have  had  its  particular  determinant.  Now  there 
are  about  300  threads  on  the  shaft  of  a  large  feather,  and  each  of  them 
bears  on  the  average  1,600  processes,  making  for  the  whole  feather  480,000 
of  these  processes.  For  one  feather  alone  there  must  have  been  480,000 
determinants,  and  for  the  whole  tail  many  millions.  And  these,  along  with  the 
determinants  for  the  detailed  parts  of  all  the  other  feathers,  and  for  the 
variable  components  of  all  organs  forming  the  body  at  large,  must  have  been 
contained  in  the  microscopic  head  of  a  spermatozoon !  "  [And  each  of  them 
must,  throughout  all  the  complex  developmental  processes,  have  preserved 
the  ability  to  find  its  way  to  the  exact  place  where  it  was  wanted  !] 

If  my  Cambridge  correspondent  is  able  to  conceive  this  process 
implied  by  the  hypothesis  of  Weismann,  I  can  only  say  that  he 
has  an  enviable  power  of  imagination. 

But  now  comes  the  strange  fact  that  an  impossibility  of  thought 
implied  by  Weismann's  hypothesis  does  not  cause  rejection  of  it, 
but  yet  is  urged  as  a  reason  for  rejecting  an  alternative  hypo- 
thesis which  does  not  imply  it.  One  objector  cannot  conceive 
that  "a  change  in  the  development  of  a  muscle  or  nerve  can 
effect  a  corresponding  change  in  that  part  of  the  germ  which  is 
destined  to  produce  a  corresponding  part  in  the  descendant " ; 
and  another  objector  says  it  is  "  very  hard  to  believe "  that  a 
functionally-changed  organ  will  so  affect  spermatozoa  and  ova 
that  "  one  particular  part  of  them  will  be  so  altered  that  the 
organisms  which  grow  up  from  them  will  be  able  to  present  the 
same  modification  on  the  application  of  a  different  stimulus."  It 
is  tacitly  assumed  by  both  that,  as  in  the  hypothesis  of  Weis- 
mann so  in  the  counter-hypothesis,  a  particular  part  of  the 
germ-plasm  gives  origin  to  a  particular  part  of  the  developed 
organism.  But  nothing  of  the  kind  is  implied.  The  nature  of 
the  counter-hypothesis  (at  any  rate  as  held  by  me)  is  entirely 
misapprehended.  Anyone  who  turns  back  to  the  chapters  in  the 
first  volume  where  the  conception  of  physiological  units  (or  con- 
stitutional units)  was  set  forth,  or  who  re-reads  the  foregoing 
appendix,  will  see  that  there  is  altogether  excluded  any  idea  of 
correlation  between  certain  parts  of  the  germ  and  certain  parts 
of  the  resulting  organism.  The  units  are  supposed  to  be  all 
alike,  and  during  the  progressive  embryological  changes  local 
groups  of  them  are  supposed  to  take  on  different  forms  and 
structures  under  the  combined  forces,  general  and  local,  brought 
to  bear  on  them.  This  conception  is  necessitated  by  all  the 
evidence.  The  fact  disclosed  by  the  experiments  of  Driesch, 
Wilson,  and  Chabry,  that  from  fractions  of  an  ovum  structures 
may  be  obtained  like  that  obtained  from  the  whole  ovum,  only 
smaller,  necessitates  it.  The  fact  that  any  sufficiently  large 
fragment  of  a  polyp  or  planarian,  no  matter  from  what  part  of 
the  body  taken,  will  develop  into  a  complete  polyp  or  planarian 
necessitates  it.  The  fact  that  from  an  undifferentiated  portion 


620  APPENDIX  G. 

of  a  plant,  even  so  small  as  a  scale,  a  complete  plant  may  arise 
necessitates  it.  And  it  is  necessitated  by  the  fact  that  among 
plants,  roots  are  produced  by  imbedded  shoots  and  shoots  by 
roots,  as  well  as  by  the  fact  that  low  animals,  such  as  hydroids, 
if  deprived  of  both  head  and  root,  will  develop  a  head  from  the 
root  part  and  a  root  from  the  head  part,  if  their  respective  con- 
ditions are  inverted.  All  this  evidence  shows  conclusively  that 
the  component  units  of  each  species,  whether  existing  in  the  germ 
or  in  the  developed  organism,  are,  when  not  yet  differentiated  by 
local  conditions,  all  alike,  and  that  the  notion  of  special  parts  of 
the  germ-plasm  correlated  with  special  parts  of  the  resulting  or- 
ganism, is  entirely  alien  to  the  hypothesis. 

"  But  how  do  the  units  of  a  modified  organ  affect  the  units  of 
the  germ  in  such  wise  that  these  produce  an  inherited  modifica- 
tion of  the  organ  ? "  will  be  asked.  This  difficulty  has  been  dealt 
with  in  §§  97rf,  970,  where  the  analogy  between  the  social  organism 
and  the  individual  organism  has  been  brought  in  aid :  serving,  if 
not  to  furnish  a  conception,  yet  to  furnish  an  adumbration.  Ke- 
garding  citizens  as  the  units  of  an  unfolding  society,  say  a  colony, 
it  was  pointed  out  that  the  nature  they  inherit  from  a  mother- 
society  gives  them  a  proclivity  towards  a  society  of  like  struc- 
ture, the  traits  of  which  are  progressively  assumed  as  the 
colony  grows  sufficiently  large  to  make  them  possible.  At  the 
same  time  it  was  pointed  out  that  while  the  influence  of  the 
entire  aggregate  on  the  individuals  is  seen  in  this  forming  of 
them  into  a  society  of  the  inherited  type,  the  influences  of  local 
circumstances,  and  of  individuals  on  one  another,  in  each  group, 
make  them  differentiate  into  appropriate  social  structures,  taking 
on  fit  occupations  and  industries :  the  implication  being  that  in 
virtue  of  their  inherited  natures  they  all  have  partial  capacities 
for  the  various  activities  they  undertake ;  so  that  an  immigrant 
clerk  sets  up  a  tavern,  a  compositor  takes  to  carpentering,  and  a 
university  man  rides  after  cattle  or  is  employed  on  a  sheep  farm. 
Evidence  was  given  in  that  place,  as  in  the  above  paragraph,  that 
the  constitutional  units  of  an  organism  similarly  have  all  of  them 
potentialities  for  taking  on  this  or  that  structure  and  mode  of 
action  which  local  conditions  determine.  It  was  further  argued 
that  as  citizens  are  continually  being  remoulded  by  their  society 
into  congruity  with  it,  and,  if  circumstances  change  them,  tend  to 
remould  their  society  ;  so  in  the  individual  organism,  there  is  this 
reciprocal  action  of  the  whole  on  the  units  and  of  the  units  on 
the  whole.  Hence  it  was  inferred  that  the  modified  units  in  any 
modified  part  tend  to  diffuse  modifications  like  their  own  through 
the  units  at  large :  being  aided  by  the  circulation  of  protoplasm, 
as  suggested  in  §§  54rf  and  97/.  And  it  was  urged  that,  however 
inconceivably  complex  such  a  process  may  be,  yet  it  seems  not 


FUNCTIONALLY-CAUSED  MODIFICATIONS.  621 

incredible  when  we  recognise  the  probability  that  an  organism  is 
more  or  less  permeable  to  undulations  propagated  by  its  mole- 
cules :  Rontgen  rays  giving  warrant.  If  such  units  throughout  the 
tissues  may  take  in  and  send  out  ethereal  waves  which  bring  it 
into  rhythmical  relations  with  others  of  its  kind  and  tend  to  pro- 
duce congruity,  it  becomes,  if  not  conceivable  still  supposable, 
that  throughout  the  circulating  protoplasm  there  goes  on  a  con- 
tinual harmonization  of  its  components — a  moulding  of  each  by 
all  and  of  all  by  each.  Should  it  be  said  that  such  a  process  is 
too  marvellous  to  be  reasonably  assumed,  the  reply  is  that  it  is 
not  more  marvellous  than  heredity  itself,  which,  were  it  not 
familiar  to  us,  would  be  thought  incredible. 

But  as  I  have  said  in  the  place  referred  to — "  At  last  then  we 
are  obliged  to  admit  that  the  actual  organizing  process  transcends 
conception.  It  is  not  enough  to  say  that  we  cannot  know  it ;  we 
must  say  that  we  cannot  even  conceive  it :  "  can  only  conceive 
the  possibility  of  a  suggested  interpretation. 

Hence  we  have  to  rely  upon  evidences  of  other  kinds.  Among 
these,  some  which  I  think  dispose  absolutely  of  the  fashionable 
hypothesis  while  they  harmonize  with  the  opposed  hypothesis, 
have  now  to  be  named.  That  their  implication  should  not  have 
been  generally  recognized  would  have  seemed  to  me  incompre- 
hensible were  it  not  that  I  have  myself  only  now  observed  this 
implication.  The  facts  are  these : — 

"  Verlot  mentions  a  gardener  who  could  distinguish  150  kinds  of  camellia, 
when  not  in  flower ;  and  it  has  been  positively  asserted  that  the  famous  old 
Dutch  florist  Voorhelm,  who  kept  above  1,200  varieties  of  the  hyacinth,  was 
hardly  ever  deceived  in  knowing  each  variety  by  the  bulb  alone.  Hence 
we  must  conclude  that  the  bulbs  of  the  hyacinth  and  the  branches  and 
leaves  of  the  camellia,  though  appearing  to  an  unpractised  eye  absolutely 
undistinguishable,  yet  really  differ."  (Darwin,  Variation  of  Animals  and 
Plants,  cfcc.,  vol.  ii,  p.  251.)  " 

More  recently  testimony  to  like  effect  has  been  given  by  Dr. 
Maxwell  Masters,  and  has  already  been  quoted  by  me  in  a  note 
to  §  286  in  illustration  of  another  truth.  He  says  concerning 
such  variations : — 

"  To  the  untrained  eye,  the  primordial  differences  noted  are  often  very 
slight ;  even  the  botanist,  unless  his  attention  be  specially  directed  to  the 
matter,  fails  to  see  minute  differences  which  are  perceptible  enough  to  the 

raiser  or  his  workmen These  apparently  trifling  morphological 

differences  are  often  associated  with  physiological  variations  which  render 
some  varieties,  say  of  wheat,  much  better  enabled  to  resist  mildew  and  dis- 
ease generally  than  others.  Some,  again,  prove  to  be  better  adapted  for 
certain  soils  or  for  some  climates  than  others ;  some  are  less  liable  to  injury 
from  predatory  birds  than  others,  and  so  on." 

In  his  Vegetable  Teratology,  p.  493,  Dr.  Masters  names  another 
fact  having  a  like  implication — the  fact  that  among  seedling 


622  APPENDIX  G. 

stocks  which  have  not  yet  flowered,  those  which  will  produce 
double  flowers  are  distinguishable.     He  says: — 

"  This  separation  of  the  single  from  the  double-flowered  plants,  M.  Chatie 
tells  us  is  not  so  difficult  as  might  be  supposed.  The  single  stocks,  he  ex- 
plains, have  deep  green  leaves  (glabrous  in  certain  species),  rounded  at  the 
top,  the  heart  being  in  the  form  of  a  shuttlecock,  and  the  plant  stout  and 
thick-set  in  its  general  aspect,  while  the  plants  yielding  double  flowers  have 
very  long  leaves  of  a  light  green  colour,  hairy  and  curled  at  the  edges,  the 
heart  consisting  of  whitish  leaves,  curved  so  that  they  enclose  it  com- 
pletely." 

What  is  the  general  truth  implied  ?  Clearly  that  there  exists 
no  such  thing  as  an  independent  local  variation.  Some  marked 
change  in  the  form  or  colour  of  a  flower  or  a  fruit  draws  atten- 
tion ;  and,  being  a  change  which  interests  the  florist  or  gardener, 
pecuniarily  or  otherwise,  not  only  draws  attention  but  usually 
monopolizes  attention :  the  natural  impression  produced  being  that 
this  variation  stands  there  by  itself — is  without  relation  to 
variations  elsewhere.  But  now  it  turns  out  that  there  are 
concomitant  variations  all  over  the  plant.  Even  in  under- 
ground bulbs  certain  appreciable  differences  go  along  with  certain 
conspicuous  differences  in  the  flowers.  And  if  along  with  a 
striking  change  in  a  flower  which  the  florist  contemplates,  there 
go  changes  all  over  the  plant  not  obvious  to  careless  observers 
but  visible  to  him,  we  must  infer  that  there  are  everywhere 
minute  differences  which  even  the  florist  cannot  perceive:  the 
whole  constitution  of  the  plant  has  diverged  in  some  measure 
from  the  constitutions  of  kindred  plants.  Every  local  variation 
implies  a  change  pervading  the  entire  organism,  manifested  in 
concomitant  variations  everywhere  else. 

If  so,  what  becomes  of  the  hypothesis  of  determinants — the 
hypothesis  that  there  is  a  special  element  in  the  germ-plasm 
which  results  in  a  special  local  modification  in  the  adult  organism  ? 
That  there  are  no  facts  supporting  it  has  been  all  along  mani- 
fest ;  but  now  it  is  manifest  that  the  facts  directly  contradict  it. 

At  the  same  time  it  may  be  remarked  that  while  the  facts  are 
wholly  incongruous  with  the  hypothesis  of  determinants  and  its 
accompanying  elaborate  speculation,  they  are  not  incongruous 
with  the  alternative  hypothesis.  Impossible  though  it  may  be 
to  imagine  the  natures  of  those  ultimate  units  peculiar  to  each 
species,  which  have  proclivities  towards  the  particular  form 
of  organization  characterizing  it,  yet  that  a  change  of  structure 
arising  in  one  part  of  the  organism  is  accompanied  by  multitudi- 
nous changes  of  structure  in  other  parts  of  the  organism,  is  not 
only  congruous  with  the  belief  that  there  exist  such  constitutional 
units,  but  yields  it  distinct  support.  For  if,  as  above  argued,  a 
conspicuous  local  variation  is  not  the  result  of  any  modification  of 
units  special  to  the  locality,  but  is  the  result  of  a  modification  of 


FUNCTIONALLY-CAUSED  MODIFICATIONS.  623 

the  units  at  large,  then  it  must  happen  that  such  modification  must 
have  its  effects  on  all  other  parts  of  the  organism  ;  so  that  there 
cannot  fail  to  result  all  those  small  concomitant  variations  above 
indicated. 

May  we  not  also  say  that  it  becomes  less  incomprehensible 
that  structural  changes  caused  by  use  and  disuse  are  inherited  ? 
If,  as  we  see,  a  local  variation  spontaneously  arising  is  accompanied 
by  multitudinous  other  local  variations,  implying  a  necessary 
correlation  between  each  local  variation  and  the  general  constitu- 
tion of  the  organism ;  then  it  may  be  argued  that  if  a  marked 
change  of  function  in  an  organ  causes  increase  or  decrease  of  it, 
this  general  correlation  implies  that  there  must  be  a  reciprocal  re- 
action between  the  part  and  the  whole,  tending  to  re-establish 
their  congruity.  The  constitution  at  large  will  in  so  far  be 
changed,  and  along  with  its  change  will  go  corresponding  changes 
in  the  sperm-cells  and  germ-cells. 

Finally  let  me  add,  not  another  argument,  but  another  fact  of 
observation,  of  the  kind  which  opponents  demand,  but  which, 
when  they  are  from  time  to  time  furnished,  are  severally  pooh- 
poohed  as  not  enough.  Each  of  them  is  spoken  of  as  a  solitary 
fact  and  slighted  as  inadequate ;  and  when  by-and-by  another  is 
named,  this  is  treated  in  the  same  way ;  so  that  the  facts  which 
if  brought  together  would  be  recognized  as  sufficient  are  never 
brought  together.  That  to  which  I  refer  is  set  forth  in  a  pamphlet 
by  M.  Leo  Errera,  Professor  at  the  University  of  Brussels, 
entitled  "  Heredite  d'un  Caractere  acquis  chez  un  Champignon 
pluricellulaire  ;  "  being  an  account  of  experiments  of  Dr.  Hunger, 
at  the  Botanical  Institute  in  Brussels.  First  enumerating  various 
instances  of  adaptations  to  climate,  as  those  of  plants  which,  fitted 
to  northern  regions,  preserve  their  constitutional  rapidity  of 
growth  and  seeding  when  brought  south,  and  do  this  for  several 
generations,  he  goes  on  to  detail  the  culture-experiments  of 
M.  Hunger,  and  sums  up  the  results  of  these  in  the  following 
words : — 

"  On  d6duit  de  la  que : 

"  1°  Les  conidies  ftAspergillus  niger  sont  adaptees  a  la  concentration  du 
milieu  ou  a  vecu  1'individu  qui  les  porte ;  cet  effet  est  encore  plus  marque 
apres  deux  generations  passees  dans  un  milieu  donne  (Exper.  I  et  II) ; 

"  2°  II  s'agit  d'une  veritable  adaptation  et  non  pas  simplement  d'un 
accroissement  de  vigueur  chez  les  conidies  provenant  des  liquides  concentres, 
car  ces  monies  conidies  germent  moins  rapidement  et  donnent  des  plantes 
moins  vigoureuses  que  les  conidies  normales  lorsqu'on  les  seme  de  nouveau 
sur  le  milieu-type:  en  s'adaptant  aux  liquides  concentres,  elles  se  sont 
desadaptees  du  liquide  normal  (Exper.  Ill) ; 

"  3°  line  generation  passee  sur  le  liquide  normal  n'efface  pas  I'innuence 
d'une  ou  de  deux  generations  anterieures  passees  sur  une  liquide  plus  con- 
centre (Exper.  IV). 

"  Tous  ces  resultats  concordent :  Us  montrent  une  legere,  mais  incontest- 
able transmission  hereditaire  de  I 'adaptation  au  milieu." 


SUBJECT-INDEX. 

(For  this  Index  as  it  appeared  in  previous  editions  the  Author  is  indebted  to  F. 
HOWARD  COLLINS,  Esq.,  of  Edgbaston,  Birmingham.  It  has  now  been  adjusted 
to  suit  the  present  revised  and  enlarged  edition.) 


ACACIA,  foliar  organs,  II,  41,  264. 

Acalephw:  environment,  I,  105; 
water  in,  I,  173. 

Acari:  special  creation  and  effects 
of,  I,  428;  direct  transformations, 
I,  706;  segmentation,  II,  111. 

Acorus  calamus,  agamic  propaga- 
tion, I,  642. 

Acquired  characters,  inheritance 
of:  functionally-produced  modi- 
fications in  plants  and  animals, 
I,  307-13,  318,  526,  541,  562,  692- 
5,  II,  618-22;  conceivability  of, 
on  the  hypothesis  of  physiologi- 
cal units,  I,  368-71,  695,  II,  618- 
22;  diminution  of  jaw,  I,  541-2, 
693;  current  views  on,  I,  559-60; 
cessation  of  selection,  I,  560-3; 
Elmer's  theory  of  orthogenesis, 
I,  5GO;  species  differentiation,  I, 
573;  location  of  mammalian 
testes,  I,  573;  tactual  perceptive- 
ness,  I,  602-8,  633,  665,  666,  672-3, 
692;  blindness  of  cave-animals,  I, 
612-3,  647-9;  coadaptation  of  co- 
operative parts,  I,  621,  663-5; 
transmission  of  disease,  I,  622-3; 
hypothesis  supported  by  telegony, 
I,  624-8,  644-6,  649-50;  views  of 
Darwin  and  neo-Darwinists,  I, 
t>30,  685,  690;  why  facts  in  sup- 
port are  meagre,  I,  632;  degrada- 
tion of  little  toe,  I,  652-3,  673; 
neuter  forms  of  social  insects,  I, 
658-9,  663-4,  670,  675;  degenerated 
instinct  in  ants,  I,  660-2;  rudi- 
mentary limbs  of  whale,  I,  669, 
692;  importance  of  question,  I, 
672,  690;  monstrous  development 
of  honey-ants,  I,  683-4;  osteology 
of  Punjabis,  I,  689;  summary  of 
evidences  in  support,  I,  692-5; 


genesis  of  vertebrate  skull,  II, 
227;  false  joints,  II,  371,  372;  con- 
ceivability of  rival  hypotheses,  II, 
618-22;  adaptation  to  environment 
in  Aspergillus,  II,  623. 

Acrogens,  the  term,  II,  55-6.  (See 
Archegoniatcw.) 

Actinophrys:  a  primary  aggregate, 
II,  76,  genesis,  II,  452. 

Actinozoa:  multiaxial  development, 
I,  166;  waste  and  repair,  I,  213, 
219;  differentiation,  I,  391;  para- 
sitism, I,  397;  integration,  II,  92; 
symmetry,  II,  189,  192;  growth 
and  genesis,  II,  444. 

Activity:  the  principle  of,  the  es- 
sential element  in  Life,  I,  113, 
114,  122;  not  inherent  in  living 
matter,  I,  120;  nutrition  and  gene- 
sis, rfsum6,  II,  497-9;  and  evolu- 
tion, II,  501^. 

Adaptation:  general  truths,  I,  227- 
33,  233-5;  botanical,  I,  227;  physio- 
logical, I,  228-33;  psychological, 
I,  229,  230-3;  structural,  func- 
tional, and  interdependence,  I, 
235-9,  240-1,  318;  social  and  or- 
ganic stability,  I,  240-2;  resume,  I, 
242-3;  to  varied  media,  I,  479-81, 
489,  556:  multiplication  of  effects, 
I,  512-3,  550;  direct  equilibration, 
I,  522-3;  natural  selection  and 
equilibration,  I,  530-5;  non-adap- 
tive specific  characters,  I,  565; 
time  required  for  effecting,  I, 
565-6;  an  obstacle  to  re-adapta- 
tion, II,  11;  of  skin  and  skele- 
ton, II,  215,  217;  outer  tissue,  II, 
312-4,  387;  skin  and  mucous  mem- 
brane differentiation,  II,  321-2; 
389;  vascular  system,  II,  343-4; 
is,  II,  352;  muscular,  II, 
625 


626 


SUBJECT-INDEX. 


368-9,  391;  persistence  of  force 
and  physiological,  II,  394;  of  re- 
productive activity  to  conditions, 
II,  411-6;  vertebrae  development, 
II,  563-6.  (See  also  Co-adapta- 
tion.) 

Africa,  effect  of  climate  on  inhabi- 
tants, I,  30. 

Agamogenesis  :  alternation  with 
gamogenesis,  I,  266-7,  272-3,  284- 
94,  336,  592,  II,  415;  parallelism  in 
karyokinesis,  I,  267-8;  a  process 
of  disintegration,  I,  276-7;  condi- 
tions determining  its  continuance, 

I,  284-94,  295-7,  330;  physiological 
units,    I,    351,    II,    613;    spontane- 
ous fission,   I,  582,  584-7,   589-92, 
595-6,  599;  remarkable  extent  of, 
under    favourable    conditions,    I, 
591-2,  640-1;  in  Actinozoa,  II,  92; 
in  Hydrozoa,  II,  102;  in  Annelida, 

II,  103;  innutrition,  II,  179-80. 
AgaricmcB,  II,  139,  257. 

Agassiz,  L.  J.  R.,  zoological  classi- 
fication, I,  380. 

Aggregates,  Animal  and  Plant  (see 
Morphology). 

Agility,  a  vital  attribute,  I,  578. 

Agrimony,  floral  symmetry,  II,  42, 
167,  170. 

Air,  In  vegetal  tissues,  II,  567-8, 
583,  591,  593. 

"  Air  plants,"  I,  208. 

Albumen:  properties,  I,  12;  Lieber- 
kiihn's  formula,  I,  13;  diffusibil- 
ity,  I,  19;  in  organic  tissues,  I,  41. 

Alcohols,  properties,  I,  10-12. 

Algce:  reproduction,  and  the  dy- 
namic element  in  life,  I,  118-9; 
multlcentral  development,  I,  163, 
164;  axial  development,  I,  165; 
locomotive  powers  of  minute 
forms,  I,  196;  uniform  tissue  and 
function,  I,  200,  586;  gamogenesis, 
I,  271,  279,  280,  283,  II,  448,  449, 
450;  fertility,  I,  582,  II,  440,  441; 
fission,  I,  584,  585;  unicellular 
forms,  II,  22;  integration  in  Gon- 
fervoidrw  and  Conjugates,  II,  25; 
pseudo-foliar  and  axial  develop- 
ment, II,  28-33,  57;  foliar  devel- 
opment, II,  76,  91;  branch  sym- 
metry, II,  145:  cell  metamor- 
phoses, II,  176;  tissue  differentia- 


tion, II,  244,  246,  251,  252,  256, 
272,  385-6;  adaptation  of  repro- 
ductive activity  to  conditions,  II, 
289;  integration,  II,  292;  indefi- 
niteness,  II,  295;  genesis  and  de- 
velopment, II,  4G3. 
Alimentary  canal:  metabolic  pro- 
cesses and  agents,  I,  68-9,  74; 
structural  traits,  I,  192;  progres- 
sive development,  I,  195;  relation 
to  environment,  I,  196;  function, 

I,  205;  segmentation  in  annelids, 

II,  125;    differentiation,    II,    301, 
302,  321-2,  323-5,   389;  specializa- 
tions in  birds,   II,   325;   in  rumi- 
nants, II,  327-9;  differentiation  of 
liver,  II,  329-33;  muscularity,  II, 
364. 

Allotropism:  of  organic  constitu- 
ents, I,  4,  9;  muscular  action,  1,59. 

Alloys,  melting  point  of,  I,  339. 

Alternation  of  generations,  mislead- 
ing application  of  term,  II,  84. 
(See  Agamogenesis  and  Gamogen- 
esis.) 

Amitosls,  occurrence  of,  In  morbid 
tissues,  I,  264. 

Ammonia:  properties,  I,  7,  9;  nerve 
stimulation,  I,  55. 

Amoeba:  central  development,  I,  163; 
a  primary  aggregate,  II,  86;  sym- 
metry of  encysted,  II,  186;  sym- 
biosis, II,  400. 

Amphibia:  classification  of,  I,  392; 
embryonic  respiratory  system,  I, 
457;  structure  and  media,  I,  483; 
limb  locomotion,  II,  15;  segmenta- 
tion, II,  122,  225;  outer  tissues, 
II,  311;  respiration,  II,  334,  338; 
Owen  on  skeleton,  II,  552,  557, 
558. 

Amphioxus:  separation  of  segmenta- 
tion spheres  of  egg,  I,  691;  em- 
bryogeny,  II,  121;  local  segmenta- 
tion, II,  125-7,  605;  genesis  of 
vertebrate  axis,  II,  213-6,  218, 
222;  development,  II,  564. 

Amphipnous  cuchia,  vascular  air- 
sacs,  II,  337. 

Anabas  scandrns,  the  climbing  fish, 
I,  480,  4S3. 

Anacharfs  (see  Floidra). 

Anresthetics,   diverse  effects  of,   I, 


SUBJECT-INDEX. 


G27 


Angrcecum,  assimilative  function  of 
root,  II,  255. 

"  Animal  Spirits,"  vitalism  and,  I, 
115. 

Animals:  nutrition  and  molecular 
rearrangement,  I,  36-7;  nitrogen- 
ous character,  I,  39-41;  sensible 
motion,  I,  57;  metabolism,  I,  62- 
77;  multiplication  of  energies,  I, 
75;  contrasted  traits  of  plants  and, 
I,  196;  what  is  an  individual?  I, 
246-7;  solar  influence,  I,  500,  556; 
geologic  changes  affecting,  I,  501- 

4,  549,   550,   556;    interdependence 
with  plants,  I,  504-6,  514,  II,  398- 
401;   complexity  of  influences  af- 
fecting,  I,   506;   geographical   iso- 
lation   and    origin    of    species,    I, 
568-9;    vital   attributes,    I,    577-9; 
distribution      and      antiquity      of 
plant  and  animal  types,   II,  297; 
mutual   dependence  of  organisms 
at  large,  II,  397-408;  hypothetical 
plant-animal    type,    II,    397;    pro- 
gressive increase  of  size,  II,  401; 
laws  of  multiplication,  II,  411-6; 
rhythm  in  numbers,  II,  419;  law 
of    weights    and    dimensions,    II, 
434. 

Animals,    domesticated:    variation, 

I,  324,   326,    560,    563,    693;    inter- 
breeding,   I,    345-7,    354,    II,    615; 
pure    and    mixed    breeds,    I,    354, 
625. 

Annelida:  phosphorescence,  I,  50; 
axial  development,  I,  165,  166;  in- 
tegration, I,  363;  larval  forms  and 
phytogeny,  I,  447,  II,  115;  seg- 
mental  fission,  I,  588-9;  seg- 
mentation, II,  98-101,  103-4,  602- 

5,  II,    107-9,    125-7;    lateral   gem- 
mation,  II,   105;   embryogeny,   II, 
119;  bilateral  symmetry,  II,   197- 
200;  genesis,  II,  444,  453. 

Annulosa:  regeneration,  I,  361-2; 
distinctive  traits,  I,  392;  origin  of 
type,  II,  98-110,  602-6;  unit  of 
composition,  II,  105;  application 
of  term,  II,  111;  vertebrate  sym- 
metry compared,  II,  203-6;  seg- 
mental  differentiation,  II,  207-9; 
unintegrated  function  in  Planaria, 

II,  37^;  development  and  genesis, 
II,  464;  nutrition  and  genesis,  II, 


490.  (See  also  Annelida  and 
Arthropoda.) 

Anthropomorphism,  former  preva- 
lence of,  I,  419. 

Ants:  utilization  of  aphids,  I,  660- 

I,  II,  403,  405;  nest-mates,  II,  405; 
castes  in  social  species,  I,  658-9, 
670,   675;   loss   of  self -feeding   in- 
stinct in  Amazons,  I,  660-1,  663- 
4;  monstrous  development  of  Hon- 
ey-ants,   I,    683;   bulk   and   fecun- 
dity,  II,  492.   (See  also  Termites.) 

Aphis:  individuality,  I,  249,  250;  II, 
603;  parthenogenesis,  I,  274-5, 
289;  fertility,  I,  582,  640-1;  II, 
476,  490;  utilized  by  ants,  I,  660- 
1;  II,  403,  405;  over-multiplica- 
tion checked  by  lady-bird,  II,  406. 

Aquatic  animals,  large  size  attained 
by,  I,  156. 

Arachnida:  avoidance  of  danger,  I, 
92;  oviparous  homogenesis,  I,  271; 
segmentation,  I,  469;  II,  113,  314; 
integration  and  homology,  II,  111, 
121;  bilateral  symmetry,  II,  198. 

Arcella:  symmetry,  II,  186;  outer 
tissue  differentiation,  II,  309. 

Archcgoniatece:  morphological  com- 
position, II,  32-5;  growth  and  de- 
velopment, II,  50-6;  tubular 
structure,  II,  58,  62;  alternating 
generation  not  distinctive,  II,  84; 
asymmetry  and  environment,  II, 
140;  integration,  II,  293,  296;  in- 
dividuation  and  genesis,  II,  441, 
451,  463. 

Archenteron:  primitive  externality, 

II,  301;  formation  of  coelom,   II, 
302. 

Archiannelida:  segmentation,  II, 
125. 

Arenicola  marina:  polytrochal  lar- 
vse,  II,  109. 

Arm:  embryogeny  of  human,  I,  169; 
vicarious  use  of,  I,  209. 

Army,  morphological  analogy,  II,  6. 

Arteries  (see  Vascular  System). 

Arthropoda:  uniaxial  development, 
I,  165;  protoplasmic  continuity,  I, 
190,  629;  excursiveness,  I,  481; 
limb  locomotion,  II,  15;  integra- 
tion and  homology,  II,  111-4,  121; 
bilateral  symmetry,  II,  197-200; 
genesis,  II,  445,  453! 


628 


SUBJECT-INDEX. 


Ascidians:  multiaxial  development, 
I,  165,  166;  functional  differentia- 
tion, I,  202;  composite  individual- 
ity of  Doliolum,  I,  247;  self-fer- 
tilization, I,  342;  integration,  II, 
94,  96,  97;  symmetry,  II,  194; 
origin  of  vertebrate  type,  II,  194, 
598,  605. 

Ascomycetes,  reproduction,  II,  450. 

Assimilation:  compared  with  rea- 
soning, I,  81-7;  a  trait  of  vitality, 

I,  577. 

Asteroidea,  radial  symmetry,  II, 
196. 

Astronomy:  growth  of  celestial 
bodies,  I,  135;  Schleideii  on  indi- 
viduality, I,  245;  evolution,  I, 
432,  435;  classification  of  stars,  I, 
444;  rhythm  of,  and  organic 
change,  I,  499-501,  557;  law  of 
equilibration,  I,  519-20;  coopera- 
tion of  structure  and  function, 

II,  3. 

Atavism:  occurrence  of,  I,  305-6, 
314;  digital  variation,  I,  321-3. 

Atoms:  use  of  term,  I,  6,  31;  ethe- 
real undulations  and  oscillations, 

I,  31-5. 

Australia:  settler's  usages,  I,  364; 
ratio  of  jaw  to  skull  in  natives,  I, 
541. 

Axillary  buds,  origin  and  develop- 
ment, II,  65-8. 

Axis:  "  neutral  "  of  mechanics,  II, 
210;  genesis  of  vertebrate,  II, 
212-6,  224-7. 

Bacteria:  fission,  I,  270;  non-nu- 
cleated, II,  20;  rate  of  increase, 

II,  443. 

Baer,  K.  E.  von:  embryological  for- 
mula, I,  171,  172,  451,  453,  461, 
466;  zoological  classification,  I, 
383;  on  animal  transitions,  I, 
480. 

Balanophor&,  inner  tissue,  II,  274. 

Bark:  varied  development,  II,  247- 
9;  physiological  differentiation, 
II,  249-50,  258,  386. 

Basidiomycetes,  reproduction,  II, 
450. 

Bat,  infertility  of,  II,  473. 

Bates,  H.  W.,  protective  mimicry 
of  butterflies,  I,  398. 


Batrachia  (see  Amphibia). 

Bean,  vascular  system,  II,  573, 
591. 

Beaver,  tail  and  co-adapted  struc- 
tures, I,  616. 

Bees  (see  Insects). 

Bigoniacece:  multiplication  I,  224, 
317,  442;  individuality,  I,  251;  de- 
velopment from  scales,  I,  282; 
symmetry,  II,  159,  166;  develop- 
ment, II,  271. 

Berkeley,  M.  J.,  indefiniteness  of 
mosses  and  ferns,  II,  296. 

Bile,  arrest  of  excretion,  I,  209. 

Bilirubin  and  biliverdine,  function 
of,  II,  330,  333. 

Biology:  definition  and  divisions,  I, 
124-5;  organic  structural  pheno- 
mena, I,  125-7;  also  functional,  I, 
127-9;  actions  and  reactions  of 
function  and  structure,  I,  129-30; 
genesis,  I,  130-1;  limited  knowl- 
edge of,  I,  131;  evolution,  I,  432, 
434;  sociological  analogies  (see 
Sociology). 

Biophprs,  Weismann's  germ-plasm 
units  (see  Weismanu). 

Birds:  flesh-eating  and  grain-eat- 
ing contrasted,  I,  68;  growth  and 
expenditure  of  force,  I,  142;  size 
of  egg  and  adult,  I,  144;  limita- 
tions on  flight,  I,  155;  self-mobil- 
ity, I,  175;  temperature,  I,  176; 
functional  and  structural  differ- 
entiation, I,  201;  food  of  starv- 
ing pigeon,  I,  215;  viviparous- 
ness,  I,  271;  heredity  and  pigeon 
breeding,  I,  305;  atavism  In 
pigeon,  I,  314;  osseous  varia- 
tion In  pigeon,  I,  321;  classifica 
tion,  I,  392;  migrations  and 
change  of  habits,  I,  399,  402,  500; 
distribution  in  time,  I,  410;  Dar- 
win on  petrels,  I,  455;  rudi- 
mentary teeth,  I,  457;  vertebrae, 
I,  471;  II,  564;  feather  develop- 
ment, I,  473;  habits  of  water 
ouzel,  I,  485;  egg  shells  and  di- 
rect equilibration,  I,  526;  bones  of 
waders  and  direct  equilibration, 
I,  527;  fertility  and  nervous  de- 
velopment, I,  598;  cellular  con- 
tinuity, I,  629;  adaptation  of 
structure  to  environment,  II,  12; 


- 

- 


SUBJECT-INDEX. 


629 


sexual  selection,  II,  269;  wing 
spurs,  II,  313;  outer  tissue  differ- 
entiation, II,  314-5,  387;  ali- 
mentary canal  development,  II, 
325,  327;  muscular  colour  and 
activity,  II,  365-9;  nutrition,  II, 
433;  cost  of  genesis,  II,  436; 
growth  and  genesis,  II,  454,  458; 
heat  expenditure  and  genesis,  II, 
468-9,  474;  activity  and  genesis, 
II,  470-2,  474;  contrasted  mam- 
malian fertility,  II,  470;  eggs  of 
wild  and  tame,  II,  478;  fertility 
of  blackbird  and  linnet  compared, 
II,  503;  Owen  on  skeleton  of,  II, 
559,  560,  561. 

Bischoff,  embryogeny  of  human 
arm,  I,  169. 

Bison,  modifications  entailed  by  in- 
creased weight  of  head,  I,  512. 

Blackbird,  contrasted  with  linnet 
in  development,  II,  503. 

Blainville,  de,  definition  of  life,  I, 
79,  93. 

Blastosphere,  independence  of  cells 
in  Echinoderm  larvae,  I,  185. 

Blastula,  definition  of  life  and  for- 
mation of,  I,  112. 

Blood:  similarity  of  iron  peroxide, 
I,  17;  metabolic  processes,  I,  69; 
segregation  of  abnormal  constitu- 
ents, I,  180;  protozoon  life  of  cor- 
puscles, I,  186-7;  morbid  changes, 

I,  221,    701;    assimilative    power 
and  organic  repair,  I,  221-2;  res- 
piratory tissue  differentiation,  II, 
310-1;   pressure   in  mammals,    II, 
340.     (See  also  Vascular  System.) 

Blow-fly,     Weismann    on    nutrition 

and  genesis  in,  I,  678-9. 
Boers,    Cape,    habits   and   fertility, 

II,  508. 

Boismont,  A.  B.  de,  on  human  fer- 
tility, II,  511. 

Bone:  growth  and  function,  I,  151; 
adaptability,  I,  230;  II,  217-8; 
function  and  weight,  I,  308,  693; 
mammalian  cervical  vertebrae,  I, 
394;  evolution  and  vertebral  col- 
umn, I,  470-1;  partial  develop- 
ment, I,  473;  size  of  head  as  in- 
fluencing, I,  512,  536-9;  direct 
equilibration  and  strength,  I, 
527;  natural  selection  and  co- 


adaptations,  I,  614-21,  674,  677; 
rudimentary  limbs  of  whale,  I, 
668,  685,  692;  inheritance  of  ac- 
quired modifications  in  Punjabis, 

I,  689;  skull  development,  II,  222; 
theory  of  supernumerary,  II,  223; 
Cope  on  origin  of  vertebrate  osse- 
ous system,  II,  225-7;  differentia- 
tion,  II,   344-56;   false  joints,    II, 
370-2;    Owen's    theory    of    verte- 
brate skeleton,  II,  548-66. 

Book-worm,  food  of,  I,  77. 

Born,  G.,  experiments  on  frog-lar- 
vae, I,  365. 

Botany,  biological  classification,  I, 
124,  125.  (See  Plants.) 

Bothriocephalus,  development,  II, 
490. 

Botryllidce:  development,  I,  166;  in- 
dependence of  components,  I,  247; 
agamogenesis,  I,  641. 

Bower,  Prof.,  on  alternation  of  gen- 
erations, II,  84. 

Brachiopoda,  rude  vascular  system, 

II,  340. 

Bradbury,  J.  B.,  on  vaso-dilators,  I, 
55. 

Brain:  natural  selection  and  mental 
evolution,  I,  553;  analysis  of  sub- 
stance, I,  596;  weight  in  higher 
animals,  I,  598-9;  size  in  civilized 
and  uncivilized,  II,  530. 

Branches   (see  Morphology). 

Branchiae  (see  Respiratory  System). 

Brass,  effect  of  antimony  on,  I,  121. 

Bread,  diamagnetism,  I,  370. 

Breeding:  heredity,  I,  304-5;  in- 
and-in,  I,  344-7,  353;  II,  615; 
pure  and  mixed,  I,  354,  625. 

Bricks,  changed  equilibrium  shown 
by,  I,  38,  42. 

Brodie,  T.  G.,  cell  chemistry,  I, 
260. 

Brownell,  Miss  J.  L.,  on  birth-rate 
in  United  States,  II,  520. 

Brown-S6quard,  on  inherited  epi- 
lepsy, I,  312,  624. 

Bryophyllum,  peculiar  proliferation, 
II,  295. 

Bryophyta,  large  size  attained  by 
some,  I,  138. 

Bryozoa,  gemmation,  I,  588. 

Budding  (see  Gemmation). 

Buds:   development,   I,   167-8;   the- 


630 


SUBJECT-INDEX. 


orles  of  heredity  and  cauline,   I, 
358-9,   360;  axillary,   II,  65-9;  ef- 
fects of  nutrition,  II,  73-4. 
Butterfly:    protective    mimicry,     I, 
398;  Instance  of  tame,  I,  684. 

CABBAGE,  varieties  of,  I,  302. 

Cactacea:  foliar  and  axial  develop- 
ment, II,  47-9;  differentiation  in, 
II,  258,  2 T6,  282;  vascular  system, 
II,  282;  dye  permeability  and  cir- 
culation, II,  571,  572;  wood  for- 
mation, II,  575,  577,  578,  580. 

"  Callus,"  budding  from,  I,  358, 
359. 

Camel,  natural  selection  and  hump 
of,  I,  534. 

Canadians,  French,  fertility  of,  II, 
509. 

Cancer,  the  definition  of  life,  I,  111; 
oesophageal,  II,  324;  and  vascular 
system,  II,  343. 

Caoutchouc,  leaf  structure,  II,  589. 

Capillaries    (see   Vascular    System). 

Capillarity,  and  vegetal  vascular 
system,  II,  279-80,  286,  568,  570, 
585,  587,  592-6. 

Carbohydrates:  instability,  I,  10; 
the  term  "hydro-carbon,"  ib. ; 
molecular  changes  in,  I,  42-3;  or- 
ganic transformation,  I,  43,  48; 
metabolic  processes,  I,  63-77, 
262-3,  II,  362. 

Carbon:  properties,  I,  3-5,  20;  com- 
pounds, I,  6,  7,  9,  10-12,  13,  24-5. 

Carbonic  acid  (carbon  dioxide): 
properties,  I,  6,  7,  9;  in  animal 
and  plant  functions,  I,  62,  214, 
II,  398;  diffusibility,  II,  331. 

Carbonic  oxide,  properties,  I,  6. 

Carnivores:  nitrogenous  food,  I,  47, 
68;  katabolic  process,  I,  71;  re- 
stricted environment,  I,  396; 
their  beneficial  effects  on  animal 
life,  II,  405-6. 

Carpenter,  W.  B. :  on  functional 
specialization,  I,  208;  reproduc- 
tion of  sea- weed,  I,  582;  vegetal 
cell  multiplication,  I,  585;  struc- 
ture and  multiplication  of  com- 
pound organisms,  I,  586-9;  on 
fundamental  traits  of  sex,  I,  595; 
nutritive  system  of  invertebrates, 
I,  595;  Maerocyrtis,  II,  450;  nutri- 


tion and  reproductive  function, 
II,  460. 

Cartilage  (see  Bone). 

Castration,  effect  of,  on  growth,  II, 
459. 

"  Castration  parasitaire,"  Julin  on, 
II,  493-6. 

Catalysis,  and  vital  metamorphosis, 
I,  39,  43. 

Cattell,  McKeen,  on  tactual  percep- 
tiveness,  I,  666. 

Caulerpa,  simulation  of  higher 
plant-forms,  II,  22. 

Cave-animals,  degeneration  of  eyes, 
I,  309,  612-3,  614,  647-9,  693. 

Cell,  the:  incomprehensibility  of 
forces  at  work  in,  I,  118;  proto- 
plasts and  their  traits,  I,  181; 
the  cell-theory,  I,  1S4,  252,  II,  17- 
21,  85;  differentiation,  I,  188-9, 
194;  the  continuity  of  protoplasm, 

I,  190-2,    194,   628-30,    II,    21;   its 
structure,    I,    253-5;    function    of 
centrosome,    I,   254-5,   257;   struc- 
ture and  function  of  nucleus,   I, 
255-6,  258-9;  karyokinesis,  I,  257- 
8;  function  of  chromatin,  I,  259- 
65;   fertilization   and   function  of 
polar  bodies,  I,  266-8;  theories  of 
heredity  based  on  theory,  I,  356; 
Weismann's    differentiation    into 
reproductive  and   somatic,   I,  622, 
628-30,  633-44;  nucleus  absent  or 
dispersed,  II,  20,  85;  morphologi- 
cal differentiation,  II,  175-7;  ani- 
mal morphology,  II,  228-30;  mor- 
phological     summary,      II,      233; 
vegetal  tissue  differentiation,   II, 
249-50,     386;     vascular     develop- 
ment, II,  279-84,  389. 

Centipede,   bilateral  symmetry,   II, 

198-200. 
Cephalopoda:    bilateral     symmetry, 

II,  203;  vascular  system,  II,  341. 
Cercarite  (see  Distoma). 

Cereus,    tissue    differentiation,    II, 

276,  283. 

Cesalpino,  I,  377. 
Cestoda  (see  Eniozoa). 
Chcetopoda,    segmentation,     II,    98, 

103,  605. 

Chaja,  wing  spurs,  II,  313. 
Change,   and   definition   of  life,   I, 

81-90,  113. 


SUBJECT-INDEX. 


631 


Charles,  R.  H.,  on  inheritance  of 
acquired  modifications  in  leg- 
bones  of  Punjabis,  I,  689. 

Chatie,  on  single  and  double  stocks, 
II,  622. 

Chemistry:  properties  of  organic 
elements,  I,  3-5,  20,  22;  of  dia- 
tomic compounds,  I,  7-10;  tria- 
tomic,  I,  10-12;  polyatomic,  I, 
12-13,  25;  traits  of  evolution,  I, 
23—4;  ethereal  undulations  and 
atomic  oscillation,  I,  31-6;  chemi- 
cal affinity  and  organic  change,  I, 
36-7,  38-43;  oxidation  and  genera- 
tion of  heat,  I,  46-9,  60;  genera- 
tion of  nerve  force,  I,  52,  60; 
metabolism,  I,  62-77;  physiology 
and  organic,  I,  127;  flesh  constitu- 
ents, I,  154;  composition  of  or- 
ganisms and  environment,  I,  173; 
organic  development  and  differen- 
tial assimilation,  I,  179-80;  chemi- 
cal units,  I,  225,  II,  612;  primi- 
tive ideas  of  elements,  I,  41 1; 
evolution  of  organic  compounds, 

I,  696-701,  703. 

Chestnut,    leaf   symmetry,    II,    149, 

153. 
Chiton:  simulation  of  segmentation, 

II,  116,  118;  symmetry,  II,  202. 
Chlorophyll:     function,     I,     65,     II, 

263;  nutrition  and  absence  of,  II, 
74;  constitution,  II,  262;  symbi- 
otic presence  in  animals,  II,  400. 

Chandrae anthus  gibbosus,  enormous 
development  of  reproductive  sys- 
tem, II,  487. 

Chordata,  affinities,  I,  466. 

Chromatin  (see  Cell). 

Circle,  the,  and  evolution  hypoth- 
esis, I,  433. 

Circulation  (see  Vascular  System). 

Cirripedia:  Darwin  on  retrograde 
development,  I,  458;  remarkable 
transformation  in  Sacculina,  II, 
494-5. 

Civilization,  human  evolution  and 
genesis,  II,  529-31. 

Cladophora:  integration,  II,  25; 
axial  development,  II,  28. 

Classification:  subjective  concep- 
tion, I,  78;  two  purposes  of,  I, 
374;  a  gradual  process,  I,  375; 
botanical,  I,  377-80,  389-90;  zo- 


ological, I,  380-9;  incomplete 
equivalence  of  groups,  I,  389, 
445-6,  448,  555,  572;  group  at- 
tributes, I,  390-3;  the  truths  in- 
terpreted, I,  393-4;  ethnologic 
and  linguistic  evolution,  I,  441-6; 
organic  evolution,  I,  443,  447, 
555;  differences  in  kind  and  de- 
gree, I,  444-6;  antecedent  struc- 
tural similarity,  I,  447,  448-9; 
Von  Baer's  formula,  I,  451-4, 
555;  organic,  not  uniserial,  II, 
115. 

Classification  of  the  Sciences,  The, 
and  evolution  and  dissolution,  II, 
5. 

Claus,  C.,  on  segmentation  in  An- 
nelids and  Chaetopods,  II,  605. 

Clover:  flower  and  axial  develop- 
ment, II,  45;  symmetry,  II,  152. 

Co-adaptation  of  cooperative  parts: 
principles  underlying,  I,  234-5, 
511-3,  514-5;  slow  operation  of 
the  process,  I,  236;  sociological 
analogy,  I,  237-40;  reversion  un- 
der original  conditions,  I,  240;  the 
analogy  continued,  ib. ;  the  case 
of  bison's  head,  I,  512;  natural  se- 
lection an  inadequate  explana- 
tion, I,  535,  614-21,  692;  Romanes 
on  "  cessation  of  selection  "  as 
effecting,  I,  560,  561-2;  Weis- 
mann's  theories,  I,  560-3,  663-5, 
670,  674-5;  natural  selection  and 
economy  of  growth,  I,  562;  phy- 
siological processes  involved,  I, 
566-7;  Wallace's  argument  from 
artificial  selection,  I,  615;  what 
are  cooperative  parts?  I,  616-7; 
"  intra-selection  "  examined,  I, 
676-8. 

Coal,  social  effects  of  supply,  I, 
238-9,  241. 

Cocoa-nut,  growth  and  fertility,  II, 
457. 

Coccospheres:  vital  problem  pre- 
sented by  protective  structures,  I, 
119;  imbricated  plates,  I,  182. 

Cockroach,  ousting  of  European 
species,  I,  399. 

Cod:  ova  of,  II,  435;  growth  and 
fertility,  II,  454. 

Codium:  symmetry,  II,  136;  tissue 
differentiation,  II,  246. 


632 


SUBJECT-INDEX. 


Ccelenterata:  rudimentary  contrac- 
tile organs,  I,  58;  vital  changes 
In  polyp,  I,  95;  axial  development, 
I,  165,  166;  environment  and 
structure,  I,  173;  self-mobility,  I, 
175;  H,  14,  15;  functional  differ- 
entiation, I,  201,  391;  inactivity 
and  waste,  I,  213;  reparative 
power,  I,  219,  224;  individuality, 

I,  246,  247,  250;  heterogenesis,  I, 
273,  277,  296;  negative  disintegra- 
tion in  Hydrozoa,  I,  276,  587;   re- 
productive tissue,  I,  281;  differen- 
tiation in  Hydrozoa,  I,  391;  classi- 
flcatory    value,    I,    446;    regenera- 
tion of  fragments,  II,  90;  integra- 
tion, II,  90,  102,  105,  124;  gemma- 
tion, II,  91;  tertiary  aggregation, 

II,  92,    95,    124;    uiolluscan    affin- 
ities, II,  115;  radial  symmetry,  II, 
188;   symmetry  of  compound,   II, 
192-3;    segmental    differentiation, 
II,  207;   physiological  differentia- 
tion   in    Hydra   and    analogy,    II, 
300;  ciliation  of  blastula,  II,  301; 
tissue     reduplication,     II,     301-2, 
389;   outer   tissue   differentiation, 
II,  309;  osmosis  in  Hydra,  II,  339; 
vascular    system    in    Hydra,    II, 
340,  376;  functional  co-ordination, 
II,  376;  symbiosis,  II,  400;   asex- 
ual genesis,  II,  443-4;  growth  and 
sexual   genesis,    II,    452;    develop- 
ment and  genesis,  II,  462;  nutri- 
tion and  genesis,  II,  476. 

Coelom,  origin  and  function,  II, 
302-3. 

Collins,  F.  Howard,  jaws  and  teeth 
of  savages  and  civilized,  I,  541. 

Colloids:  T.  Graham  on,  I,  15-8; 
diffusibility,  I,  18-21;  organic,  I, 
21,  25,  26;  pliability  and  elas- 
ticity, I,  27;  capillary  affinity,  I, 
28;  isomerism,  I,  59;  instability, 
I,  350;  molecular  mobility  and  dif- 
fusibility, II,  331;  instability  of, 
and  nerve  differentiation,  II,  356- 
61;  and  muscular  tissue,  II,  361-4. 

Colonies,  autogenous  development 
and  parallel  in  heredity,  I,  366- 
8;  II,  620. 

Colour:  sensation  of,  I,  54;  phoano- 
gamic,  II,  75,  265-6;  light  and 
Vegetal,  II,  261-2;  floral  fertiliza- 


tion, II,  267-9;  sexual  selection, 
II,  269;  activity  and  muscular,  II, 
365-9;  physio'logical  units  and 
mixture  of,  in  offspring,  II,  616, 
617. 

Conimensalism,  organic  integration 
as  displayed  in,  II,  402-4. 

Composite:  floral  symmetry,  II,  173. 

Condor,  weight  of,  I,  155. 

ConfcrvoidccB,  1,  279,  280;  II,  25,  28, 
449.  (See  Alga.) 

Conjugated;,  II,  449.     (See  Alga.) 

Conjugation,  in  Alga,  I,  279;  in  Pro- 
tozoa, I,  280;  II,  452;  can  fission 
persist  without?  I,  637;  relation 
to  growth,  II,  449. 

Connective  tissue,  Hertwig's  classi- 
fication, I,  189. 

Constitutional  units,  I,  369.  (See 
Physiological  Units.) 

Consumption,  hereditary  transmis- 
sion, I,  307. 

Co-ordination  of  actions  (see  Life). 

Cope,  E.  D.,  on  origin  of  vertebrate 
structure,  II,  225-7. 

Cormophyta:  slight  internal  differ- 
entiation, H,  273;  vascular  sys- 
tem, II,  280. 

Corpuscula  tactus,  their  function,  I, 
75. 

Correspondence,  use  of  word,  I,  97. 
(See  Life.) 

Cousin-marriages,   I,   345,  II,  615. 

Cow:  what  prompts  her  to  mumble 
a  bone?  I,  120. 

Cow-parsnip  (see  Heracleum). 

Crab  (see  Crustacea). 

Creation  (sec  Special  creation). 

Crinoidea,  symmetry,  II,  195-6. 

Crocodile,  cqntinuous  growth,  I, 
154,  292. 

Crookes,  Sir  W.,  hypothetical 
chemical  unit  "  protyle,"  I,  22,  23. 

Cruciferce,  floral  symmetry,  II,  164, 
171. 

Crustacea:  locomotion  of  lobster,  I, 
175;  regeneration  of  limbs,  I,  224, 
300,  589,  II,  76;  homogenesis,  I, 
271;  genesis  and  nutrition  in 
Daphnida,  I,  290-1;  growth  and 
genesis,  I,  292;  degeneration  of 
eye  in  cave-inhabiting,  I,  309,  614, 
648;  hermit  crab  parasite,  I,  397; 
changes  of  media,  I,  401,  481-2; 


SUBJECT-INDEX. 


633 


retrograde  development  in  cirri- 
pedes,  I,  458;  segmentation,  I, 
468-9,  II,  114;  Darwin  on  jaws 
and  legs,  I,  471;  survival  of  cirri- 
pedes,  I,  517;  integration  and 
homology,  II,  111-4,  121,  603;  bi- 
lateral symmetry,  II,  198-201; 
eyes,  II,  318;  dermal  structure  of 
hermit  crab,  II,  322,  387;  fer- 
tility, II,  453;  nutrition  and  gene- 
sis in  parasitic  species,  II,  487; 
"  castration  parasitaire,"  II, 
493-6. 

Crystalloids:  Prof.  Graham  on,  I, 
15-8;  diffusibility,  I,  18-21;  or- 
ganic, I,  21-2,  26. 

Crystals:  simulation  of  life  in 
"  storm  glass,"  I,  96;  growth,  I, 
135-7,  577;  segregation,  I,  179, 
221,  223;  equilibration,  I,  337; 
physiological  units  and  polarity,  I, 
701-6;  time  and  formation,  II,  77. 

Ctcnodrilus,  segmental  individual- 
ity, II,  103,  603,  604. 

Cube;  bilateral  symmetry,  II,  132. 

Cunningham,  J.  T.,  I,  vi;  II,  vi; 
on  non-adaptive  specific  charac- 
ters, I,  565;  food  of  blow-fly  lar- 
vae, I,  678;  arthropod  segmenta- 
tion, II,  114;  egg-production  of 
Conger,  II,  425. 

Cuttle-fish,  individuality  of  Hecto- 
cotylus,  I,  250. 

Cuvier,  zoological  classification,  I, 
381. 

Cyanogen,  properties,  I,  7,  9. 

Cyclichthys,  dermal  structure,  II, 
306. 

DALTELL,  Sir  J.,  regeneration  in 
Dasychone,  I,  361;  propagation  of 
Hydra,  II,  476. 

Daphnida,  heterogenesis  and  nutri- 
tion, I,  290-1. 

Darwin,  C.:  Origin  of  Species,  I, 
129,  II,  528;  natural  selection  and 
function,  I,  308-9,  693:  atavism, 
I,  314;  osseous  variations  in  pig- 
eons, I,  321;  plant  variation  and 
domestication,  I,  325;  "  spontane- 
ous variation,"  I,  328,  697;  floral 
fertilization,  I,  340,  II,  168,  267, 
407,  608;  Intercrossing  and  self- 
fertilization,  I,  344,  345;  inter- 


crossing I,  347,  611,  669;  his  the- 
ory of  pangenesis  examined,  I, 
356-62,  370,  372;  plant  fertiliza- 
tion and  distribution,  I,  397; 
habits  of  birds,  I,  400;  distribu- 
tion and  natural  barriers,  I,  402, 
476;  disappearance  and  non-reap- 
pearance of  species,  I,  406;  dis- 
tribution in  time  and  space,  I, 
410;  linguistic  classification,  I, 
442;  classification  of  organisms,  I, 
443;  classification  and  descent,  I, 
448;  on  petrels,  I,  455;  suppres- 
sion of  organs,  I,  457;  develop- 
ment of  Cirrhipedia,  I,  458;  jaws 
and  legs  of  Crustacea,  I,  471; 
aborted  organs,  I,  474,  563;  rela- 
tions of  species  in  Galapagos 
archipelago,  I,  478;  opinions  of  E. 
Darwin  and  Lamarck,  I,  491;  the 
term  "  survival  of  the  fittest,"  I, 
530;  indirect  equilibration  by 
natural  selection,  I,  530-5;  in- 
heritance of  acquired  characters, 
I,  535-42,  560,  630,  685,  690;  Wal- 
lace on  natural  selection  in  man, 

I,  553;  misleading  connotations  of 
term  "  natural  selection,"  I,  609, 
695;  caste  gradations  and  jaws  of 
driver  ants,  I,  658;  attachment  of 
climbing  plants,  II,  276-7;  vegetal 
fructification,      II,      294;      earth- 
worm,   II,    402;    animal    sterility 
and    domestication,    II,    480,    488; 
variation  in  hyacinth  and  camel- 
lia, II,  621. 

Darwin,  Dr.  E.,  modifiability  of  or- 
ganisms, I,  490,  492-7. 

Death:  an  arrest  of  vital  corre- 
spondence, I,  102;  only  limit  to 
vegetal  growth,  I,  153;  cessation 
of  coordination  of  actions,  I,  578, 
579;  Weismann's  hypothesis,  I, 
636-8;  physiological  integration, 

II,  374,  392;  cause  of  natural,  II, 
413;  relation  to  births,  II,  417. 

Definiteness:  of  vital  change,  I,  87- 
90,  106,  109;  developmental,  I, 
178;  functional,  I,  212;  segrega- 
tion of  evolution,  I,  514-6. 

Definition,  difliculties  of,  I,  78;  II, 
17. 

Degeneracy,  morphological  obscura- 
tions due  to,  II,  12,  53. 


634 


SUBJECT-INDEX. 


DcndroUum  (see  Orchids). 

Dcsmidiacece:  unicellular,  II,  21; 
linear  and  central  aggregation,  II, 
23;  natural  selection  and  sym- 
metry, II,  134,  133;  morphological 
differentiation,  II,  177;  tissue,  II, 
2-14;  genesis,  II,  440,  449. 

Determinants,  Weismann's  germ- 
plasm  units  (see  Germ-plasm). 

Development:  an  increase  of  struc- 
ture, I,  162,  II,  461;  primarily 
central,  I,  162,  166;  uni-  and  mul- 
ti-central, I,  163-4,  166-7;  axial, 
I,  164,  167;  uni-  and  multi-axial, 

I,  165-6;    a    change    to    coherent 
definite   heterogeneity,    I,    167-70. 
179;  Von  Baer's  formula,  I,  171-2; 
individual      differentiation      from 
environment,  I,  172-8;  cell  forma- 
tion,   I,    225;    discontinuous,    and 
agamogenesis,  I,  275;  Prof.   Hux- 
ley's  classification,   I,   276;   socio- 
logical parallel  to  autogenous,  I, 
364-8,  II,  620;  retrograde,  I,  457- 
8:  inequalities  among  co-operalive 
parts,  I,  617;   "  heterochrouy,"  I, 
655;  continuous  and  discontinuous 
vegetal,      II,     52;     summary     of 
physiological,    II,    384-00;    nutri- 
tion and  genesis,  resume,  II,  407- 
9;  evolution,  II,  501-5;  commence- 
ment of  genesis,   II,  506;  of  ver- 
tebrate limbs,   II,   553.     (See  also 
Multiplication.) 

Development  Hypothesis,  The,  I,  417. 
Dialects  (see  Language). 
Dialysis,  and  diffusibility,  I,  19,  20. 
Diastase,    decomposition    of,    I,    38, 

40. 
Diatomacece:  tissue,  II,  244;  genesis, 

II,  440,  448. 

Diatomic  compounds  (see  Chem- 
istry). 

Dicotyledons:  growth,  I,  139,  143, 
II,  63-4,  69-72,  78,  82-3;  uniaxial 
development,  I,  165;  stem  and 
leaf  functions,  II,  257;  mechani- 
cal stress  and  wood  formation,  II, 
277;  growth  and  genesis,  II,  451. 

Differentiation  (see  Morphology  and 
Physiology). 

Difflugia:  primary  aggregate,  II,  86- 
7;  symmetry,  II,  186;  outer  tissue 
diffrronibilion,  II,  SCO. 


Diffusion,  of  colloids  and  crystal- 
loids, I,  18-20;  II,  331. 

Digestion:  action  of  nitrogenous 
compounds,  I,  69;  obesity,  IX, 
480-4;  fertility,  II,  514. 

Dimorphism:  floral,  I,  534;  sexual, 
in  parasites,  I,  315;  social  insects 
(see  Insects). 

Dinosaurs,  size  of,  I,  139. 

Diphycs:  individuality,  I,  246;  sym- 
metry, II,  192. 

Disease:  segregation  of  blood  con- 
stituents, I,  179;  changes  in 
blood  from,  I,  221,  701;  heredity, 

I,  306-7,    312-3,    622-3;    belief    in 
supernatural  origin,  I,  419;  para- 
sitism   and    special    creation,    I, 
427;   morbid   products  as   specific 
characters,    I,    567;    telegony,    I, 
646;  dermal  structure,  II,  oOU;  in- 
testinal muscular  hypertrophy.  II, 
325;    indigestion    and    alimentary 
canal  development,  II,  328;  jaun- 
dice and  bilirubin,  II,  330;  locali- 
zation of  excretion,  II,  331;  mem- 
branes  in   inflammatory,    II,   343; 
osseous  differentiation  in  rickets, 

II,  352;    fatty    degeneration,    II, 
482. 

Disintegration,  physiological  (see 
Physiology). 

Distoma:  metagenesis,  I,  273-4;  dis- 
integration of  genesis,  I,  276; 
cycle  of  generations,  II,  489. 

Distribution:  physical  limits,  I,  396; 
organic  environment,  I,  396-8; 
parasitic  conditions,  I,  397-8;  sim- 
ultaneity of  agencies  affecting, 
I,  398;  mutual  encroachments  of 
species,  I,  398-401,  477,  489;  facts 
disproving  pre-adaptation  to  habi- 
tats, I,  401-3,  411-2;  of  animals 
and  plants  in  time,  I,  404-11,  412; 
ousting  of  native  species  In  New 
Zealand,  I,  477;  local  influences, 
I,  477-9,  489;  through  varied 
media,  I,  479-85,  489,  556;  past 
and  present  organic  forms,  I, 
485-9,  556;  complex  organization 
and,  II,  296-7. 

Division  of  labour,  physiological 
(see  Labour). 

Dog:  contrasted  lives  of  tortoise 
and,  I,  103,  104;  inherited  habits, 


SUBJECT-INDEX. 


635 


I,  309,  573;  abnormal  digits,  I, 
324;  Interbreeding  of  divergent 
varieties,  I,  565;  decrease  of  jaw, 

I,  615,  693;  telegony,  I,  645;  con- 
ditions affecting  fertility,  II,  474, 
479. 

Dohrn,  theory  of  vertebrate  struc- 
ture, II,  606. 

Doliolum,  combination  of  individu- 
alities, I,  247. 

Domestication  (see  Animals). 

Doubleday,  E.,  on  nutrition  of  gene- 
sis, II,  510-2. 

Driesch,  separation  of  segmentation 
spheres  of  Echinus  ovum,  I,  691; 

II,  618. 

Dropsy  (see  Disease). 

Drosera:  individuality,  I,  251;  pro- 
liferous growth,  II,  75. 

Du  Bois-Ileymond,  E.  H.,  elec- 
tricity from  muscles  and  nerves, 

I,  50. 

Dumas,   antithesis  of  animals   and 

plants,  I,  62. 
Dwarfs,  Hindu  family  of,  I,  316. 

EAR,  development  of  vertebrate,  II, 

318,  320. 
Earth,  climatic  rhythm  and  organic 

change,  I,  499-501,  557. 
Earth-worm:     bilateral     symmetry, 

II,  199,  200;  mould  production,  il, 
402. 

Echinococcus  (see  Entozoa). 

Echinodcrmnta:  independence  of 
blastosphere  cells,  I,  185;  proto- 
plasmic continuity  in  embryos,  I, 
190;  separation  of  segmentation 
spheres  of  ovum,  I,  691;  II,  618; 
symmetry,  II,  191,  195-6. 

Economy:  of  growth  in  natural  se- 
lection, I,  536,  562;  a  trait  of  or- 
ganic evolution,  II,  501,  504. 

Ectoderm:  functional  differentia- 
tion, I,  202,  203;  functional  vicarl- 
ousness,  I,  209;  reproductive  func- 
tion, I,  281. 

Effects,  Multiplication  of:  varia- 
tion, I,  329-30,  333;  organic  evo- 
lution, I,  511-4,  515,  517,  549, 
557,  II,  405-6;  morphological  de- 
velopment, II,  7-9,  234;  physio- 
logical differentiation,  II,  390-1, 
392. 


Eggs  (see  Embryology). 

Eimer,  T.,  theory  of  orthogenesis, 
I,  563-4. 

ElasmobranchU:  protoplasmic  con- 
tinuity, I,  C29;  segmentation,  II, 
126. 

Electricity;  genesis  in  organic  mat- 
ter, I,  50-2,  60;  muscular  ac- 
tion, I,  59;  incomprehensibility, 
I,  121. 

Elephant:  fertility,  I,  583,  599,  II, 
459,  506;  cerebro-spinal  system,  I, 
508,  599. 

Elk,  Irish,  horns  and  correlated 
parts,  I,  537,  674. 

Eloidca  canadensis:  individuality,  I, 
248;  enormous  agamic  multiplica- 
tion, I,  G42. 

Elongation,  and  locomotion  in  ani- 
mals, II,  15. 

Embryology:  as  aiding  biology,  I, 
125-6;  simulated  growth,  I,  136; 
Initial  and  final  organic  bulks,  I, 
143,  158,  161;  foetal  flesh  constitu- 
ents, I,  154;  human  arm  develop- 
ment, I,  169;  Von  Baer's  for- 
mula, I,  170-2,  451-4.  466:  em- 
bryonic heat,  I,  177;  spherical  or- 
ganic form,  I,  177;  unit-life  In 
multlcellular  organisms,  I,  185-6; 
functional  differentiation,  I,  203; 
individuality,  I,  24(5-7:  unspecial- 
ized  reproductive  tissue,  I,  279- 
83,  317;  changes  following  im- 
pregnation, I,  283-4:  nutrition  and 
vegetal  growth,  I,  285-8,  295-7; 
and  animal  growth,  I,  289-94, 
295-7;  physiological  units  and 
heredity,  I,  317-9;  variation  and 
parental  functional  condition,  I, 
324;  uterine  environment,  I,  327-8; 
physiological  units  and  variation, 
I,  330-4,  458;  fertilized  and  un- 
fertilized ova,  I,  340-1;  hermaph- 
rodism,  I,  341-2,  344;  sociologi- 
cal parallel,  I,  366-8:  evolution 
hypothesis,  I,  434,  436,  453,  454, 
555;  petrel  development,  I,  455; 
substitution  and  suppression  of 
organs,  I,  456-8,  466,  472-3;  struc- 
tural proclivities  of  physiological 
units,  I,  458;  abridgment  of 
stages,  I,  458-9.  464:  disappear- 
ance of  intermediate  forms,  I, 


636 


SUBJECT- INDEX. 


459-60,  463;  pre-adaptation,  I, 
461-3;  discrimination  of  species 
in  early  stages,  I,  461;  anomalous 
persistence  of  ancestral  traits,  I, 
4G3-5;  phylogeny,  I,  466;  egg-shell 
function,  I,  527;  genesis  of  grades 
in  social  insects,  I,  654-6,  658-9, 
679-80;  determination  of  sex,  I, 
657;  order  of  development  quali- 
fied by  needs,  I,  679;  osteology  of 
Punjabis,  I,  689;  direct  transfor- 
mations and  physiological  units, 

I,  706:    transformation    of    blas- 
tema, II,  20;  arrest  of  growth  and 
innutrition,  II,  73:  development  of 
segmented  animals,  II,  100-2,  602; 
adaptive  vertebrate  segmentation, 

II,  118-23,  124,  223-4,  605-<3;  ani- 
mal cell  morphology,  II,  228;  pri- 
mary differentiations  of  germinal 
layers,    II,    300-2;  lung    develop- 
ment,  II,   333-4;   mammalian  ova 
maturation,  II,  342-3:  movements 
of  ova,  II,  356,  363;  modifications 
In  mole,  II,  391;  genesis  and  nu- 
trition, II,  424,  425;  fish  ova,   II, 
435,  454;  cost  of  genesis,  II,  435- 
6;  number  of  birds'  eggs,  II,  454- 
6,  478;  heat  and  genesis,  II,  468, 
474:  activity  and  genesis  in  birds, 
II,    470-2,    474;    vertebrate    limb 
development,   II,  553;  ossification 
In    vertebrates,    II,    556;    Owen's 
vertebrate  theory,  II,  563;  devel- 
opment of  vertebras,  II,  564.     (See 
also  Multiplication.) 

Embryology  of  conceptions,  I,  451. 

Emigrants,  type  of  organization 
among,  I,  364,  II,  620. 

Endoderm:  functional  differentia- 
tion, I,  202,  203;  functional  vicari- 
ousness,  I,  209. 

Endogen,  application  of  term,  II, 
R2,  78,  82.  (See  Monocotyledons.) 

Energy:  evolution  of,  in  animals,  I, 
71-7;  organic  growth  and  expen- 
diture, I,  141;  functional  transfer, 
I,  201-6;  chromatin  as  the  source 
of,  in  fcaryoklnesls,  II,  261-5. 
(See  also  Force.) 

Entozoa:  metagenesis,  I,  273,  641; 
self-fertilization,  I,  342;  distribu- 
tion, I,  398;  and  special  creation, 
I,  428;  fission  in  simple  types,  I, 


584;  nutrition  and  genesis,  I,  641; 
II,  488;  direct  transformation,  I, 
706;  integration,  II,  102;  seg- 
mentation, II,  107,  108;  interde- 
pendence and  organic  integration, 
II,  404. 

Environment:  degree  of  life  and 
complexity  of,  I,  104-7;  relation 
to  organic  structure  and  func- 
tion, I,  172-8;  II,  12-5;  adaptation 
to  varied  media  an  evidence  of 
evolution,  I,  479-81,  556;  influence 
of  solar  system,  I,  500,  556;  in- 
herited adaptation  to,  II,  623. 

Holis,  branchiae,  II,  118. 

Epidermis  (see  Skin). 

Epilepsy:  definition  of  life  and 
movements  in,  I,  112;  heredity,  I, 
312. 

Epithelium:  ciliated,  I,  187;  Hert- 
wig's  classification,  I,  189;  repro- 
ductive function,  I,  280;  "  pave- 
ment "  and  "  cylinder,"  II,  229. 

Epizoa:  distribution,  I,  398;  special 
creation  and  effects  of,  I,  428:  In- 
terdependence and  organic  inte- 
gration, II,  404;  nutrition  and 
genesis,  II,  487. 

Equilibration:  variation  and  law  of, 
I,  326,  334;  molecular  arrange- 
ment, I,  337^5;  of  organic 
change,  I,  348,  547,  557;  direct 
and  indirect,  I,  519-22,  573:  adap- 
tation by  direct,  I,  522-3,  551, 
557;  nutrition,  defence,  and  fer- 
tilization of  plants,  I,  523-5;  di- 
rect of  animals,  I,  525-8,  551, 
557;  natural  selection  and  indi- 
rect, I,  530-4,  552,  557;  of  natural 
selection,  I,  543-7,  552-3,  557; 
Increasing  importance  of  direct,  I, 
553;  of  forces  acting  on  species, 
I,  571-2,  II,  417-20;  phenomena 
not  accounted  for  by,  I,  573;  tis- 
sue differentiation,  II,  245;  gene- 
sis of  nervous  system,  II,  307-8; 
functional,  II,  391-4;  laws  of  mul- 
tiplication, II,  411-6;  in  human 
and  social  evolution,  II,  537.  (See 
also  Acquired  characters  and 
Natural  selection.) 

Errera,  L.,  on  inherited  adaptation 
to  environment  In  Aspergillus,  II, 
CIS. 


SUBJECT-INDEX. 


63T 


Ethnology:  heredity,  I,  303-4,  310; 
plasticity  of  mixed  races,  I,  354; 
primitive  ideas,  I,  417;  evolution 
and  classification,  I,  441-3,  446; 
natural  selection,  I,  553. 

Euphorbiacew:  foliar  and  axial  de- 
velopment, II,  47-8;  physiological 
differentiation,  II,  258;  dye  per- 
meability and  circulation,  II,  571; 
wood  formation,  II,  575,  577, 
578;  foliar  vascular  system,  II, 
589-92,  596. 

Evaporation:  organic  change,  I,  28; 
vegetal  circulation,  II,  587. 

Evolution:  chemical  elements  and 
compounds,  I,  22-4,  67;  primordial 
form  of  living  matter,  I,  63-4, 
181;  II,  21-2;  definitions  of  life,  I, 
107-10;  growth  the  primary  trait 
of,  I,  135;  comprehends  growth 
and  development,  I,  162;  illustra- 
tions in  development,  I,  167-70, 
178-9;  progressive  structural  dif- 
ferentiation, I,  181-4,  192-6,  211- 
2;  life  before  organization,  I,  210; 
heterogeneity  of  function,  I,  211; 
stability  of  species,  I,  242,  515, 
518;  individuality,  I,  247;  cell-or- 
ganization, I,  262;  genesis,  hered- 
ity, and  variation  resulting  from, 
I,  354-5;  period  required  for  or- 
ganic, I,  407,  565-6;  contrasted 
with  special-creation  hypothesis, 
I,  415,  431-40;  derivation  of  hy- 
pothesis, I,  431,  439,  554;  increas- 
ing belief  in,  I,  431-3,  439;  ex- 
periences supporting  conceivabil- 
ity,  I,  433-5,  439;  direct  evidence, 
I,  435-7,  439;  malevolence  not  im- 
plied by,  I,  437-9;  evidence  from 
classification,  I,  443,  444,  449, 
466,  555;  embryology,  I,  451-3, 
466;  substitution  and  suppression 
of  organs,  I,  456-8,  466,  472-3;  in- 
sect segmentation,  I,  468-9;  ver- 
tebral column  development,  I, 
470-2;  rudimentary  organs,  I, 
472-5;  adaptation  to  varied  media, 
I,  479-85,  556;  growth  of  the  the- 
ory of  organic,  I,  490-8;  instabil- 
ity of  the  homogeneous,  a  cause, 
I,  509-11,  516,  550;  multiplication 
of  effects,  I,  511-14,  517-8,  550,  II, 
405;  segregation,  and  heteroge- 


neity and  definiteness  of,  I,  514-8, 
550;  natural  selection  and  general 
doctrine  of,  I,  543-8,  557;  factors 
tabulated,  I,  551;  inductive  evi- 
dences summarized,  I,  555-6;  sur- 
viving disbelief  in  France,  I,  559; 
current  theories  of  organic,  I, 
559-74;  Elmer's  theory  of  ortho- 
genesis, I,  563-^;  Gulick  on 
mouotypic  and  polytypic,  I,  569; 
phenomena  unexplained  by  the- 
ories, I,  573-4;  inorganic  and  the 
System  of  Philosophy,  I,  693; 
"  spontaneous  generation,"  I,  C9J- 
701,  702;  dissolution  and  problems 
of  morphology,  II,  4-6;  morphol- 
ogy and  formula,  II,  7-9,  231-5; 
difficulties  of  definition,  II,  17; 
cell-doctrine,  II,  17-21,  85;  unicel- 
lular origin  of  plants,  II,  21-2; 
resume  of  plant  morphology,  II, 
78-80;  origin  and  differentiation 
of  phaenogamic  type,  II,  83; 
physiological  problems,  II,  239-43; 
tissue  differentiation,  II,  244-6, 
385;  integration  of  organic  world, 
II,  396,  406;  race  and  individual 
multiplication,  II,  428-30;  declin- 
ing fertility  and  human,  II,  431, 
529-30;  individuation,  genesis, 
and,  II,  501-5;  human  life,  pro- 
spective, II,  522-5;  forces  influ- 
encing human,  II,  525-8;  future 
of  population,  II,  532-7;  self-suf- 
ficingness  of,  II,  537;  vertebral, 
II,  563-6. 

Excretion:  genesis  of  organs  of,  II, 
303;  localization  of,  II,  331-3. 

Exogen,  application  of  term,  II,  82. 
(See  Dicotyledons.) 

Expenditure  (see  Multiplication). 

Eye,  the:  molecular  transforma- 
tions in  visual  process,  I,  75-6; 
progressive  development,  I,  195, 
II,  317-9;  waste  and  repair,  I, 
218;  transmitted  defects,  I,  306, 
311,  694;  degeneration  in  cave- 
animals,  I,  309,  612-3,  614,  647-9, 
693;  late  development  in  insects, 
I,  658;  migration  in  flat  fishes,  II, 
205. 

FABRE,  J.  H.,  nutrition  and  sex  in 
Osmia  tricornis,  I,  657. 


638 


SUBJECT-INDEX. 


False  joints,  I,  230;  theories  of 
heredity  and,  I,  3(52,  364,  II,  371-2. 

Fats,  the:  physical  and  chemical 
properties,  I,  10-12;  non-nitrogen- 
ous, I,  41;  action  of  bile,  II,  330. 

Fatty  degeneration,  and  failing  vi- 
tality, I,  41. 

Feathers,  development,  I,  474,  II, 
314-6. 

Feet,  heredity  and  size,  I,  311. 

Ferments,  changes  and  nitrogen- 
ous character  of,  I,  38. 

Ferns:  foliar  development  and  nu- 
trition, II,  76;  inner  tissue  differ- 
entiation, II,  273;  indeflniteness, 
II,  296;  genesis,  II,  441,  463. 

Fertility,  the  General  Law  of  Ani- 
mal, I,  577-601.  (See  Multiplica- 
tion.) 

Fertilization:  unit-life  of  generative 
elements,  I,  185-6;  the  function  of 
chromatin,  I,  260,  263-5;  extru- 
sion of  polar  bodies,  I,  266-8;  na- 
ture and  functions  of  generative 
elements,  I,  279-83,  317,  334,  342, 
593-7;  differentiation  and  varia- 
tion effected  by,  I,  330-2;  the  es- 
sential object  of,  I,  340-1,  II,  614- 
6;  hermaphrodlsm  and  self-,  I, 
341-2;  crossing  and  its  effects,  I, 
343-7;  isolation  of  species  in  re- 
spect of,  I,  570;  floral  (see  Flow- 
ers). 

Ficus,  foliar  structure,  II,  589,  596. 

Fingers:  embryogeny  of  human,  I, 
169;  heredity  and  abnormal,  I, 
305,  314,  321-3;  autogenous  de- 
velopment of  supernumerary,  I, 
363;  rudimentary,  I,  473. 

Fishes:  sizes  of  ova  and  adult,  I, 
143-4;  growth  of  pike,  I,  154, 
292;  size  and  environment,  I,  156; 
temperature,  I,  174;  self-mobility, 
I,  175;  continuity  of  blastoineres, 
I,  214,  II,  327;  genesis,  I,  271,  II, 
435,  436;  conditions  affecting 
genesis,  I,  292-3,  583,  598,  599,  II, 
454;  classification,  I,  392;  change 
of  media,  I,  401,  480;  distribution 
in  time,  I,  408-9;  climbing  spe- 
cies, I,  480,  482;  migrations,  I, 
500;  dermal  structure,  I,  526,  II, 
305-6,  315,  387;  Cunningham  on 
non-adaptive  specific  characters, 


1,  565;  elongation  and  locomotion, 

11,  15;  segmentation,  II,  122,  225; 
bilateral     symmetry,     II,     203-5; 
eyes    of    Plcuroncctidw,     II,    205; 
genesis    of    vertebrate    axis,    II, 
212-6,  218-21,  225;  ossification  of 
paleozoic,  II,  218;  respiratory  or- 
gans, II,  334-8;  activity  and  mus- 
cular colour,   II,   365-9;   Owen  on 
skeleton,  II,  552,  557,  558-60,  562, 
564. 

Fission  (see  Agamogenesis). 

Flint,  Austin,  on  telegony,  I,  644. 

Flounder,  symmetry  and  eyes,  II, 
205. 

Flower,   Sir  W.,  on  ferret,  II,  480. 

Flowers:  pollen  propulsion  in  or- 
chids, I,  57;  nature  of  reproduc- 
tive elements,  I,  283;  insect  fer- 
tilization, I,  340,  525,  II,  168,  174, 
267,  407;  self-  and  mutual  fertili- 
zation, I,  342-5,  570;  Darwin  on 
hoinologies,  I,  472;  direct  equi- 
libration and  fertilization,  I,  524- 
5;  dimorphism,  I,  534;  foliar  ho- 
mology  of  petals,  II,  43-6;  sym- 
metry, II,  132,  161,  162-4,  170, 
174,  608;  fertilization  and  sym- 
metry, II,  164-70;  clusters  and 
components,  II,  170-4;  nutrition 
and  inflorescence,  II,  179-80,  541- 

2,  546-7;  tissue  differentiation,  II, 
265-9;     separation     of     ancestral 
traits  in  hybrids,  II,  616-7. 

Fly,  beneficial  parasitism,  II,  406. 

Food  (see  Nutrition). 

Food-cavity,  genesis  and  develop- 
ment of,  I,  188,  195. 

Foraminifcra:  form,  I,  173;  primary 
aggregate,  II,  87,  124;  progress- 
Ing  integration,  II,  89-90,  124. 

Force:  action  on  like  and  unlike 
units,  I,  5;  expenditure  and  or- 
ganic growth,  I,  149-54,  161;  func- 
tional accumulation,  transfer,  and 
expenditure,  I,  198-9,  201-3,  391; 
waste  and  expenditure,  I,  214-5; 
distribution  during  strain,  II,  209- 

12.  (See  also  Energy,  and  Persist- 
ence of  Force.) 

Fossils  (see  Palaeontology). 

Foster,  Sir  M.,  on  storage  of  glyco- 
gen,  I,  70,  74;  Increase  of  weight 
In  hybernating  dormouse,  I,  214. 


SUBJECT-INDEX. 


639 


Fowls  (see  Oallinacew). 

Foxglove:    abnormal    development, 

I,  287,   II,   46;  floral  distribution, 

II,  141;  nutrition  and  growth,  II, 
1T9. 

France:  surviving  disbelief  in  or- 
ganic evolution,  I,  559;  rate  of 
multiplication,  II,  509,  512. 

Frankland,  Sir  E.,  on  isomerism  of 
protein,  I,  700. 

Fraser,  Col.  A.  T.,  on  family  of 
Hindu  dwarfs,  I,  316. 

Fries,  E.,  multiplication  of  Reticu- 
laria,  I,  582,  II,  450. 

Frog:  vitality  of  detached  heart,  I, 
111;  of  larval  fragments,  I,  365. 

Fry,  Sir  E.,  on  alternation  of  gen- 
erations, II,  84. 

Fuci:  cell  multiplication,  II,  27;  un- 
differentiated  outer  tissue,  II,  256. 

Function:  as  a  basis  of  classifica- 
tion, I,  124-9,  129-31;  simultane- 
ous progress  of  structure  and,  I, 
197,  211;  divisions  of,  I,  198-200, 
391;  correlative  complexity  of 
structure,  I,  200,  210-1;  progres- 
sive differentiations,  I,  201-4;  con- 
comitant integration,  I,  205-8; 
specialization  and  vicariousness, 
I,  208-10;  formula  of  evolution,  I, 
211;  diminished  ability  and  over- 
work, I,  215-6;  growth  and  in- 
creased, I,  228-33,  234-5;  Inter- 
dependence of  social  and  organic, 

I,  237-9,    240-2;     structure    and 
heredity,  I,  306-13,  318-9  (see  Ac- 
quired   Characters);    aids    natural 
selection,  I,  308;  organic  interde- 
pendence,  I,  318-9;  parental  con- 
dition and  variation,   I,  324,  326; 
variation    and    altered,    I,    325-6, 
333—4;    as    causing    variation,    I, 
334-5;      effect     on     physiological 
units,  I,  353-4,  II,  620;  zoological 
classification,    I,    391-3;    multipli- 
cation of  effects,   I,   512;   law  of 
equilibration,   I,  519-22,  557;  cor- 
relation   of    changes    in,    I,    529; 
structural  effects  of  changing,   I, 
541-2;  structural  cooperation,   II, 
3,  217;  vicarious  vegetal,  II,  270; 
vicariousness    and    specialization, 

II,  293:    epidermic   structure,    II, 
312-4,   387;  structure  and  muscu- 


lar, II,  369,  391;  adaptive  bone- 
structures,  II,  370-1;  equilibra- 
tion and  adaptation,  II,  392;  per- 
sistence of  force  and  adaptation, 
II,  394.  (See  also  Physiology.) 
Fungi:  nitrogenous  character,  I, 
40;  development,  I,  163,  164,  165; 
conjugation,  I,  279,  II,  449;  fis- 
sion, I,  584,  585;  integration,  II, 
24-5,  293;  symmetry,  II,  137^0, 
146:  puff-ball  tissue,  II,  246,  252, 
386;  tissue  differentiation,  II,  256; 
inner  tissue,  II,  279;  indefinite- 
ness,  II,  295;  growth  and  genesis, 
II,  459;  nutrition  and  genesis,  II, 
487. 

GalUnaccw:  conditions  affecting  fer- 
tility, II,  454-5,  469,  471;  mascu- 
line traits  of  old  hens,  II,  493. 

Gall-:  definition  of  life  and,  I,  III; 
Hertwig  on,  I,  690. 

Galton,  F.,  on  variation  outside  the 
mean,  I,  669. 

Gamogenesis:  homogenesis,  I,  270, 
271,  336;  heterogenesis,  I,  270, 
336;  independence  of  offspring,  I, 
278;  reproductive  tissue,  I,  279- 
84;  vegetal  nutrition,  I,  285-8, 
293;  II,  39;  animal  nutrition,  I, 
289-94,  297;  when  and  why  does 
it  recur?  I,  294-7,  336-40;  effect 
on  species,  I,  347-9;  leaf  forma- 
tion, II,  39;  alternating  genera- 
tion in  liverworts,  II,  80-4;  mol- 
luscan  homogenesis,  II,  116,  117- 
8;  vertebrate,  II,  118;  growth,  II, 
266.  (See  also  Fertilization,  and 
Multiplication.) 

Gasteropoda  (see  Mollusca). 

Geddes  and  Thompson,  on  the  de- 
termination of  sex,  I,  657. 

Gelatine,   nutritive  value  of,   I,  77. 

Gemmation:  and  genesis,  I,  272-6; 
theories  of  heredity  and,  I,  361; 
annulose,  II,  100-5,  106. 

Generalization,  impossibility  of  per- 
fect, I,  450. 

Generation,  and  genesis:  the  words, 
I,  269. 

Genesis  (see  Multiplication). 

Gcntiana:  floral  arrangement,  II, 
608-11. 

Genus:  indefinite  value,  I,  389,  446; 


640 


SUBJECT-INDEX. 


Instability  of  homogeneous  and 
heterogeneity  of,  I,  509-11,  515, 
517-8,  550,  557. 

Geology:  growth  displayed  in,  I, 
135,  136;  distribution  in  time,  I, 
404-11,  412;  special  creation,  I, 
419,  420;  evolution,  I,  432,  437; 
record  congruous  with  evolution, 
I,  485-9,  556;  organic  influence  of 
changes,  I,  501-3,  549,  550,  557; 
climatic  influence  of  changes,  I, 
503;  time  required  for  organic 
evolution,  I,  565-6;  rise  of  insect 
and  plant  relations,  II,  407;  hu- 
man evolution  and  changes,  II, 
534. 

Geometry,  evolution  illustrated  by, 
I,  433-4. 

Germ-cell:  unspecialized  nature,  I, 
279-83,  317;  dissimilarity,  I,  330, 
332,  334,  342;  equilibrium,  I,  340. 
(See  also  Fertilization.) 

Germ-plasm,  Weismann's  theory  of, 

I,  357-8;   inconsistent  with  plant 
embryogeny,  I,  359;   regeneration 
of  lost  limbs,  I,  362;  variations  in 
peacock's  tail  feather,  I,  372,  695; 

II,  618-9;   alleged    differentiation 
of  reproductive  and  somatic  cells, 
I,  622,  628-30,  633-44,  646;  origin 
of  variations  in  neuter  insects,  I, 
659,    663-5,    671,    675;    correlated 
variations  in  stag,  I,  677;  insuper- 
able difliculties,  I,  682;  conceiva- 
bility   of   hypothesis,    I,    695;   II, 
619;  correlated  variations  in  culti- 
vated plants,  II,  621-2. 

Ghost-theory,  Vitalism  and,  I,  114. 
Giraffe,  co-adaptation  of  structures, 

I,  615. 
Gizzard,  development  of  birds,  II, 

320. 
Glass,     molecular     rearrangement, 

I,  337,  352,  704. 

Glove,  strain  analogy,  II,  575. 
Glycogeu,  in  animal  metabolism,  I, 

70,  72. 
Goethe,  J.  W.  von:  foliar  homology, 

II,  43-4,  543,  544,  archetypal  hy- 
pothesis, II,  122;  vegetal  fructifi- 
cation and  nutrition,  II,  180;  the- 
ory of  supernumerary  bones,   II, 
223:  on  the  skull,  II,  561. 

Gold,  effect  of  bismuth  on,  I,  121. 


Gorilla,  callosities,  II,  312. 

Gould,  J.,  Birds  of  Australia,  II, 
469. 

Gout  (see  Disease). 

Grafting,  Bora's  experiments  with 
frog  larvse,  I,  365. 

Graham,  T.,  properties  of  water,  I, 
9,  II,  359;  colloids  and  crystal- 
loids, I,  15-8,  II,  356;  their  dif- 
fusibility,  I,  18-20;  sapid  and  in- 
sipid substances,  I,  53. 

Gramince:  foliar  surfaces,  II,  61, 
263;  floral  symmetry,  II,  165; 
physiological  differentiation,  II, 
257. 

Graminivores,  food  contrasted  with 
that  of  carnivores,  I,  68. 

Grassi,  on  food-habits  of  Termites, 
I,  686. 

Gravity:  its  ultimate  incomprehen- 
sibility, I,  121;  vegetal  circula- 
tion, II,  586.  (See  also  Specific 
Grav'ty.) 

Gregarina:  central  development,  I, 
163;  primary  aggregate,  II,  87; 
symmetry,  II,  186. 

Grimaux,  on  artificial  proteids,  I, 
39. 

Growth:  organic  and  inorganic,  I, 
135-7;  simulation  of,  I,  136;  limits 
to,  I,  137,  155-7;  structural  com- 
plexity, I,  138-40,  145-7,  161;  nu- 
trition, I,  140,  147-9,  161;  expen- 
diture of  energy,  I,  141-3,  161; 
initial  and  final  bulks,  I,  143-4, 
157-60,  161;  final  arrest  of,  I, 
149-55,  639;  where  unceasing,  I, 
154;  resume  with  generalizations, 
I,  161;  defined,  I,  162;  II,  461; 
increased  function,  I,  228-33,  234- 
5;  functional  interdependence,  I, 
235-9,  240;  nutrition  and  vegetal, 
I,  293,  294-7,  336,  II,  39;  hetero- 
genesis  and  animal  nutrition,  I, 
289-93,  296,  336;  homo-  and  hetero- 
genesis  and  natural  selection,  I, 
294-8;  of  acrogens,  II,  56;  cylin- 
drical form  of  vegetal,  II,  56-64; 
endogenous,  II,  60-2,  78;  exogen- 
ous, II,  63-4,  78;  plant  differentia- 
tion, II,  129-131;  tissue  differen- 
tiation, II,  370;  formation  of 
adaptive  bone-structures,  II,  370- 
2;  progressive  increase  of  size 


SUBJECT-INDEX. 


641 


with  evolution,  II,  401-2;  vegetal, 
and  asexual  genesis,  II,  439-42; 
animal,  and  asexual  genesis,  II, 
442-5;  antagonistic  to  asexual 
genesis,  II,  446;  vegetal  and  sex- 
ual genesis,  II,  448-51;  animal 
and  sexual  genesis,  II,  452-6,  495; 
antagonistic  to  sexual  genesis,  II, 
457-8;  nutrition  and  genesis,  re- 
sume, II,  497-9;  evolution  and, 
II,  501-5;  commencement  of  gen- 
esis, II,  506;  fertilization  and 
restoration  of  growth-energy,  II, 
613. 

Gulick,  T. :  on  monotypic  and  poly- 
typic  evolution,  I,  569;  physio- 
logical selection,  I,  569-71. 

Gunpowder,  nitrogenous  instability, 
I,  8,  43. 

Gymnotus,  electricity  of,  I,  51. 

Oyrodactylus  elcgans,  rapid  succes- 
sion of  generations,  I,  641;  II, 
488. 

HABIT,  change  of,  in  plants,  I,  308. 

Haemal,  term  applied  to  female  ele- 
ment, I,  594-5. 

Hairs:  non-conductors  of  heat,  I, 
526;  vegetal,  and  natural  selec- 
tion, I,  532;  development,  II,  314- 
6;  tactual  organs,  II,  317. 

Hand:  embryogeny,  I,  169;  heredity 
and  size  of,  I,  311;  distribution  of 
veins,  I,  364. 

Hardy,  W.  B.,  I,  vil;  II,  vi. 

Hare:  activity  and  muscular  col- 
our, II,  365;  expenditure  and 
genesis,  II,  472. 

Hart,  J.  A.,  on  "  Parasol  "  ants,  I, 
687-8. 

Haviland,  G.  D.,  collection  of  Ter- 
mites, I,  687. 

Haystack,  chemical  action  in,  I,  74. 

Head,  structural  influence  of  size, 
I,  512,  537. 

Hearing:  the  sense  of,  I,  54;  multi- 
plying agencies,  I,  75. 

Heart  (see  Vascular  System). 

Heat:  action  on  di-  and  trl-atomic 
compounds,  I,  7-8,  10-12,  23,  24; 
on  colloids  and  crystalloids,  I,  26; 
organic  changes  from  evaporation, 
I,  29;  chemical  decomposition  by, 
I,  33;  organic  oxidation,  I,  46-9, 
87 


60;  growth  and  organic,  I,  152-3; 
animal,  vegetal,  and  environment, 
I,  174-5,  177;  alloy  melting  points, 

I,  339;  organic  effects  of  rhythm 
in   terrestrial,   I,   498,   557;   effect 
on    physiological    units,     I,    705; 
respiration  in  fishes,  II,  337;  ani- 
mal preservation,   II,   434;   verte- 
brate expenditure  and  genesis,  II, 
468-9,  474;  insect  genesis,  II,  476; 
seasonal    variations    and    genesis, 

II,  484-5;  in  germination,  II,  615. 
Hebrew  idea  of  creation,  I,  421. 
Hectocotylus,  individuality,  I,  250. 
Hellin,     D.,     on     multiparity     and 

twin-births,  II,  457. 

Hen,  what  prompts  her  to  pick  up 
egg-shell  fragments?  I,  120. 

Henslow,  Rev.  G.,  inheritance  of 
functionally-produced  changes,  I, 
560. 

Hepaticce:  Schleiden  on,  II,  51,  52; 
continuous  and  discontinuous  de- 
velopment, II,  52<  phyletic  homo- 
logles,  II,  80—4;  meaning  of  so- 
called  alternating  generation,  II, 
84;  vascular  system,  II,  280;  gene- 
sis and  development,  II,  463. 

Heredity:  structural  modification, 
I,  232;  function  of  cell-nucleus  in, 
258-59;  general  truths,  I,  301-4; 
transmission  of  congenital  pecul- 
iarities, I,  304-7;  structure  and 
altered  function,  I,  307-13,  318-9 
(see  also  Acquired  Characters) ; 
atavism,  or  recurrence  of  ances- 
tral traits,  I,  314;  sex  limitation, 
I,  314-6;  physiological  units,  re- 
sume, I,  350-5;  II,  612-6;  Darwin's 
and  Weismann's  theories  exam- 
ined, I,  356  et  seq.,  559-61;  II,  622; 
true  theory  must  include  plants, 
I,  358;  inadequacy  of  theory  of 
physiological  units,  I,  360-1;  so- 
ciological parallel,  I,  366-8:  natu- 
ral selection  (q.  v.),  I,  545-7,  553, 
557;  ethnology  and  natural  selec- 
tion, I,  553;  unsolved  problems,  I, 
573-4;  mutilations,  I,  631;  ulti- 
mate process  incomprehensible,  I, 
695;  cell  doctrine,  II,  19;  physio- 
logical development,  II,  242;  wood 
formation,  II,  287:  tissue  differ- 
entiation, II,  304,  312-4;  respira- 


642 


SUBJECT-INDEX. 


tory  system,  II,  311;  osseous  dif- 
ferentiation, II,  351;  muscular 
adaptation,  II,  367;  persistence  of 
force  and  physiological  adapta- 
tion, II,  394;  vegetal  vascular  sys- 
tem, II,  574,  582,  588,  596. 

Hermaphrodism,  I,  340-3. 

Hertwig,  O. :  on  Weismann's  germ- 
plasm  theory,  I,  690;  cell  charac- 
ters, I,  691;  meaning  of  fertiliza- 
tion, II,  613. 

Hertwig,  R.,  classification  of  tis- 
sues, I,  189. 

Heterochrony  of  development,  I, 
655. 

Heterogeneity:  in  chemical  evolu- 
tion, I,  23-4;  of  vital  changes,  I, 
84-90;  of  development,  I,  170,  178; 
functional,  I,  204-8,  211-2;  of  or- 
ganic matter,  I,  350-5;  organic 
and  instability  of  homogeneous,  I, 
509-11,  517,  549,  557:  segregation 
accompanying,  I,  514-6,  517-8, 
550. 

Heterogenesis:  occurrence,  I,  270, 
272-5,  336;  animal  nutrition,  I, 
289-91,  295-7;  natural  selection,  I, 
295-8;  heredity,  I,  301. 

Hindus:  food,  I,  68;  dwarf  family, 

I,  31G. 

Histology  (see  Physiology). 

Hofmeister,  sporophytic  generation 
of  Archegoniates,  II,  80. 

Hollyhock,  floral  symmetry,  II,  167, 
169,  170. 

Homogeneous,  instability  of  the: 
variation,  I,  330,  334,  342;  evolu- 
tion, I,  509-11,  517,  549,  557;  mor- 
phological development,  II,  7-9, 
234;  direction  of  vegetal  growth, 

II,  181;  radial  symmetry,  II,  190; 
physiological    differentiation,    II, 
384,  392. 

Homogenesls  (sec  Gamogenesis). 

Homology,  simulation  of,  by  anal- 
ogy, II,  14. 

Hooker,  Sir  J.  D.,  I,  ix;  European 
plants  In  New  Zealand,  I,  477; 
plant  distribution,  I,  479;  adapta- 
tion of  plants  to  varied  media,  I, 
484;  plant  growth,  II,  56;  Balano- 
phorw  and  Rafflesiaceoe,  II,  274; 
structural  complexity,  II,  295, 
297;  relative  antiquity  and  dis- 


tribution of  plants  and  animals, 
II,  297;  bean  vascular  system,  II, 
574. 

Hooker,  Sir  W.,  on  fructification  in 
Jungermanniaccat,  II,  52. 

Horns,  natural  selection  and  corre- 
lated variation,  I,  537,  567,  674, 
677. 

Horse:  ancestral  types,  I,  409;  fer- 
tility, I,  598;  weight  of  brain,  I, 
599;  quagga  markings,  I,  624,  627. 

Husbandry,  co-ordination  of  actions 
iu,  I,  96,  579. 

Hutchinson,  Sir  J.,  hereditary 
syphilis,  I,  623. 

Huxley,  T.  H.,  I,  ix;  "  continuous  " 
and  "  discontinuous  "  develop- 
ment, I,  164;  classification  of  de- 
velopment, I,  276;  hermaphrodism, 
I,  344;  zoological  classification,  I, 
383;  on  "  Persistent  Types,"  I, 
408-9;  ancestral  equine  types,  tb. ; 
segmentation  of  articulates,  I, 
468-9,  II,  113;  agamic  multiplica- 
tion of  Aphis  and  Entozoa,  I,  640- 
1;  II,  476;  cell  doctrine,  II,  21;  ver- 
tebrate embryo,  II,  119,  120;  mol- 
luscan  symmetry,  II,  202;  tegu- 
mentary  organs,  II,  314,  315;  ver- 
tebrate sensory  organs,  II,  318, 
319;  Chondi-acanthus,  II,  487; 
Owen's  vertebrate  theory,  II,  563. 

Hyacinth:  lateral  spike,  II,  42; 
symmetry,  II,  141,  162. 

Hybernation,  waste  and  repair  in, 
I,  214-5. 

Hybrids,  separation  of  ancestral 
traits  in,  II,  616-7. 

Hydro-carbons:  properties,  I,  6-9; 
the  term  carbo-hydrates  (q.  v.),  I, 
10. 

Hydrochloric  acid,  in  gastric  juice, 
I,  69. 

Hydrogen:  chemical  and  physical 
properties,  I,  3-5;  compounds,  I, 
6,  8,  9,  10-12;  12-13. 

Hydrozoa  (see  Ccelentcrata). 

Hymcnoptera  (see  Insects). 

Hypertrophy  (see  Disease). 

Hypospadlas,  telegonlc  transmis- 
sion, I,  646. 

Hypostasls  of  a  relation,  exemiili- 
fied  in  explanations  of  fertiliza- 
tion, II,  613. 


SUBJECT-INDEX. 


643 


IDEAS  (see  Psychology). 

Individuality:  the  botanical,  I,  244- 
6;  the  zoological,  I,  246-7;  the  fer- 
tilized germ  product,  I,  248-9; 
definition  of  life,  I,  250. 

Individuation:  and  genesis,  I,  583- 
4;  II,  428-30,  499;  total  cost,  II, 
435-7;  genesis  and  evolution,  II, 
501-5,  529,  530. 

Infusoria:  functional  specialization, 
I,  391;  primary  aggregate,  II,  87; 
asymmetry,  II,  187,  188;  differ- 
entiation, II,  299,  385;  genesis,  II, 
442,  446,  452. 

Injuries,  repair  of  animal,  I,  219, 
222-4,  316,  II,  102,  611. 

Insanity,  inherited,  I,  314. 

Insects:  temperature,  I,  47,  174; 
phosphorescence,  I,  49;  self-mo- 
bility, I,  175;  parthenogeuesis,  I, 
274-5,  277,  294,  592,  640;  growth 
and  reproduction,  I,  292;  species 
distribution  determined  by  pres- 
ence of,  I,  396-7;  eyes  of  cave-in- 
habiting, I,  309,  612-3,  614,  647-9, 
693;  persistent  types,  I,  408;  re- 
trograde development,  I,  458; 
segmentation,  I,  408-9;  II,  114; 
aborted  organs,  I,  474;  East  In- 
dian distribution,  I,  478;  floral  fer- 
tilization, I,  525;  II,  168-9,  40P  7, 
608;  appliances  for  cleaning  an- 
tennae, I,  651;  eyes,  I,  658,  II, 
318;  integration  and  homology,  II, 
111-3,  121;  bilateral  symmetry,  II, 
198;  sexual  selection,  II,  269; 
eyes,  II,  318;  environment,  II, 
433;  cost  of  genesis,  II,  436,  437; 
development  and  genesis,  II,  461; 
nutrition  and  genesis,  II,  476, 
490-2. 

Insects,  Social,  origin  of  caste- 
gradations  in,  I,  654-65,  670,  674, 
675,  678-84,  686-8. 

Instability  of  the  homogeneous  (see 
Homogeneous). 

Instinct:  organic  evolution  and  co- 
ordination of,  in  mason-wasp,  I, 
5/4;  a  vital  attribute,  I,  578;  loss 
of  self-feeding,  in  Amazon  ants,  I, 
660-1,  663^. 

Integration:  in  chemical  evolution, 

I,  23;  morphological  composition, 

II,  4-6;  arthropod,  II,  111-4,  121: 


physiological,  in  plants,  II,  292-5, 
295-8,  390;  of  organic  world,  II, 
396-408;  genesis,  II,  424,  426-9. 

Intelligence,  a  vital  attribute,  I, 
579. 

Internodes:  varied  development,  II, 
45;  nutrition  and  length,  II,  178-9. 

Intestine  (see  Alimentary  Canal). 

Intra-selection,  Roux's  theory  of,  I, 
562,  676-8. 

Irish,  nutrition  and  genesis,  II,  510. 

Iron:  colloidal  form  of  peroxide,  I, 
17,  20;  molecular  rearrangement, 
I,  337,  704;  vegetal  absorption,  II, 
573. 

Iron  industry,  interdependence  of 
social  function,  I,  237-41. 

Isolation,  and  species  differentia- 
tion, I,  568-9. 

Isomerism:  of  organic  constituents, 
I,  4,  9,  25;  tri-  and  poly-atomic 
compounds,  I,  11,  13,  25;  muscular 
action,  I,  59;  organic  evolution,  I, 
700,  703;  differentiation  of  nerve- 
tissue,  II,  356-60,  361;  of  muscu- 
lar tissue,  II,  361-4. 

JACKSON,  J.  Hughlings,  on  inherit- 
ance of  nervous  peculiarities,  I, 
313,  694. 

Jaundice  (see  Disease). 

Jaws,  of  uncivilized  and  civilized, 
I,  541-2,  612,  693. 

Johnson,  G.  Lindsey,  on  inherited 
myopia,  I,  694. 

Jones,  T.  Rymer,  on  fission,  I,  585, 
590. 

Julin,  C.,  on  "  castration  parasi- 
taire  "  in  Crustaceans,  II,  493-6. 

Jungermanniaccce:  morphology,  II, 
33-4;  relations  of  high  and  low 
types,  II,  35,  55;  continuous  and 
discontinuous  development,  II, 
52-5,  92:  tubular  structure,  II,  58, 
62;  proliferous  growth,  II,  67,  91; 
colour,  II,  75,  265:  symmetry,  II, 
140;  fertility  and  growth,  II,  441. 

Jussieu,  A.  de,  plant  classification, 
I,  378. 

KABYOKINESIS,  I,  257,  259,  263-5. 

Kerner,  A.,  on  cauline  buds.  I,  358; 
plant-classification  in  Natural  His- 
tory of  Plants,  I,  378-9. 


644 


SUBJECT-INDEX. 


Kidd,  Benj.,  his  acceptance  of 
Weismaimism,  I,  690. 

Kitto,  Dr.,  his  visual  memory  and 
deafness,  I,  230. 

Klebs,  on  Hydrodictyon,  I,  288;  Vau- 
chcria,  II,  84. 

Klein,  E.,  multiplication  of  Bac- 
teria, II,  443. 

Korschelt,  E.,  annulose  segmenta- 
tion, II,  103,  601-3,  605;  Arenicola 
larvse,  II,  109. 

LABOUR,  physiological  division  of, 
I,  204,  207,  591,  II,  373;  its  mean- 
ing and  Weismann's  fallacious  in- 
terpretation, I,  634-5. 

Lacaze-Duthiers,  on  origin  of  annu- 
lose type,  II,  110. 

Lamarck:    zoological    classification, 

I,  382:    opinions    of    E.    Darwin 
and,  I,  491,  493-7;  neo- Darwinists 
and,  I,  630-1. 

Lami  nariacew:  pseudo-foliar  and 
axial  development,  II,  30;  tissue, 

II,  247,  256,  272. 

Language:  and  evolution,  I,  442, 
444,  446;  perceptiveness  of  tongue- 
tip,  I,  607. 

Lankester,  Sir  E.  Ray,  absence  of 
nucleus  in  Archerina,  I,  183;  di- 
versity of  Protozoa,  ib.;  zoological 
classification,  I,  387;  blindness  of 
cave-animals,  I,  647-8,  649. 

Laugh,  definition  of  life  and,  I,  112. 

Laurel,  leaves  of,  II,  149,  249. 

Leaves:  growth  of  shoot,  I,  168;  de- 
velopment and  aggregation,  II, 
37^12,  76;  stem-like  stalks,  II,  41; 
homologies,  II,  42,  75-7,  83;  nu- 
trition and  compound,  II,  42; 
foliar  and  axial  development,  II, 
46-50,  541-7;  "  adnate,"  II,  58; 
proliferous  growth,  II,  67,  91;  nu- 
trition and  development,  II,  76-8; 
symmetry,  and  of  branches,  II, 
148-50,  151;  size  and  distribution 
of  leaflets,  II,  152-5;  transition 
from  compound  to  simple,  II,  155- 
8;  unsymmetrical  form,  II,  158- 
9;  natural  selection  and  distribu- 
tion, II,  179;  morphological  sum- 
mary, II,  234-5;  tissue  differentia- 
tion, II,  247;  distribution,  II,  249; 
outer  tissues  of  stem  and,  II, 


256-9,  270,  386;  distribution  of 
stomata,  II,  260-1;  wax  deposit 
on,  II,  260,  261;  light  and  colour, 
II,  261-2;  superficial  differentia- 
tion, II,  263-5,  270,  387;  abortive 
in  parasitic  plants,  II,  274;  sub- 
merged, in  aquatic  plants,  II, 
274-5;  inner  tissue  differentiation, 
II,  278,  388;  vascular  tissue  dif- 
ferentiation, II,  286,  288,  388;  dye 
absorption  and  circulation,  II, 
570-4,  577;  vascular  system,  II, 
588-92,  596;  arrangement,  II,  608- 
11. 

Lcpidoptcra  (see  Insects). 

Lcpidosircn:  ossification,  II,  218; 
respiration,  II,  338;  skeleton,  II, 
553,  555,  500. 

Lcpidostcus:  armour,  I,  526;  air 
bladder,  II,  334. 

Leroy-Beaulieu,  Pierre,  on  Austra- 
lian miners'  usages,  I,  304. 

Lcssonia:  Hooker  on  growth,  II,  56; 
branch  symmetry,  II,  140. 

Lewes,  G.  H.,  definition  of  life,  I, 
80. 

Lichens:  tissue,  I,  586;  cell  multi- 
plication, II,  27;  Hooker  on 
growth,  II,  56;  tubular  structure, 
II,  57;  integration,  II,  293;  dual 
nature,  II,  399;  reproduction,  II, 
450. 

Liebig,  Baron,  nitrogenous  food 
stuffs,  I,  47-8. 

Life:  co-ordination  of  actions,  I, 
79,  89,  577-80;  defined  by  Schell- 
ing,  I,  78,  178;  Kicheraud,  I,  79; 
De  Blainville,  I,  79,  93;  Lewes,  I, 
80;  definition  yielded  by  contrast- 
ing most  unlike  kinds,  I,  81-S; 
changes  showing,  I,  91;  vital  ac- 
tions and  environment,  I,  92-3; 
resulting  addition  to  conception, 
I,  93,  326;  Comte's  definition,  I, 
93;  correspondence  of  external 
and  internal  relations,  I,  93-6, 
100;  II,  523;  continuous  adjust- 
ment of  such  relations,  I,  99;  com- 
pleteness proportionate  to  corre- 
spondence, I,  101-4,  109,  349; 
length  and  complexity,  I,  103; 
complexity  of  environment  and 
degree  of,  I,  104-6;  definitions  of 
evolution  and,  I,  107-10;  deaden- 


SUBJECT-INDEX. 


645 


cies  of  formula,  I,  112-3;  activity 
the  essential  element,  I,  113;  hy- 
pothesis of  independent  vital 
principle  examined,  I,  114-7;  dif- 
ficulties of  physico-chemical  the- 
ory, I,  117-20;  ultimate  incompre- 
hensibility, I,  120-3,  373:  validity 
of  conclusions  reached,  I,  123;  Is 
organization  produced  by?  I,  197; 
precedes  organization,  I,  210; 
definitions  of  individuality  and,  I, 
250;  effect  of  incident  forces  on, 
I,  348-9,  355:  length  in  individuals 
and  species,  I,  422;  equilibration 
of,  I,  547,  557;  final  formulation 
of  definition,  I,  580;  co-ordination 
of  actions  and  sexual  differentia- 
tion, I,  593;  "  absolute "  com- 
mencement of,  I,  699,  702;  inte- 
gration and  augmentation,  II, 
42G;  prospective  human,  II,  522-5. 

Light:  influence  on  organisms,  I, 
30-6,  II,  433;  nitrogenous  plants, 
I,  40;  organic  phosphorescence,  I, 
49;  heliotropism,  I,  92,  II,  160; 
effects  on  organic  matter,  I,  149; 
plant  adaptation,  I,  227;  rhyth- 
mical variation  of,  and  organic 
life,  I,  499,  557;  vegetal  influ- 
ences, II,  130,  131,  147,  149,  158; 
influence  on  flowers,  II,  167-8, 
608-11;  vegetal  tissue-differentia- 
tion, II,  253-5,  258,  259;  action  on 
leaves,  II,  260-4 ;  on  plant  vascular 
system,  II,  288,  297,  586;  develop- 
ment of  sensory  organs,  II,  320. 

Liliacece,  floral  symmetry,  II,  170. 

Lime,  leaf  forms,  II,  158,  159. 

Lindley,  J.,  plant  classification,  I, 
377. 

Linnseus,  C.,  classificatory  system, 
I,  377,  380. 

Linnet,  contrasted  with  blackbird 
in  development,  II,  503. 

Liver:  metabolic  processes,  I,  69, 
70;  vitality  of  excised,  I,  111;  de- 
velopment, II,  329-33. 

Liver-fluke  (see  Distoma). 

Liverworts  (see  Hcpaticce). 

Lizard,  regeneration  of  lost  tail,  I, 
360. 

Locomotion  (see  Motion). 

Logic,  reasoning  and  definition  of 
life,  I,  81-6. 


Logwood,  vegetal  staining,  II,  569- 
74,  577-81,  584. 

Longevity,  and  complexity  of  life, 
I,  102-3. 

Lubbock,  Sir  J. :  on  growth  and 
genesis  in  insects  and  crusta- 
ceans, I,  292;  aquatic  flies,  I,  400. 

Lungs  (see  Respiratory  System). 

Lymphatic  system:  amreboid  cells, 
I,  187;  structural  traits,  I,  192, 
193. 

MACBRIDE,  E.  W.,  I,  vi,  II,  vi; 
zoological  phyla,  I,  386-7;  arthro- 
pod segmentation,  II,  114;  cteni- 
dia  of  slug,  II,  117;  conjugation 
of  Paramoecium,  II,  452. 

Macrocystls  pyrifera,  gigantic  sea- 
weed, I,  121. 

Magenta,  vegetal  staining,  II,  569- 
74,  577-81,  584. 

Magnetism:  muscular  action,  I,  .59; 
incomprehensibility,  I,  121. 

Maillet,  B.  de,  modifiability  of  or- 
ganisms, I,  490,  496. 

Mammalia:  temperature  and  mole- 
cular change,  I,  30;  nutrition  and 
growth,  I,  141;  expenditure  of 
force,  I,  142,  156;  flesh  constitu- 
ents, I,  154;  temperature,  I,  174, 
177;  self-mobility,  I,  175;  func- 
tional and  structural  differentia- 
tion, I,  201;  heart-function,  I,  206; 
viviparous  homogenesis,  I,  271; 
variation  and  uterine  environ- 
ment, I,  327;  classification,  I,  392; 
cervical  vertebrae,  I,  394,  II,  564; 
aquatic  types,  I,  400;  fossil  re- 
mains and  rate  of  evolution,  I, 
407;  ancient  and  modern  forms 
contrasted,  I,  408-10;  embryonic 
respiratory  system,  I,  456;  sup- 
pression of  teeth,  I,  457;  arrested 
development,  I,  473^4;  simulated 
homologies,  I,  485;  natural  selec- 
tion and  inactive  parts,  I,  534; 
re-development  of  rudimentary 
organs,  I,  563;  location  of  testes 
and  current  theories,  I,  573;  fer- 
tility and  development,  I,  583,  II, 
465;  fertility  and  nervous  develop- 
ment, I,  598-9;  locomotion  and 
elongated  form,  II,  15;  symmetry, 
11,204;  tegumentary  structure,  II, 


64:6 


SUBJECT-INDEX. 


314;  circulation,  II,  340;  vascular- 
Ity  and  ova-rnaturatlon,  II,  342-3; 
activity  and  muscular  colour,  II, 
305 -9;  functional  integration,  II, 
375;  outer  tissue  differentiation, 
II,  387;  growth  and  genesis,  II, 
456,  459;  comparative  fertility, 
II,  465,  470;  heat  expenditure  and 
genesis,  II,  467-9;  activity  and 
fertility,  II,  472;  nutrition  and 
genesis,  II,  479-80. 
Man:  effect  of  climate  on  vigour,  I, 
30;  flesh  and  grain  eaters  com- 
pared, I,  68;  longevity  and  life,  I, 
103;  complex  environment,  I,  105; 
embryogeny  of  arm,  I,  169;  fer- 
tility and  conditions  affecting  it, 

I,  300,    570,   583,   II,   484,    506-21; 
inheritance    of    functionally    pro- 
duced changes,  I,  310-3,  541,  605, 
608,    612,    652,    673,    689,    693-4; 
heredity  and  sex,   I,  315-6;  func- 
tion of   bilirubin,   I,   330;   cousin- 
marriages,  I,  346,  II,  615;  primi- 
tive notions,  I,  417-9;  inutility  of 
Appendix  vermiformis,    I,   474;    di- 
minution of  jaw,  I,  541,  612,  693; 
co-ordination   of  actions   greatest 
in,   I,   579;   fundamental  traits  of 
sex,  I,  594-7;  obesity,  I,  594;  sub- 
stance   and    weight    of    brain,    I, 
596,    599;    distribution   of   tactual 
perceptiveness,     I,     602-8,    665-6, 
672-3,  692;  telegony,  I,  625,  644-5; 
degradation  of   little  toe,   I,   652, 
673;   transmitted  osteological  pe- 
culiarities   of    Punjabis,    I,    689; 
traits  of  twin-bearing  women,  II, 
457;  comparative  mammalian  fer- 
tility,   II,    465;    future    evolution, 

II,  522-37.      (See   also    Language 
and  Sociology.) 

Manatee,  nallless  paddles,  I,  473. 

Manx  cats,  I,  303. 

Marchantiacea::  symmetry,  II,  140; 
outer  tissue  differentiation,  II, 
252. 

Marmot,  hibernation  and  waste,  I, 
214-5. 

Marriage  (see  Multiplication). 

Marsh,  O.  C.,  on  telegony,  I,  644. 

Masters,  M.  T.,  on  foliar  homology, 
n,  46-7;  selection  of  Inconspicu- 
ous variations  in  plants,  II,  298, 


621;  separation  of  ancestral  con- 
stitutions in  plant  hybrids,  II, 
616;  single  and  double  stocks,  II, 
622. 

Matter,  incomprehensibility  of  in- 
teractions, I,  121-2. 

Mechanics:  tranverse  strains,  II, 
209-12;  genesis  of  vertebrate  axis, 
II,  212-6,  216-8,  224,  225-7;  osse- 
ous differentiation,  II,  345-51;  dis- 
integrated motion,  II,  375;  anal- 
ogy from  locomotive,  II,  517-9; 
future  human  evolution,  II,  524; 
strain  and  vegetal  structure,  II, 
574-88,  592-6. 

Mcditsce:  contractile  functions,  I, 
58;  II,  374;  individuality,  I,  248; 
heterogenesis,  I,  273;  fertility,  I, 
582;  strobilization,  I,  592;  sym- 
metry, II,  188-91. 

Mehnert,  E.,  on  feet  of  peutadac- 
tyle  vertebrates,  I,  401. 

Mensel's  salt,  temperature  and  iso- 
merism,  I,  77. 

Metabolism:  antithesis  between 
plants  and  animals,  I,  62-3;  evo- 
lution-hypothesis and  primordial, 
I,  63-4;  in  plants,  I,  C4--7;  ani- 
mals, I,  67-77;  nervo-muscular  ac- 
tivities, I,  71-7;  summary,  I,  77; 
cell  processes,  I,  201. 

Metals:  remarkable  interactions  of 
some,  I,  121;  melting  of  alloys,  I, 
339;  atomic  re-arrangement,  I, 
352. 

Metamerism  (see  Segmentation). 

Mctazoa:  cellular  structure,  I,  184, 
194,  II,  21;  subordination  of  units, 
I,  185-7;  general  characters  of  tis- 
sues, I,  188-9;  protoplasmic  con- 
tinuity, I,  190-2,  194,  628;  genesis 
of  food  cavity  and  visual  organ, 
I,  195;  Weismann's  differentiation 
theory,  I,  637-43. 

Meteorology:  non-vital  changes 
shown  in,  I,  82,  84;  crystalliza- 
tion of  "  storm  glass,"  I,  96;  spe- 
cial creation,  I,  419;  rhythm  in, 
and  organic  change,  I,  499-501, 
557;  variations  due  to  geologic 
change,  I,  503. 

Microstomida,  segmental  reproduc- 
tion, II,  102. 

Migration:    of    animal    species,    I, 


SUBJECT-INDEX. 


647 


396-401,  411;  solar  Influences,  I, 
500;  part  played  by,  in  organic 
evolution,  I,  568;  causes  of,  II, 
533-4. 

Milk,  heat  and  supply  of,  II,  468. 

Milne-Edwards,  H.,  "  physiological 
division  of  labour,"  I,  204;  Weis- 
mann's  erroneous  application  of 
it,  I,  634;  on  ocular  structure,  II, 
318. 

Mind  (see  Psychology). 

Mitosis  (see  Karyokinesis). 

Mobility,  molar  and  molecular,  I, 
14;  environment  and  self  mobil- 
ity, I,  177. 

Mohl,  on  phsenoganiic  growth,  II, 
82. 

Mole,  modifications  due  to  habits, 
II,  891. 

Molecules:  mechanically  considered, 
I,  14;  stability,  I,  337-40;  nerve- 
differentiation,  II,  353-61,  379-82. 

Mollusca:  axial  development,  I,  165; 
genesis,  I,  271,  II,  444;  her- 
maphrodism,  I,  341;  classiflcatory 
traits,  I,  392;  distribution  in 
time,  I,  405,  408,  410,  446-7; 
trochophore  and  its  relationships, 

I,  447,    II,    108,    109,    115;    devel- 
opment,   I,    460;    amphibious   and 
terrestrial,   I,    481;   indirect   equi- 
libration,   I,    534;    secondary    ag- 
gregation,   II,    115-7;    symmetry, 

II,  201-3;    outer   tissue,    II,    310, 
387;   alimentary   system,    II,   325; 
vascular  system,  II,  340-1. 

Molluscoida,  II,  598.  (See  Polyzoa 
and  Tunicata.) 

Monocotyledons:  growth,  I,  138, 
139,  143;  uniaxial  development,  I, 
165;  cotyledonous  germination  and 
endogenous  growth,  II,  59-62,  69- 
72,  82-3,  181-2;  absence  of  helical 
phyllotaxy  in  Ravenala,  II,  182; 
surface  contrasts,  II,  257;  outer 
leaf  tissue,  II,  263;  wood  forma- 
tion, II,  278;  growth  and  genesis, 
II,  451. 

Monstrosities,  in  plants,  II,  78,  541, 
546;  vertebrate,  II,  118. 

Morgan,  T.  H.,  on  regeneration  of 
Planaria,  II,  102,  611. 

Morphology:  facts  comprised  in,  I, 
125-6;  morphological  units,  I,  190- 


2,  225;  rudimentary  organs,  I, 
472-5,  556;  structural  and  func- 
tional co-operation,  II,  3,  239;  in- 
tegration, II,  4-6,  181-96;  change 
of  shape,  II,  6;  formula  of  evolu- 
tion, II,  7-9;  as  interpreted  by 
phylogeny,  II,  10-6;  evolution  and 
cell  doctrine,  II,  17-21. 
Morphology,  Animal:  evolution  and 
segmentation  of  Articulata,  I,  468- 
9;  vertebral  column  development, 

I,  470;  simulated  homologies,   II, 
14-5;  primary  aggregates,  II,  85- 
8,  123-4;  secondary,  II,  88-91,  124; 
tertiary,  II,  91-3;  integration  and 
independence  of  individuality,  II, 
93-9,  124;  annulose  segmentation, 

II,  98-101,    106-10,    125-7,    602-7; 
progressive  annulose   integration, 
II,  100-5,  111-5,  121,  124,  223;  un- 
integrated     molluscan    form,     II, 
115-7;     vertebrate     segmentation 
and  integration,  II,  117-23,  124-7, 
223-4,     602,     606-7;     motion     and 
symmetry,      II,      183-5;      symme- 
try of  primary  and  secondary  ag- 
gregates,     II,     186,      187-91;     of 
compound   Ccelentcrata,   II,   192-4; 
simulation    of    plant    shapes,    II, 
192;    symmetry    of    Pol'jzoa    and 
Tunicata,     II,     194;     of    PlatyJifl- 
minthcs  and  Echinoderms,  II,  195- 
7;    of    Annulosa,    II,    197-201;    of 
molluscs,     II,     201-3;     of     verte- 
brates,   II,   203-6,   208;   similarity 
of  animal  and  plant,  II,  203;  cell- 
shapes,  II,  228-30;  evolution  and 
generalizations    summarized,     II, 
231-5.     (See  also  Structure.) 

Morphology,  Vegetal:  simulated 
homologies,  II,  13-4;  unicellular 
plants,  II,  21;  aggregation  and  in- 
tegration, II,  22-6,  78-9;  pseudo- 
foliar  development,  II,  26-8; 
pseud-axial,  II,  28-9;  pseudo- 
foliar  and  axial,  II,  30-2;  com- 
position of  Archegoniates,  II,  33- 
5;  leaf  development  and  aggrega- 
tion, II,  37-42,  75-8;  foliar  homo- 
logies, II,  42-6,  75-8;  foliar  and 
axial  development,  II,  46-50,  541- 
7;  growth  and  development  of 
Archegoniates,  II.  50-6;  of  Phse- 
nogams,  II,  56-64,  78-80;  axil- 


648 


SUBJECT-INDEX. 


lary  bud  development,  II,  65-9; 
phsenogamic  modes  of  growth,  II, 
69-72;  ho  mo  logics,  II,  73-5,  SO-4; 
development  of  foliar  into  axial 
organs,  II,  75-8;  resume,  II,  78- 
80;  criticisms  and  replies,  II,  80- 
4;  can  plant-shapes  be  formu- 
lated? II,  128;  growth  and  differ- 
entiation, II,  129-31;  kinds  of 
symmetry,  II,  131-3;  symmetry  of 
primary  aggregates,  II,  134-7;  of 
secondary,  II,  137-40;  tertiary,  II, 
140-3;  symmetry  and  environing 
influences,  II,  143-4;  symmetry  of 
branches,  II,  145-8;  leaf  and 
branch  symmetry,  II,  148-50; 
phsenogamic  unit  homology,  II, 
151;  size  and  distribution  of  leaf- 
lets, II,  152-5;  transition  from 
compound  to  simple  leaves,  II, 
155-8;  unsymmetrical  leaf  devel- 
opment, II,  158-9;  differentiation 
of  homologous  units,  II,  159-60; 
floral  symmetry,  II,  161-74;  cell- 
differentiation  and  metamorpho- 
sis, II,  175-7;  nutrition  and  differ- 
entiation, II,  178;  and  inflores- 
cence, II,  179;  helical  growth  of 
phsenogams,  II,  180-1;  summary 
of  symmetry,  II,  234;  stress  and 
structure,  II,  275-9,  388.  (See  also 
Structure.) 

Morton,  Lord,  quagga-marked  foal, 
I,  624. 

Moser,  impressions  produced  by 
light  on  metals,  I,  352. 

Mosses:  varied  development,  II,  50- 
1,  52;  homologies,  II,  80,  81;  in- 
deflniteness,  II,  296;  multiplica- 
tion, II,  441. 

Moth,  clothes,  food  of  larva,  I, 
77. 

Motion:  organic,  and  environment, 
I,  75-7,  175-8,  196;  of  animals  and 
waste,  I,  214,  220;  simulation  of 
locomotive  structures,  II,  15. 

Motor  organs,  differentiation  of,  I, 
262. 

Mountains:  climatic  effects,  I,  504; 
growth  of  trees  on,  II,  142. 

Mouse:  fertility  of,  II,  421,  473; 
tapeworm  parasitism,  II,  490; 
compared  with  rat,  II,  503-4. 

Mucor,  II,  22,  123. 


Mucous  membrane,  differentiation, 
II,  321-2,  389. 

Multiplication:  decline  of  fertility 
with  evolution,  I,  103,  II,  431; 
vitalism,  I,  116;  phenomena  classi- 
fied, I,  130;  the  term  "  genesis," 

I,  269:  processes  classified,  I,  270- 
6,   336,  583;  a  process  of  disinte- 
gration,  I,   276;   reproductive  tis- 
sue in  gamogenesis,  I,  278-84;  nu- 
trition   and    growth,     I,     285-94, 
295-7,    299;    natural    selection,    I, 
295-8;    hermaphrodism,    I,    340-4: 
in-and-in  breeding,  I,  344-7;  phy- 
siological units,  I,   350-5;   law  of 
race-maintenance,  I,  581;  II,  420- 
3,   430;   effect  of  mental   applica- 
tion, I,  597,  II,  511-4,  516-9,  530; 
individuation    antagonistic    to,    I, 
598-COO,     II,    428-30,     435-7,     499, 
501-5;    checks   put   by   carnivores 
on,   II,   405;   four  factors   in  rate 
of,   II,   416,   435;   destructive   and 
preservative    forces,    II,    417-20; 
rhythm  of  species,  II,  419;  nutri- 
tion and  disintegration  of,  II,  424, 
425,  430;  integration  and  genesis, 

II,  426-8;    influence    of    environ- 
ment, II,  432-3;  and  variations  of 
expenditure,     II,     433-5;     growth 
and   asexual   genesis,    II,    439-46; 
asexual  and  sexual  distinguished, 
II,     448;      sexual     genesis     and 
growth,   II,   448-58,   495;   and   de- 
velopment,   II,    4(51-5;    plant    ex- 
penditure, II,  467;  animal  expen- 
diture,   II,    468-72;    nutrition    In 
plants,  II,  475,  511;  in  animals,  II, 
476-84,    511;    seasonal    variations, 
II,    484-5;    nutrition,    resume,    II, 
486,    497-9;    nutrition    and    piira- 
sitlc,     II,     486-90;     reversion     to 
agamogenesis,    II,    490-2;    human 
fertility,  II,  506-10;  Doubleday  on, 
II,    510-2;    civilized    and    uncivil- 
ized,  II,   514-6;   human  evolution 
and    decline    In,    II,    529-31;    the 
future    of    population,    II,    532-7: 
equilibration    and    evolution,    II, 
537. 

Muscle:  electrical  contrasts,  I,  50: 
action  of,  I,  59;  metabolism,  I,  70, 
71-4:  definition  of  life  and  actions 
of,  I,  112-3:  growth  and  function, 


SUBJECT-INDEX. 


C49 


I,  151,  155;  development,  I,  170; 
Hertwig's  classification  of  tissues, 
I,  389;  functional  differentiation, 

I,  203-4;  waste  and  repair,  215-7; 
inodiflability  and  adaptability,   I, 
228-9,  230,  232;   correlated  varia- 
tions,   I,   53&-9,   614-21,   676,   693; 
resistance  to  strains,   I,   639;   ac- 
tion on  bones  in  Punjabis,  I,  689: 
differentiation,  II,  361-9;  activity 
and  colour,  II,  365-9:  integration, 

II,  376,   382:   equilibration   In  ac- 
tion, II,  393;  activity  and  fertility 
in  birds,  II,  470-2;  future  human 
evolution,   II,   523;   origin  of  ver- 
tebrate type,  II,  598-600. 

Music:  limited  adaptability  of  voice 
and  ear,  I,  231;  inheritance  of 
faculty,  I,  311-2,  694. 

Mutilations,  the  question  of  their 
inheritance,  I,  631. 

Mycctozoa,  growth  and  reproduction, 
I,  298-9. 

Myocommata  (myotomes),  and  ver- 
tebrate skeleton,  II,  216,  217-8, 
222. 

Myopia,  inheritance  of,  I,  306,  694. 

Myrianida  fasciata,  I,  361;  II,  445. 

Myriapoda:  gemmation,  I,  589;  seg- 
mentation, I,  590,  II,  113,  114, 
601;  degenerated  eyes  of  cave-in- 
habiting, I,  649;  integration  and 
homology,  II,  111-4;  genesis,  II, 
445. 

Myxothallophijta,  I,  378. 

NAILS,  mammalian,  I,  473. 

Nais:  regeneration  of  detached 
parts,  I,  219,  361. 

Narcissus,  separation  of  ancestral 
traits  in  hybrids,  II,  617. 

Natural  selection:  structural  modifi- 
cation, I,  211;  in  cell  processes,  I, 
263-4;  multiplication,  I,  295-8; 
aided  by  function,  I,  308-10;  spe- 
cial creation,  I,  426-7:  the  term 
"  survival  of  the  fittest,"  I,  530; 
indirect  equilibration,  I,  530-5, 
552-3,  557,  571;  changes  unex- 
plained by,  I,  535-42,  571,  II,  371; 
tendency  to  economy,  I,  536,  502; 
decrease  of  jaw,  I,  541,  693;  gen- 
eral doctrine  of  evolution,  I,  543- 
8,  557;  unceasing  operation,  I, 


552;  human  races,  I,  553;  current 
views,  I,  559-60;  panmhtia  and 
cessation  of  selection,  I,  560-3: 
intra-selection,  I,  562,  676-8; 
Elmer's  theory  of  orthogenesis,  I, 
564;  Mr.  Cunningham's  criticism, 
I,  565-6;  location  of  mammalian 
testes,  I,  573;  co-ordinated  in- 
stincts of  mason-wasp,  I,  574; 
tactual  perceptiveness,  I,  603-8, 
633,  646,  665,  671,  672,  692;  errone- 
ously identified  with  artificial  se- 
lection, I,  609,  695;  reversed  se- 
lection, I,  611;  blindness  of  cave- 
animals,  I,  613,  614,  647-8,  693;  co- 
adaptation  of  co-operative  parts, 
I,  614,  621,  663-5,  670,  674,  675, 
689,  692;  where  operative,  I,  632; 
Weismann  on  conceivability  of 
process,  I,  651;  degeneration  of 
little  toe,  I,  652-3,  673;  genesis  of 
caste  gradations  In  social  insects, 
I,  654-60,  663,  670,  675,  684;  self- 
feeding  instinct  in  ants,  I,  660-2, 
670;  rudimentary  organs,  I,  667-9, 
671,  692;  horns  of  stag,  I,  676-8, 
692;  musical  faculty,  I,  694;  the 
neo-Darwinian  position  reviewed, 

I,  694-5;  vegetal  nutrition,  II,  51- 
2;  upright  vegetal  growth,  II,  56- 
7;   endogenous   growth,    II,    57-8; 
exogenous,   II,   64;  Navicula  sym- 
metry,   II,    135;    foliar,    II,    158; 
foliar    distribution,    II,    167,    179; 
floral  fertilization  and  symmetry, 

II,  168-70,  608-11;  helical  phscno- 
gamic  growth,  II,  181;  Echinodcr- 
mata  and  bilateral  symmetry,  II, 
197;  vertebrate  structure,  II,  214- 
20,  227;  phsenogamic  tissue  differ- 
entiation,   II,    248:    physiological 
differentiation,  II,  252,  256;  root- 
lets of  ivy,   II,  254:  stomata  and 
foliar  surfaces,  II,  261,  262;  floral 
fertilization,  II,  268-9;  sexual  se- 
lection, II,  269;  vegetal  tissue  dif- 
ferentiation, II,  279;  wood  forma- 
tion, II,  287-8,  290;  animal  tissue 
differentiation,   -II,    304-8;    evolu- 
tion of  nervous  system,  II,  307-8; 
respiratory   system,   II,   311;   der- 
mal callosities,  II,  312-4;  sensory 
organ  complexities,   II,   321;   skin 
and   mucous   membrane   differen- 


650 


SUBJECT-INDEX. 


tiation,  II,  322;  localization  of  ex- 
cretion, II,  333;  respiratory  or- 
gans of  fishes,  II,  335-8;  heart 
and  vascular  system,  II,  341,  344; 
osseous  differentiation,  II,  355; 
also  muscular,  II,  363,  368-9; 
"  false  joints,"  II,  371;  Insect  nu- 
trition and  genesis,  II,  499;  econo- 
mics of  evolution,  II,  501-5;  au- 
thor's enunciation  of  survival  of 
the  fittest  in  1852,  II,  528-9;  evils 
of  interference  with,  in  man,  II, 
532-3;  vegetal  tissue  formation, 
II,  582,  594-6;  origin  of  verte- 
brate type,  II,  599. 

Nature,  more  complex  than  sup- 
posed, I,  252,  450. 

Jfavicula,  symmetry,  II,  134-5. 

"  Nebular  Hypothesis,"  I,  23. 

Negation,  inconceivability  of,  the 
ultimate  test  of  truth,  I,  675. 

Negroes,  telegony  in  United  States, 
I,  644-5. 

Ncmerticke:  continuing  vitality  of 
pilidium,  I,  250;  bilateral  sym- 
metry, II,  195. 

Neo-Darwlnists,  and  Lamarck,  I, 
630;  their  position  reviewed,  I, 
694-5. 

Nerves:  electrical  phenomena,  I,  51; 
generation  of  nerve-force,  I,  52-6, 
60;  corpmcula  tactus,  I,  75;  Hert- 
wig's  classification  of  tissues,  I, 
189:  structural  traits,  I,  192,  193; 
environment  and  structure,  I, 
196;  differentiation,  I,  203;  II, 
355-61;  vasomotor  system,  I,  206; 
vicarious  function,  I,  209;  activity 
and  waste,  I,  216;  adaptability,  I, 
229,  232,  236;  parallelism  In  cell- 
processes,  I,  260-2;  heredity,  I, 
313;  effects  of  severance,  I,  349; 
relative  development  in  men  and 
women,  I,  594;  analysis  of  brain 
substance,  I,  596;  individuation 
and  development  of,  I,  598,  599, 
600;  distribution  of  tactual  per- 
ceptlveness,  I,  603-8,  633,  646, 
605-6,  671,  672/692;  alleged  cost- 
liness of  tissue,  I,  662;  instinct 
degeneration  in  ants,  ib. ;  "  sensa- 
tion areas,"  I,  6(56;  segmentation 
in  Annelids,  II,  125;  ectodermal 
derivation,  II,  303-4;  cooperating 


factors  in  evolution  of,  II,  307-8; 
differentiation  from  muscle,  II, 
363.  (See  also  Psychology.) 

Nervousness,  hereditary  transmis- 
sion, I,  307. 

Neurine,  I,  594,  597. 

Neuter-insects  (see  Insects). 

New  Zealand:  invasion  of  alien  spe- 
cies, I,  477;  kinship  of  past  and 
present  forms,  I,  489. 

Nitrogen:  properties,  I,  3-5,  20,  24; 
compounds  and  their  properties, 

I,  6,  8,  9,  12-14,  25-6,  39,  41,  42-3, 

II,  250;    organic    importance,    I, 
42-3;  evolution  of  heat  and  oxida- 
tion, I,  47;  violent  organic  effects 
of  compounds,  I,  54-5;  function  in 
metabolism,    I,    63-4,    66,    68-70; 
presence  in  protoplasm,  I,  66;  ac- 
tion in  digestion,  I,  69;  fat  accu- 
mulation and  fertility,  II,  483. 

Nitro-glycerine,  violent  effects  of, 
I,  55,  122. 

Notochord:  segmentation,  II,  125, 
218-22;  formation,  II,  217-8,  COO; 
in  Permian  vertebrates,  II,  225. 

Noumenon,  life  not  manifested  as, 
I,  5SO. 

Nuclein,  II,  21. 

Nucleus:  central  development,  I, 
163;  in  simple  organisms,  I,  183; 
phenomena  exhibited  by,  I,  255- 
8;  current  hypotheses  of  function, 
I,  258-9;  properties  and  function 
of  chromatin,  I,  259-65;  fusion  in 
fertilization,  I,  283-4;  function  in 
unicellular  reproduction,  I,  595-6; 
absence  of,  II,  20-1;  diffused 
form,  II,  85;  macro-  and  micro- 
nucleus  in  conjugation,  II,  452. 

Nutrition:  organic  molecular  re-ar- 
rangement, I,  36;  nitrogenous  and 
non-nitrogenous,  I,  47-8,  68,  71-4, 
77,  II,  362;  food  assimilation  and 
reasoning,  I,  81;  needful  for  vital 
change,  I,  94;  relation  to  growth, 
I,  140,  143,  144,  147-9,  150,  157, 
161;  expenditure  of  energy,  I, 
157,  391;  fluid,  I,  208;  vegetal 
fructification,  I,  267;  II,  266; 
vegetal  growth  and  genesis,  I, 
293,  294-7,  336;  animal  growth 
and  genesis,  I,  289-93,  295-7,  :«f,; 
conditions  qualifying  antagonism 


SUBJECT-INDEX. 


651 


of  growth  and  genesis,  I,  299; 
competition  among  parts  of  an  or- 
ganism for,  I,  562,  566,  676;  sex 
differentiation,  I,  594-3;  cell-mul- 
tiplication, I,  638;  differentiation 
of  neuter  insects,  I,  655-00,  670, 
674,  686-8;  monstrous  ant  forms, 

I,  683-4:  leaf-development,  II,  39, 
42,  73-8;  vegetal  development,  II, 
51-2,   178,   276;   axillary  buds,   II, 
65-9,  73-4;  effect  on  animal  aggre- 
gation, II,  93;  internodes  and  in- 
florescence,    II,     178-80;     helical 
phsenogamic  growth,   II,   181;   ac- 
tion of  bile,   II,  330;  osseous  de- 
velopment,  II,   349,   353;   genesis, 

II,  419,  422,  427,  435-7,  452;  par- 
ental   loss   in   feeding   young,    II, 
424,     429;     diverse     sources,     II, 
433;    Carpenter    on    reproduction 
and,  II,  4GO;  animal  development 
and  genesis,  II,  465;  expenditure 
and  genesis,  II,  468;  variations  of 
genesis,    II,    475-80,    511;    obesity 
and  genesis,   II,   480-4,   511;   gen- 
eral doctrine  of  genesis,  II,  486; 
genesis    and    vegetal    parasitism, 
II,    486;   also   animal,    II,    487-90, 
495;     insect    genesis,     II,     490-2: 
genesis,  resume,!!,  497-9;  and  evo- 
lution,    II,     501-4;    of    blackbird 
and    linnet,    II,    503;    genesis    in 
human    race,    II,    508-10,    514-6; 
Doubleday   on,    II,    510-2;    future 
human  evolution,  II,  526,  531;  flo- 
ral monstrosities,  II,  542,  546,  547. 

OBESITY,  nutrition  and  genesis,  II, 

480-4,  511. 

Odoriferous    glands,    natural    selec- 
tion and,  I,  534. 
Odours:  floral  fertilization,  II,  268- 

9;  animal  protection,  II,  434. 
Offspring:  parental  loss  entailed  by 

nurture,  II,  424,  429;  influence  of 

age  on,  II,  507. 
Oken,  L.,  archetypal  hypothesis,  II, 

122;     theory     of     supernumerary 

bones.    II,   223;   on  the  skull,   II, 

561. 
Oliver,     F.     W.,     classification     of 

plants,  I,  378-9.   . 
OpJtryotrocha    pucrilis,    ciliation    of 

segments,  II,  109. 


Orchids:  pollen  propulsion,  I,  57; 
leaf  formation  in  Dcndrobium,  II, 
60-1;  aerial  roots  and  physiologi- 
cal differentiation,  II,  255,  257; 
foliar  surface,  II,  264. 

Organic  matter:  properties  of  ele- 
ments, I,  3—5,  22;  of  compounds, 
I,  5-13,  25;  molar  and  molecular 
mobility,  I,  12-14;  colloid  and 
crystalloid  form,  I,  15-8,  25; 
their  diffusibility,  I,  18-21,  26;  ex- 
treme complexity,  I,  21;  laws  of 
evolution  and  genesis  of,  I,  22^; 
modiflability,  I,  27,  44;  capillarity 
and  osmosis,  I,  28;  effects  of  heat, 
I,  29;  of  light,  I,  30^;  nitrogen- 
ous, I,  39^3;  oxidation  and  evo- 
lution of  heat,  I,  46,  60;  genesis 
of  electricity,  I,  50-2,  60;  sensible 
motions  in,  I,  59;  transformations 
and  persistence  of  force,  I,  61; 
metabolism,  1,62-77;  artificial  pro- 
duction of  compounds,  I,  G4;  con- 
trasted with  inorganic  matter,  I, 
107-8;  incomprehensibility  of  vital 
changes  in,  I,  122;  instability,  I, 
149,  508;  phosphorus  in  cell-or- 
ganization, I,  260-1;  heteroge- 
neity, I,  350-5;  "  spontaneous  gen- 
eration "  and  evolution  of,  I,  696- 
701;  cell-doctrine  and  evolution 
of,  II,  17-21. 

Organization  (see  Structure). 

Ormerod,  Dr.,  on  sex  and  nutrition 
In  wasps,  I,  656. 

Orthogenesis,  Eimer's  theory  of,  I, 
563-4. 

Osmosis:  organic  effects,  I,  28,  29; 
in  animals,  I,  58;  in  vascular  sys- 
tem, II,  339;  in  vegetal  tissue,  II, 
568,  575,  577,  585,  592-6. 

Osteology  (see  Bone). 

Ovum  (see  Egg  and  Fertilization). 

Owen,  Sir  R.:  metagenesis  and  par- 
thenogenesis, I,  273-4;  fossil 
mammals,  I,  410;  human  para- 
sites, I,  427;  continuous  operation 
of  creative  power,  I,  492;  fission 
in  Infusoria,  I,  584,  5S5,  595-6; 
parthenogenesis,  I,  r>92;  theory  of 
vertebrate  skeleton,  II,  123,  548- 
66;  theory  of  supernumerary 
bones,  II,  223;  Eschricht  on 
Ascaris,  II,  488. 


652 


SUBJECT-INDEX. 


Oxalis:  radial  symmetry,  II,  152; 
foliar  surface,  II,  264. 

Oxen:  comparison  with  sheep,  I, 
158,  160;  cerebro-spinal  system,  I, 
508. 

Oxidation  (see  Oxygen). 

Oxygen:  properties,  I,  3-5,  20,  22; 
compounds,  I,  6-7,  10-13,  22,  24-5; 
a  crystalloid,  I,  21;  combining 
power  and  atomic  weight,  I,  33; 
organic  change  from,  I,  37;  heat 
generation,  I,  46-9;  phosphores- 
cence, I,  49;  nerve  force  depend- 
ent on,  I,  53;  animal  metabolism, 
I,  72,  73;  necessary  to  animal  life, 
I,  94-5,  577;  activity  and  amount 
inhaled,  I,  214. 

PACKARD,  A.  S.,  on  eyes  of  cave- 
animals,  I,  648-9,  693. 

Paget,  Sir  J.,  blood  changes  in 
small-pox  and  scarlatina,  I,  221, 
701. 

Palaeontology:  distribution  in  time, 
I,  404-11,  412;  special  creation,  I, 
425;  congruity  with  evolution  hy- 
pothesis, I,  485-9,  556;  relations 
of  present  to  extinct  species,  II, 
10-11;  scarcity  of  remains,  II, 
34-5;  secondary  thickening  in 
plants,  II,  56;  Cope  on  osteology 
of  Permian  Vertebrates,  II,  225-6. 

Pangenesis,  Darwin's  theory  of,  I, 
356,  357,  359,  360,  362,  372. 

Panmixia,  Weismann's  hypothesis 
of:  its  relation  to  Romanes'  "  ces- 
sation of  selection,"  I,  560;  al- 
leged selective  process  denied,  I, 
561-3,  667,  685;  distribution  of 
tactual  perceptlveness,  I,  608; 
rudimentary  eyes  of  cava-fauna, 
I,  612-3,  647;  Romanes  on  pro- 
cess, I,  649,  667;  degeneration  of 
self-feeding  instinct  in  Amazon- 
ants,  I,  660-2,  670;  rudimentary 
limbs  of  whale,  I,  668-9,  685;  a 
pure  speculation,  I,  671;  markings 
on  leg-bones  of  Punjabis,  I,  689. 

Puramcecium:  parasite  infesting,  I, 
427;  reproduction,  II,  443,  452. 

Parasites:  sexual  dimorphism,  I, 
315;  limits  to  distribution,  I,  397; 
special-creation  and,  I,  427-9,  438; 
retrograde  development,  I,  457, 


II,  12;  aphis  and  ant,  I,  660-1,  II, 
403,  405;  as  an  integrating 
agency,  II,  402-4;  its  comparative 
recency,  II,  404;  nutrition  and 
genesis  in  vegetal,  II,  486;  in  ani- 
mal, II,  487-90,  493;  "  castration 
parasitaire "  in  crustaceans,  II, 
493-6. 

Parasol  Ants,  origin  of  classes,  I, 
687^8. 

Parthenogenesis:  occurrence,  I, 
274-5;  alternating  with  gamo- 
genesis,  I,  289-91;  Owen  on,  I, 
592;  laws  of  multiplication,  II, 
415;  in  articulate  animals,  II,  445. 

Pasteur,  L.,  silk-worm  diseases,  I, 
622-3. 

Peacock :  theories  of  heredity  and 
structure  of  tail  feather,  I,  372-3, 
695,  II,  618-9. 

Pear,  foreright  shoots,  I,  287. 

Peloria:  in  gloxinia,  II,  166;  phaeno- 
ganis,  II,  180. 

Penguin,  dermal  structure,  II,  314. 

Pepsin,  I,  69. 

Pericyclic  libres  of  monocotyledons, 
II,  278. 

Pcripatus  capenste,  protoplasmic 
continuity,  I,  629. 

Peri-visceral  sac,  function  and  dif- 
ferentiation, I,  391. 

Perkin,  W.  II.,  I,  vi. 

Persistence  of  force,  corollaries 
from:  properties  of  compounds, 
I,  3;  organic  transformation,  I, 
60;  growth,  I,  150;  organic  en- 
ergy, I,  220;' variation,  I,  335; 
genesis,  heredity,  and  variation, 

I,  354-5;  morphological  summary, 

II,  235;  vegetal  tissue  differentia- 
tion, II,  245;  physiological  devel- 
opment,  II,  394. 

Petals:  foliar  homology,  II,  43-6; 
"  adnate,"  II,  58. 

Petrels,  Darwin  on,  I,  455. 

Phssnogams:  production  of  sperma- 
tozoids,  I,  186;  morphological 
composition,  II,  37-79;  leaf  tran- 
sitions, II,  37-42;  foliar  homolo- 
gies,  II,  42-9;  origin  of  type.  II, 
49-84;  vertical  growth,  II,  r>6-.f.4; 
axillary  buds,  II,  66;  cotyledon- 
ous  germination  and  endogenous 
growth,  II,  69-72;  axial  honiolo- 


SUBJECT-INDEX. 


653 


gies,  II,  73-5;  irregular  develop- 
ment, II,  75-8;  degree  of  com- 
position, II,  78;  reproductive  ho- 
mology,  II,  80-^;  uni-  and  multi- 
axial  symmetry,  II,  141-3;  unit  of 
composition,  II,  151;  helical 
growth,  II,  181;  secondary  thick- 
ening, II,  247;  tissue  and  leaf  dif- 
ferentiation, II,  247-9,  387;  also 
bark  and  cambium,  II,  249-50, 
386;  also  outer  tissue,  II,  252, 
256-9,  270,  386-7;  wax  deposit  on 
leaves,  II,  260-2;  differentiation 
of  inner  tissues,  II,  273-5,  388; 
vascular  system  development,  II, 
280-4,  388;  integration,  II,  293-5, 
296,  390;  insect-fertilization,  II, 
407;  multiplication,  II,  441,  442; 
genesis  and  growth,  II,  451,  457; 
and  development,  II,  464;  and  nu- 
trition, II,  476,  477,  511;  substi- 
tution of  axial  for  foliar  organs, 
II,  541-7. 

Phenomenon,  life  manifested  as,  I, 
580.  . 

Philology  (see  Language). 

Phoronis,  individuality,  II,  444. 

Phosphorescence,  organic,  I,  49. 

Phosphorus:  allotropic,  I,  4;  in  cell 
physiology,  I,  259-62;  cerebral  ac- 
tivity, I,  596-7;  organic  evolution, 
I,  703. 

Photogenes,  visibility  of,  I,  218. 

Phylogeny:  as  interpreting  mor- 
phology, II,  10-12;  difficulties  of 
affiliation,  II,  34-5.  (See  Embry- 
ology and  Evolution.) 

Physiological  Units:  -definition,  I, 
226;  genesis,  I,  280-1,  316;  hered- 
ity, I,  315-9;  variation,  I,  330, 
331-2,  333,  II,  619:  stability,  I, 
340:  II,  614;  self-fertilization,  I, 
342-4,  353;  interbreeding,  I,  345, 
353,  II,  615:  recapitulation  of  hy- 
pothesis, I,  350-5;  H,  612-7; 
structural  proclivities/ I,  3G2,  364, 
369-71,  II.  613,  622'.'  sociological 
analogy,  I,  364,  II,  620;  complex- 
ity in  organized  types.  I,  368-70; 
re-named  "  constitutional  units," 
I,  369;  telegony,  I,  650;  "  me- 
chanical theory,"  I,  701-6;  mor- 
phological development,  II,  7-9; 
cell  doctrine.  II,  17-21:  develop- 


ment, II,  76;  "false-joints,"  II, 
371-2;  dissociation  of  ancestral 
traits  in  hybrids,  II,  616-7;  in- 
heritance of  acquired  characters, 
II,  618-23. 

Physiological  division  of  labour 
(see  Labour). 

Physiological  Selection,  I,  509-71. 

Physiology:  and  psychology,  I,  127; 
subdivisions,  I,  128:  vicarious 
function,  I,  208;  primitive  inter- 
pretations, I,  417;  multiplication 
of  effects  exemplified,  I,  512,  II, 
390;  relations  to  morphology,  II, 
3,  239-41;  evolutionary  interpre- 
tation of  phenomena,  II,  241-5, 
384-95;  ultimate  inconceivability 
of  processes,  II,  372;  correlated 
integration  and  differentiation,  II, 
373. 

Physiology,  Animal:  metabolism,  I, 
67-77;  vertebrate  internal  sym- 
metry, II,  108;  tissue  differentia- 
tion in  Protozoa,  II,  299,  385;  pri- 
mary tissue  differentiation,  II,  300- 
2,  382,  389;  natural  selection  and 
tissue  differentiation,  II,  304-8; 
outer  tissue  in  Cozlenterata,  II, 
309-10;  respiratory  organs,  II, 
310-1,  333-8;  differentiation  of 
animal  epidermic  tissue,  II,  312-4, 
387;  development  of  tegumeutary 
organs,  II,  314-6;  of  sensory,  II, 
317-20:  inner  and  outer  tissue 
transition,  II,  321-2,  389;  aliment- 
ary canal  differentiation,  II,  323- 
5;  gizzard  development  in  birds, 
II,  325;  alimentary  canal  of  rumi- 
nants, II,  327-9;  differentiation  of 
liver,  II,  329-33;  of  animal  vascu- 
lar system,  II,  339-44;  of  osseous 
system,  II,  344-55 ;  of  nerve  tissue, 
II,  355-61;  of  muscle,  II,  361-9; 
differentiation  and  integration,  II, 
373-6;  in  vascular  system,  II,  376- 
9,  383;  in  nerves,  II,  379-82;  origin 
of  development,  II,  384;  differen- 
tiation and  instability  of  homo- 
geneous, II,  384-9,  392:  summary 
of  development,  II,  384-94;  mul- 
tiplication of  effects,  II,  390-1, 
392;  equilibration,  II,  391-4.  (See 
also  Function.) 

Physiology,    Plant:   metabolism,    I, 


654 


SUBJECT-INDEX. 


62-7;  tissue  differentiation  in  sec- 
ondary aggregates,  II,  246,  385; 
in  phsenogams,  II,  247-9,  386;  in 
bark  and  cambium,  II,  249-50, 
386;  in  free  and  fixed  surfaces,  II, 
251-6,  270,  386;  outer  stem  and 
leaf  tissue,  II,  256-0,  270,  386;  su- 
perficial differentiation  in  leaves, 
II,  260-4,  270,  387;  floral  tissue 
differentiation.il,  265-9;  outer  tis- 
sue, resume,  II,  270:  inner  tissue 
differentiation,  II,  273-5,  388; 
supporting  tissue,  II,  275-9,  285-8, 
388;  vascular  system  develop- 
ment, II,  273-5,  279-84,  2S5-8, 
388:  inner  tissue,  summary,  II, 
288-91,  388;  integration,  II,  292-8; 
differentiation  and  instability  of 
homogeneous,  II,  384-9,  392;  mul- 
tiplication of  effects,  II,  390-1, 
392;  equilibration,  II,  391-4;  cir- 
culation and  wood  formation,  II, 
564-97;  dye  permeability,  II,  569- 
74,  577-81,  584,  586.  (See  also 
Function.) 

Pickering,  J.  W.,  on  artificial  pro- 
teids,  I,  39. 

Pig:  colour  of  muscles,  I,  365-6; 
telegony,  I,  627;  fertility  of  do- 
mestic and  wild  sow,  II,  479-80. 

Pigeons:  food  of  starving,  I,  215; 
heredity  and  variation,  I,  305, 
321,  615;  atavism,  I,  314;  fertility, 
II,  471-2,  478. 

Pike,  unceasing  growth,  I,  154,  292. 

Pique-gouffe,  commensal  relations 
with  buffalo,  II,  403. 

Plagiochila,  evolution  of  stem,  II, 
62. 

Planaria:  integration,  II,  101-2; 
Morgan  on  regeneration,  II,  102, 
611;  segmentation,  II,  107;  sym- 
metry, II,  195:  unintegrated  func- 
tion, II,  373. 

Plants:  influence  of  heat,  I,  29; 
effect  of  solar  rays,  I,  31-6,  500, 
557;  chemical  composition,  I,  40- 
1;  heat  generation,  I,  47;  phos- 
phorescence, I,  49;  electricity,  I, 
51;  sensible  motion,  I,  56-7,  58; 
metabolism,  I,  62-7,  70;  vital 
changes,  I,  86,  87,  91,  94;  simula- 
tion by  crystals,  I,  96;  vital  ad- 
justments, I,  102:  length  and  com- 


plexity of  life,  I,  103-4;  biological 
classification,  I,  125;  growth.  I, 
136,  138,  140,  143,  145-9,  153,  160- 
1,  II,  401-2;  development,  I,  163- 
5,  167-70,  272;  weight,  tempera- 
ture, and  self-mobility,  I,  174; 
function,  I,  174-8;  structure,  I, 
194-6,  II,  21;  animal  structure 
contrasted,  I,  196;  function  and 
structure,  I,  200;  vicarious  func- 
tion, I,  208-9;  waste  and  re- 
pair, I,  213,  220;  physiological 
units,  I,  225-6,  317,  3GO;  adap- 
tation, I,  227;  what  is  an  in- 
dividual? I,  244-6,  250-1;  gene- 
sis, I,  270,  271,  272-3,  274,  276-8, 
279-85;  relation  of  nutrition  to 
growth  and  genesis,  I,  284-9,  294, 
295-300,  642,  II,  39;  ovule  homo- 
logues,  I,  288;  natural  selection, 
I,  294-8,  532,  533,  II,  51;  heredity, 

I,  301-4,  308,  358-GO;  variation,  I, 
320,  323-4,  325-6;  fertilization,  I, 
340-5;     classification,     I,     377-80, 
389-90;    distribution,    I,    396-400, 
401-3,   404-12,   478-9,   556;   special 
creation    and   parasitism,    I,    428; 
evolution  hypothesis,   I,   434,  443, 
449-50;     rudimentary    organs,     I, 
474,  475,  556;  varied  media,  I,  484, 

II,  32;  alien  and  native  species  in 
New  Zealand,   I,   477;   E.   Darwin 
and  Lamarck  on  evolution  of,   I, 
490-8;  geologic  changes  affecting, 
I,   501-3,  557;  interdependence  of 
animals    and,    I,    504-6,    514,    II, 
398;   complexity  of  influences  af- 
fecting,   I,    506;    direct   equilibra- 
tion,   I,    523-5;    indirect,    I,    532, 
533;    seed    distribution,    I,    546; 
wood  development,  II,  285-7,  289, 
567-97;  Interdependence,   II,.  402- 
3,  404;  insect  relations,  II,  406-7; 
adaptation  and  multiplication,  II, 
411-6;    rhythm    in    numbers,    II, 
419;  growth  and  asexual  genesis, 
11,439-42;  growth  and  sexual  gene- 
sis, 11,448-51;  expenditure,  II,  467; 
horticulture,  nutrition,  and  gene- 
sis, II,  477;  tree  development,  II, 
553;  circulation  and  wood  forma- 
tion, II,  567-92;  dye  permeability 
and  circulation,  II,  569-74,  577-81, 
584,    586;    rtitume    on    circulation 


SUBJECT-INDEX. 


655 


and  wood  formation,  II,  592-7. 
(See  also  Multiplication,  Mor- 
phology, and  Physiology.) 

Plasmodium,  dissolution  of,  I,  185. 

Plato,    iS«a  of,   II,   530. 

Platyhclminthcs:  transverse  fission, 
II,  101;  segmented  and  non-seg- 
mented types,  II,  102,  107;  sym- 
metry, II,  195,  197;  multiplication 
and  growth,  II,  488-9. 

Plethora,  fertility  and,  II,  480-4, 
511. 

Plcitrococcaccce,  unicellular  form,  II, 
21,  134. 

Plcuronectidw:  symmetry  and  loca- 
tion of  eyes,  II,  205;  outer  tissue, 
II,  387. 

Plumatclla:  metagenesis,  I,  277; 
symmetry,  II,  195. 

Podostcmacca;,  undeveloped  circula- 
tory system,  II,  274. 

Polar  bodies,  hypothesis  concerning 
extrusion  of,  I,  266-8. 

Polarity,  organic,  of  physiological 
units,  I,  226,  315,  317,  332,  350-1, 
701-6. 

Polyatomic  compounds  (see  Chem- 
istry). 

Polych&tw,  anomalous  development 
in  Myrianida,  I,  361. 

Polycytharla,  integration,  II,  90, 
124. 

Polygastrica,  aggregation,  I,  586. 

Polymerism:  of  compounds,  I,  9,  11, 
25;  nerve  tissue,  II,  356. 

Polypori,  symmetry  aiid  environ- 
ment, II,  139. 

Polyps  (see  Ccelenterata). 

Polyzoa:  size,  I,  140;  multiaxial  de- 
velopment, I,  165;  structural  in- 
definiteuess,  I,  173:  functional  dif- 
ferentiation, I,  202;  trochophoral 
kinship,  I,  447;  integration,  II, 
93-4,  96,  124;  symmetry,  II,  194, 
207;  vascular  system,  II,  340; 
gemmation,  II,  444. 

Poor  Laws,  and  natural  selection, 
II,  532. 

Population,  A  Theory  of,  I,  265,  577- 
601,  II,  411. 

Potato:  simulated  growth,  I,  136; 
vicarious  function  of  tuber.  I,  209, 
II,  255:  sub-species,  I,  302;  dye 
absorption,  II,  279. 


Preservation:  fertility  and  self-,  I, 
581;  II,  423,  430;  nutrition,  II, 
493. 

"  Progress;  its  Law  and  Cause," 
theory  of  species  differentiation, 

I,  568. 

Projectiles,   factors  In  flight  of,   I, 

450-1. 
Proteids:  metabolic  function,  I,  67, 

68,     69,     72,     76;     complexity    of 

molecule,  I,  122. 
Protein:  evolution,  I,  23,  24;  isomer- 

Ism,  I,  700,  703,  704. 
Proteus,  degeneration  of  eye,  I,  613. 
Protodrilus,  intestine  segmentation, 

II,  125. 

Protophyta:  internal  movements,  I, 
56;  limit  of  growth,  I,  138;  devel- 
opment, I,  164;  structure,  I,  173, 
181-3;  self-mobility,  I,  175;  indi- 
viduality, I,  245;  multiplication, 
I,  270,  276,  279,  581,  584-5,  II, 
439,  462;  genesis  and  nutrition,  I, 
i95;  unicellular,  II,  21;  central  ag- 
gregation, II,  24;  symmetry,  II, 
134;  tissues,  II,  244,  249;  primary 
differentiation,  II,  385;  primordial 
type,  II,  398;  symbiosis,  II,  400. 

Protoplasm:  self -increasing  func- 
tion of  primordial,  I,  63-4;  plant 
metabolism,  I,  65-7;  complexity, 
1,122,253-5;  differeutiatkm  in  sim- 
ple organisms,  I,  182-3;  continuity 
and  inter-circulation,  I,  190-2, 
371,  629,  II,  21,  620;  "  streaming," 

I,  253;   structure,   I,   253-5.      (See 
also  Cell.) 

Protozoa:  inorganic  components,  I, 
17;  locomotion,  I,  58,  175,  II,  14; 
vital  changes  shown  by,  I,  94; 
limitation  of  growth,  I,  138;  de- 
velopment, I,  164;  structure,  I, 
173,  181-3;  incipient  differentia- 
tion, I,  198,  391,  II,  299,  309;  mul- 
tiplication, I,  270,  276,  279,  280, 
582,584;  11,442,451-2;  genesis  and 
nutrition,  I,  295;  distribution,  I, 
396;  parasites  infesting,  I,  427; 
Weismann's  hypothesis  of  immor- 
tality, I,  637;  "  spontaneous  gen- 
eration," I,  697-701;  non-nucle- 
ated, II,  20;  primary  aggregate, 

II,  86-7,  124:  progressing  integra- 
tion, II,  89-91,  124;  symmetry,  II, 


656 


SUBJECT-INDEX. 


186;  primordial  plant-animal  type, 
II,  397-8:  symbiosis,  II,  400. 
Protyle,  hypothetical  chemical  unit, 

I,  22,  23. 

Pseud-axial    development,    vegetal, 

II,  28-9,  30. 

Pseudo-foliar  development,  vegetal, 
II,  26-8,  30. 

Psychidw:  parthenogenesis,  I,  275; 
sexual  dimorphism,  I,  683. 

Psychology:  reasoning  and  defini- 
tion of  life,  I,  81^8;  correspond- 
ence shown  by  recognition,  I,  95; 
contrasted  with  physiology,  I, 
127;  departments  of,  I,  127-8; 
vicarious  function,  I,  209;  waste 
and  repair  In  sensory  organs,  I, 
217;  sensory  adaptability,  I,  229, 
231,  232;  inheritance  of  sensory 
defects,  I,  306;  musical  talent,  I, 
311-2;  intellectual  progress  and 
special-creation  hypothesis,  1,417; 
special-creation  a  pseud-idea,  I, 
420,  429,  433,  554;  legitimacy  of 
evolution-hypothesis,  I,  433-5, 
439,  554;  embryology  of  ideas,  I, 
450,  457;  persistent  formative 
power  unrepresentable,  I,  492;  E. 
Darwin's  and  Lamarck's  theory  oJ 
desires,  I,  494;  natural  selection 
and  brain  evolution,  I,  553;  gene- 
sis and  cerebral  activity,  I,  594, 
II,  512^,  516-9,  530;  heredity  and 
distribution  of  tactual  perceptive- 
ness,  I,  602-8,  646,  665-6,  672, 
692;  inconceivability  of  the  nega- 
tion, I,  675;  vitiation  of  evidence, 
II,  88;  repetition  and  perception, 
II,  143;  differentiation  of  sensory 
organs,  II,  317-20;  differentiation 
of  nerve  tissue,  II,  355-61:  func- 
tional integration,  II,  376;  also  in- 
tegration, II,  380-2;  equilibration 
of  nerve  discharge,  II,  393;  human 
fertility  and  nerve  development, 
II,  466,  532;  future  human  evolu- 
tion, II,  523-5,  527;  human  evolu- 
tion and  genesis,  II,  529-31;  future 
mental  development,  II,  535;  origin 
of  vertebrate  type,  II,  598-600. 

Pteridophyla:  size  attained  by,  I, 
138,  139;  homologies,  II.  80-1,  82; 
frond  surface  differentiation,  II, 
200. 


Ptcropoda:  bilateral  symmetry,  II, 
201;  dermal  respiration,  II,  310. 

Ptyaline,  metabolic  function,  I,  69. 

Punjabis,  inheritance  of  acquired 
osteological  peculiarities,  I,  689. 

Pyrosomidcc :  phosphorescence,  I,  47; 
integration,  I,  588,  II,  97. 

QUAGGA,  telegenic  transmission  of 
markings  to  offspring  of  mare,  I, 
624,  627,  646. 

Quills,  development,  II,  314-6. 

RABBIT:  activity  and  muscle  col- 
our, II,  365;  over-running  checked 
by  weasels,  II,  405;  expendi- 
ture and  genesis,  II,  472. 

Radial,  definition,  II,  148. 

Radiolaria:  unicentral  development, 

I,  163;  secondary  aggregation,  II, 
88;  symmetry,  II,  187. 

Radula,  development  of  roots  from 
leaflets,  II,  34. 

Rafflcsiacew:  homogenesis,  I,  272; 
tissue  differentiation,  II,  274;  nu- 
trition and  genesis,  II,  486. 

Rat  (see  Rodentia). 

Rathke,  H.,  on  vertebrate  embryo, 

II,  119. 

Ray,  J.,  plant  classification,  I,  378. 

Reasoning,  compared  with  assimila- 
tion, I,  81-7. 

Recapitulation,  embryological,  I, 
453. 

Regeneration  (see  Repair). 

Rejuvenescence,  and  sexual  fertili- 
zation, I,  637;  II,  613. 

Remak,  R.,  vertebrate  embryo,  II, 
120. 

Repair:  continuity  of,  I,  216-9;  ani- 
mal injuries,  I,  219,  222-4,  II,  102, 
611;  deductive  interpretation,  I, 
221-2;  theories  of  heredity  and 
regenerative  phenomena,  I.  360-1. 

Repetition  of  like  parts.  II.  126. 

Reproduction  (see  Multiplication). 

Rcptilia:  growth  and  expenditure  of 
force,  I,  142;  sizes  of  ova  and 
adult,  I,  144;  longevity  of  croco- 
dile, I,  154;  temperature,  I,  174; 
waste,  I,  214;  distinctive  charac- 
ters, I,  392;  distribution  in  time, 
I,  409,  412;  vertebral  segmenta- 
tion, I,  470;  rudimentary  limbs  of 


SUBJECT-INDEX. 


657 


s,  I,  473;  fertility  and  devel- 
opment, I,  583,  598,  599;  regenera- 
tion, I,  589;  elongated  form,  II, 
15;  supernumerary  vertebrae,  II, 
123,  564;  bilateral  symmetry,  II, 
203,  204;  Cope  on  segmentation  in 
extinct,  II,  225,  226;  activity  and 
muscular  colour,  II,  365;  func- 
tional integration,  II,  375;  outer 
tissue  differentiation,  II,  387; 
Owen  on  skeleton,  II,  560. 

Resistance  of  media  to  locomotion, 
II,  15. 

Respiratory  System:  effect  of  light, 

I,  31;   organic  re-arrangement,   I, 
37;  cutaneous,  I,  209;  air-cells  of 
lungs,  I,  254;  embryonic  branchiae 
of    salamander,    I,    457;    differen- 
tiation,   II,    310-1,    333-8;   physio- 
logical integration,  II,  374-5,  382; 
vascular  differentiation  and  inte- 
gration, II,  377. 

Retrograde  metamorphoses,  in  ani- 
mals, II,  12. 

Retzius,  G.,  superficial  nerve-end- 
ings, I,  666. 

Reversed  Selection,  I,  611,  612. 

Khabdospheres,  calcareous  armour 
and  dynamic  element  in  life,  I, 
119. 

Rhizoids,   foliar  expansions,   II,  50. 

RMzopoda:  structure,  I,  173;  undif- 
ferentiated  function,  I,  200;  a  pri- 
mary aggregate,  II,  86;  symmetry, 

II,  186;   tissue  differentiated,   II, 
299,    385;    motion   of   sarcode,    II, 
356;  symbiosis,  II,  400. 

Rhythm:  astronomic  and  organic,  I, 
499,  557;  law  of  equilibration,  I, 
520-1;  in  multiplication,  II,  419. 

Richeraud,  Baron  A.,  definition  of 
life,  I,  79. 

Riley,  C.  V.,  on  telegony,  I,  645; 
Termites,  I,  680,  681;  pouch  of 
Honey-ants,  I,  684. 

Rodentia:  incursions,  I,  399;  Ameri- 
can types,  I,  403;  fertility  and 
development,  I,  583,  599. 

Rivinus,  plant  classification,  I,  S77. 

Rokitansky,  on  false  joints,  I,  230. 

Romanes,  G.  J.:  on  "cessation  of 
selection,"  I,  560-2;  isolation  and 
species  differentiation,  I,  569; 
"  physiological  selection,"  I,  569- 


71;  panmixia,  I,  649,  667;  influence 
of  a  previous  sire  on  progeny,  I, 
649. 

Rontgen  rays,  I,  121,  II,  621. 

Roots:  developed  from  leaflets,  II, 
34;  physiological  differentiation, 
II,  253-5,  270;  nutrition  from 
leaves,  II,  274;  size  and  function, 
II,  276. 

Rotiferw:  latent  vitality  of  desic- 
cated, I,  117;  trochopore,  II,  108, 
109;  molluscan  relationship,  II, 
115;  fertility  and  size,  II,  453,  459. 

Roux,  W. :  "intra-selection,"  I,  676; 
functional  adaptation,  II,  354. 

Rudimentary  organs:  the  definition 
of  life  and,  I,  112,  natural  selec- 
tion and  eyes  of  cave  fauna,  I, 
309,  612-4,  647-9,  693;  evolution 
hypothesis,  I,  472-5,  556;  limbs  of 
whale,  I,  668-9,  685,  693. 

Ruminants,  alimentary  canal  de- 
velopment, II,  327-9. 

SALAMANDER,   embryonic  branchiae, 

I,  457. 
Salmonidce,  reproduction  and  growth, 

I,  291-3,  II,  454. 

Salpidce:  heterogenesis,  I,  272,  277; 
integration,  I,  588,  II,  97. 

Sap  (see  Vascular  system). 

Sarcina:  central  aggregation,  II,  24; 
fertility,  II,  440. 

Savage,  Dr.,  on  "  Heredity  and 
Neurosis,"  I,  313. 

Scenedesmus,   individuation,  II,  24. 

Scent:  natural  selection  and  keen- 
ness of,  I,  610;  floral  fertilization, 

II,  268-9;   animal   protection,    II, 
434. 

Schelling,   E.   W.  J.  von,   definition 

of  life,  I,  78,  178. 
Schleiden,   J.   M.,   on  individuality, 

I,  245;  on  liverworts,   II,   50,   52; 

algal  indefiniteness,  II,  296. 
Science,   complex  revelations  of,  I, 

252,  369,  450. 
Scyphomedusw,      strobilization,      II, 

108. 
Sea:  changes  and  movements  in,  I, 

83;  life  in,  lower  than  terrestrial, 

I,    104;    distribution,    I,   396,    517; 

change   of   media    caused   by,    I, 

481;  geologic  influence,  I,  502. 


658 


SUBJECT-INDEX. 


Seals:  nail-bearing  toes,  I,  473;  vi- 

brissw,  II,  317. 
Seasons:    reproductive    periodicity, 

I,  299;  variations  of  genesis  with, 

II,  484-5. 

Sedgwick,  Adam:  on  continuity  of 
protoplasm  in  animals,  I,  190, 
629,  II,  21;  zoological  classifica- 
tion, I,  387;  discrimination  of  spe- 
cies in  embryonic  stages,  I,  461; 
persistence  of  ancestral  traits,  I, 
463-4;  Archiannelidan  segmenta- 
tion, II,  109. 

Sedgwick,  Wm.:  heredity  and  sex, 
I,  305,  314;  telegenic  transmission 
of  hypospadias,  I,  646. 

Seeds:  nitrogenous,  I,  40;  tempera- 
ture of  germinating,  I,  47,  II,  615 ; 
vitalism  and  latent  vitality  of,  I, 
116-7;  variation  in  environment, 
I,  327;  natural  selection  among,  I, 
532. 

Segmentation  (metameric):  special 
creation  hypothesis,  I,  468-9; 
Huxley  on  number  of  somites  in 
higher  articulates,  ib.;  in  annu- 
lose  animals,  II,  98-110,  111-5, 
601-5;  simulated  molluscan,  II, 
116;  in  vertebrates,  II,  125-7, 225-7, 
606-7;  in  elasmobranchs,  II,  126. 

Segregation:  of  growth,  I,  136;  of 
like  units,  I,  179;  organic  repair, 
I,  221;  variation,  I,  331,  334; 
heterogeneity,  and  deflniteness  of 
evolution,  I,  514-6,  517-8;  mor- 
phological development,  II,  7-9; 
physiological  units,  II,  616. 

Self-fertilization,  animal  and  vege- 
tal, I,  341-4,  353. 

Senses,  the  (see  Psychology). 

Sex:  in  Ascidian  colonies,  I,  247; 
limitation  of  heredity  by,  I,  314-6; 
correlated  traits,  I,  371-2,  513;  nu- 
trition and  determination  of,  in 
social  insects,  I,  655-€0,  678-84, 
686-9;  neural  and  haemal  traits,  I, 
683;  differentiation  of  organs,  II, 
303;  castration  and  growth,  II, 
459;  Julin  on  "  castration  parasi- 
taire  "  in  crustaceans,  II,  493-6; 
the  object  of  fertilization,  II,  613. 
(See  also  Fertilization.) 

Sexual  Selection  (see  Natural  Selec- 
tion). 


Sharp,  D. :  on  insect  somites,  I,  469; 
food  habits  of  Termites,  I,  686-7. 

Sheep:  contrasted  with  oxen,  I, 
158,  160;  crossing  of  English  and 
French  breeds,  I,  625;  nutrition 
and  genesis,  II,  480. 

Sherrington,  Prof.,  on  effects  of 
nerve  severance,  I,  349. 

Ship-building,  interdependence  of 
social  functions,  I,  237-9,  241. 

Shipley,  A.  E.:  segmentation  of 
Microstomida,  II,  102;  Protodrilus, 
II,  125. 

Silica,  colloid  and  crystalloid,  I,  16. 

Silicic  acid:  properties,  I,  16;  isom- 
erism,  I,  59. 

Silicon,  allotropic,  I,  4. 

Silkworm  disease,  I,  622-3. 

Simulation:  of  homology  by  anal- 
ogy, II,  14,  485;  of  segmented 
structure  by  molluscs,  II,  116. 

Siphonophora,  specialization  of  com- 
ponent polyps,  II,  95. 

Sirenia,  simulated  fish  form,  I,  485. 

Size  (see  Growth). 

Skeleton,  vertebrate  (see  Vcrte- 
brata). 

Skin:  respiratory  function,  I,  209; 
adaptability,  I,  228,  II,  312-4, 
387;  transmitted  peculiarities,  I, 
306;  Wallace  on  distribution  of 
sensitiveness,  I,  646-7;  differen- 
tiation, II,  215,  217,  304-7;  tegu- 
mentary  development,  II,  314-6, 
387;  differentiation  of  sensory  or- 
gans, II,  317-20;  and  mucous 
membrane,  II,  303-4,  321-2,  389. 

"  Skin  friction,"  and  locomotion  of 
aquatic  animals,  I,  156. 

Skull  (see  Vcrtcbrata). 

Sleep,  repair  favoured  by,  I,  216. 

Small-pox,  blood  changes  from,  I, 
221. 

Smith,  Prof.  W.,  on  fertility  of 
diatomacccr,  II,  440. 

Smith,  W.  P.,  on  telegony  in  calves 
and  foals,  I,  645. 

Smith,  W.  W.,  on  habits  of  Tetra- 
morium,  I,  660. 

Snakes  (see  Rcptilia). 

"  Social  organism,"  author's  essay 
on,  I,  363,  676. 

Sociology:  environment  and  degree 
of  life,  I,  105-6;  functional  dif- 


SUBJECT-INDEX. 


659 


ferentiation,  I,  204;  division  of 
labour,  I,  207,  363-4,  367;  func- 
tional interdependence,  I,  237-9, 
240-2;  autogenous  development  of 
units  in  colonies,  I,  364,  367-8,  II, 
620;  belief  in  social  evolution,  I, 
432;  natural  selection,  I,  553,  II, 
532;  integration  and  differentia- 
tion, II,  378-9;  effects  of  popula- 
tion, II,  535-6;  equilibration,  II, 
537. 

Soil,  dependence  of  plant  evolution 
on,  II,  402. 

Solarium  jasminoidcs,  organs  of  at- 
tachment, II,  276. 

Solar  system,  autogenous  develop- 
ment illustrated  by  distribution 
of  forces  in,  I,  366. 

Sole,  symmetry  and  location  of 
eyes,  II,  205. 

Soma-plasm,  Weismann's  theory  of 
differentiation  from  germ-plasm, 
I,  357,  622,  628-30,  633^4. 

Somites  (see  Segmentation). 

Special  creation:  and  evolution,  I, 
412,  415,  431;  improbabilities,  I, 
418-9,  430,  439,  554;  inconceiva- 
bility, I,  420,  429,  431,  554;  of  in- 
dividuals and  species,  I,  421-4; 
the  implication  of  beneficence,  I, 
425-9;  summary,  I,  429,  554;  Von 
Baer's  formula,  I,  451-6;  verte- 
brate skeleton,  II,  551,  556,  565. 

Species:  adaptation  and  stability,  I, 
242;  hereditary  transmission,  I, 
301-4;  variation  in  wild  and  culti- 
vated, I,  323-5,  326,  693;  ga  mo- 
genesis  and  life  of,  I,  347-9;  phy- 
siological units,  I,  362,  364,  369- 
71,  458,  II,  613;  indefiniteness,  I, 
389,  445,  572;  special  creation,  I, 
422-4;  instability  of  homogeneous, 
and  differentiation  of,  I,  509-11, 
515,  517-8,  550,  557;  persistence 
of,  I,  516,  518,  II,  10-11;  natural 
selection  and  equilibration,  I, 
543-8,  553,  557;  non-adaptive  char- 
acters, I,  565;  morbid  products  as 
marks  of,  I,  567;  migration  and 
isolation  as  causes  of  differentia- 
tion, I,  568-9;  increasing  multi- 
formity of  aggregate,  II,  396. 

Specific  gravity,  of  organisms  and 
environment,  I,  174,  177. 


(see  Fertilization). 


Spermatozoa 

Sperm-cell 

Sphere:  tendency  of  units  to  form, 

I,  15;  the  embryonic  form,  I,  177; 
symmetry,  II,  131. 

Spheroid,  symmetry,  II,  132. 

Spiders   (see  Arachnida). 

Spine  (see  Tcrtebrata). 

Sponge:  structure  and  dynamic  ele- 
ment in  life,  I,  119;  multicentral 
development,  I,  164;  units  and  ag- 
gregate, I,  185;  reproductive  tis- 
sue, I,  283;  integration,  I,  586;  II, 
90,  383;  physiological  differentia- 
tion, II,  300,  386;  development 
and  genesis,  11,463;  analogy  from, 

II,  576. 

Spontaneous  generation:  and  hete- 
rogenesis,  I,  270;  and  evolution, 
I,  696-701,  703. 

Stag,  horns  and  correlated  struc- 
tures, I,  567,  670,  676-7,  692. 

Stamens,  and  foliar  homology,  II, 
44. 

Starches:  properties,  I,  11;  trans- 
formations, I,  66,  68,  69,  70,  II, 
593. 

Star-fishes  (see  Asteroidea). 

Statoblasts,  of  Plumatella,  I,  277. 

Steenstrup,  on  "  Alternate  Genera- 
tion," I,  592. 

Sterility  (see  Multiplication). 

Stickleback:  ova,  II,  454;  bothrio- 
cephalus  in,  II,  490. 

Stomach  (see  Alimentary  canal). 

Stomata,  distribution,  II,  260-1. 

Straight  line,  and  evolution  hypoth- 
esis, I,  433. 

Strain:  compression  and  tension  of, 

I,  151,     II,     209-12;    relation    to 
mass,  I,  155-7;  vegetal  structure, 

II,  574-88,    592-6;    origin   of   ver- 
tebrate type,  II,  600. 

Strawberry:  multiaxial  develop- 
ment, I,  166;  multiplication,  II, 
441. 

Strength,  a  vital  attribute,  I,  578. 

Structure:  appliances  for  generat- 
ing motion,  I,  75-7;  biological 
classification,  I,  125-7,  129;  size 
and  organic,  I,  137;  growth  and 
complexity,  I,  138,  145,  161;  rela- 
tion to  environment,  1, 172-8, 195-6; 
of  unicellular  organisms,  I,  181-3; 


660 


SUBJECT-INDEX. 


multicellular,  I,  183-90;  Hertwig's 
classification  of  tissues,  I,  189; 
continuity  of  units,  I,  190-2;  sys- 
tems of  organs,  I,  192;  division 
into  universal  and  particular,  I, 
193-4;  general  truths,  I,  194-5; 
plant  and  animal,  contrasted,  I, 
195-6;  precedence  of  function  or, 
I,  197,  211;  correlative  complexity 
of  function  and,  I,  200,  211;  pro- 
gressive concomitant  differentia- 
tion, I,  201-4;  physiological  units, 

I,  225-6,  362,  364,  369-71,  II,  613; 
social    and    organic    interdepend- 
ence,   I,   235-42;    varied  by   func- 
tion,  I,  334,  535;  II,  217  (see  Ac- 
quired     Characters);      zoological 
classification,   I,   390-2;   equilibra- 
tion, I,  521,  557;  progress  of,  and 
genesis,  I,  590-1;  II,  462;  coopera- 
tion with   function,   II,   3;   evolu- 
tion  and   increased,    II,   4;   retro- 
grade     metamorphosis,      II,      12; 
simulated    homologies,    II,    13-14; 
earliest    organic    forms,     II,     19; 
cylindrical  vegetal,  II,  57-62;  per- 
manence and  complexity,  II,  295, 
296;   function  and   epidermic,    II, 
312-4,  387;  and  muscular,  II,  369, 
391;  adaptation  and  equilibration, 

II,  392;  persistence  of  force  and 
physiological  adaptation,   II,   394; 
evolution,    II,    501-4.      (See    also 
Morphology.) 

Struggle,  for  nutriment  among  com- 
ponents of  an  organism,  I,  562, 
676;  for  existence  (see  Natural 
Selection). 

Struthers,  Sir  J. :  on  heredity,  I, 
305,  314;  digital  variation,  I,  321; 
rudimentary  limbs  of  whale,  I, 
668. 

Strychnine,  effects  of,  I,  54,  55. 

Sturgeon,  size  of  ova  and  adult,  I, 
144. 

Sugars:  properties,  I,  10-11;  trans- 
formations, I,  38,  40,  66,  69,  70, 
II,  593. 

Suicide,  hereditary  tendency  to,  I, 
307. 

Sulphur:  allotropic,  I,  4,  59;  organic 
evolution,  I,  703. 

Sun  (see  Light). 

Survival  of  the  Fittest,  the  expres- 


sion,   I,    530,    610.      (See    Natural 

Selection.) 

Swan,  vertebrae  of  neck,  II,  123. 
Swiftness,     a     vital     attribute,     I, 

578. 
Syllia  ramose,  lateral  branching,  I, 

166,  361,  II,  105,  108. 
Symbiosis,  II,  399,  400. 
Symmetry  (see  Morphology). 
Syphilis,  hereditary  transmission,  I, 

623. 

TACTUAL  Perceptiveness,  heredity 
and  the  distribution  of,  I,  602-8, 
633,  665,  666,  672,  692. 

Tcenia  (see  Entozoa). 

Tansley,  A.  G.,  I,  vi,  II,  vi;  adapta- 
tion of  reproductive  activity  to 
conditions  in  Algce,  I,  288-9; 
shapes  of  Caulerpa,  II,  22;  stem- 
thickening  in  extinct  Thallo- 
phytes,  II,  56;  natural  selection 
and  leaf-distribution,  II,  179. 

Tape-worm  (see  Entozoa). 

Taste,  dependent  on  chemical  ac- 
tion, I,  54. 

Teeth:  hereditary  transmission,  I, 
306;  suppression  of  mammalian,  I, 
457;  of  uncivilized  and  civilized,  I, 
541,  693. 

Tegumentary  organs,  origin  of,  I, 
314-6. 

Telegony,  or  the  influence  of  a  pre- 
vious sire  on  offspring,  I,  624-7, 
644-3,  649-50. 

Temperature  (see  Heat). 

Tension  (see  Strain). 

Termites:  fertility,  I,  583,  II,  493; 
late  development  of  sexual  or- 
gans, I,  680;  nutrition  and  differ- 
entiation of  forms,  I,  681. 

Tctramorium,  utilization  of  aphides 
by,  I,  660-1. 

Thallophyta:  size,  I,  138,  139;  low 
co-ordination  of  parts,  I,  164; 
pseudo-foliar,  II,  28;  "  transition 
place,"  II,  30;  simulation  of 
higher  types,  II,  32;  secondary 
thickening  in  extinct  species,  II, 
56;  sexual  and  asexual  genesis,  II, 
84.  (See  also  Alga.) 

Tickling,  physiology  of,  I,  76. 

Tide  (see  Sea). 

Time,  as  a  factor  in  growth,  II,  77. 


SUBJECT-INDEX. 


661 


Tissue,  Hertwig's  classification,  I, 
189.  (See  Physiology.) 

Tongue,  perceptiveness  of  tip,  I, 
606-8,  665,  672-3. 

Tortoise:  contrasted  life  of  dog 
and,  I,  103-4;  natural  selection 
and  carapace,  I,  534. 

"  Transcendental  Physiology,"  I, 
176. 

Tree,  as  symbolizing  phylogeny,  I, 
428,  452-3.  (See  Plants.) 

Trematoda:  agamogenesis,  I,  277; 
parasitism,  I,  428;  alternate  gen- 
eration, I,  592. 

Trembley,  A.,  on  the  polyp,  I,  223. 

Trichinosis,  in  Germany,  I,  428. 

Trochophore,  phyletic  relationships 
shown  by,  I,  447,  II,  108-9. 

Tubicola:  development,  II,  100;  bi- 
lateral symmetry,  II,  197. 

Tunicata:  gemmation,  I,  588,  II,  445; 
alternate  generation,  I,  592;  inte- 
gration, II,  93-4;  tertiary  aggre- 
gation, II,  124;  symmetry,  II, 
194-5. 

Tunny,  size  of  ova  and  adult,  I, 
144. 

Turbcllaria:  segmentation,  II,  102; 
symbiosis,  II,  400. 

Turnip:  chlorophyll  in  roots,  I,  209, 
II,  254;  vascular  system,  II,  281, 
284,  578,  591,  596. 

Twins:  similarity  of,  I,  324;  traits 
of  women  bearing,  II,  457. 

"  Types,  persistent,"  Huxley  on,  I, 
408. 

ULCEK,  dermal  structure,  II,  306. 

Ultimate  Reality,  incomprehensibil- 
ity of,  I,  120. 

Viva:  cell  multiplication,  II,  26; 
outer  tissue,  II,  256. 

Umbelliferw:  floral  symmetry,  II, 
171;  axial  and  foliar  organs,  II, 
541-6. 

United  States:  cases  of  telegony,  I, 
644-5;  birth  rate,  II,  520. 

Units:  differentiation  and  dissimi- 
larity, I,  20;  "  protyle,"  I,  22-3; 
shapes  in  higher  types,  I,  164; 
differential  assimilation,  I,  180; 
primordial  organic,  I,  181;  mor- 
phological composition,  I,  184-7, 
194,  252,  II,  5,  7-9,  21,  79,  85-6; 


segregation  and  organic  repair,  I, 
221-2,  222-6;  chemical,  morpho- 
logical, and  physiological,  1, 225-6; 
II,  612;  stability,  I,  339;  instabil- 
ity and  heterogeneity  of  organic, 

I,  350;    Darwin's    gemmules,     I, 
356-60,     362,     372;     Weissmann's 
germ-plasm  (q.  v.)  i&.;  sociological 
comparison,  I,  363-8;  specific  pro- 
clivities   in    embryogeny,    I,    458; 
phaenogamic,  II,  73,  151;  annulose, 

II,  105;    incident    force    and    ho- 
mologous,   II,   159;    morphological 
summary,  II,  233.     (See  also  Phy- 
siological units.) 

"  Universal  Postulate,"  I,  675. 
Unsymmetrical,  definition,  II,  131. 
Urea,  muscular  energy  and  excre- 
tion, I,  72. 

VAN  BENEDEN,  P.  3.,  on  Tania,  II, 
103. 

Variation:  digital,  I,  331;  effects  of 
parental  conditions,  I,  324;  of  al- 
tered function,  I,  325,  334,  693; 
dissimilarity  of  initial  conditions, 

I,  327-32,  333;  "  spontaneous,"  I, 
328,  513,  697,  II,  529;  persistence 
of    force,     I,     335;     physiological 
units,   I,   348-54,    360,    369,    371-3, 

II,  614-7,      622-3;      Weismann's 
germ-plasm      theory,      I,      357-8, 
372-3,  671,  677;  II,  622;  equilibra- 
tion and  vegetal,  I,  523-5;  Weis- 
mann's panmixia  theory,  I,  561-3, 
649,  667-9,  671,  685;  reproductive 
organs,    I,   570;    natural   selection 
and   concomitant,    I,    614-21,    653, 
664,  674,  692;  and  disused  organs, 
I,    648,    668;    plus    and    minus,    I, 
667,    685;   Masters   on   correlated, 
in   plants,    II,    298,   621-2;    equili- 
bration of  favourable,  II,  394. 

Vascular  System:  effects  of  vegeto- 
alkalies,  I,  55;  nutrition,  I,  146, 
148;  embryonic  development,  I, 
169;  structural  traits,  I,  192,  193; 
function,  I,  199;  of  Ascidians,  I, 
202;  functional  differentiation  and 
integration,  I,  205-6;  organic  re- 
pair, I,  217,  221-2;  effect  of  func- 
tion, I,  229,  234-5,  236;  equilibra- 
tion, I,  535;  community  in  com- 
pound organisms,  I,  588;  develop- 


662 


SUBJECT-INDEX. 


ment  of  vegetal,  II,  273-5,  279-84, 
285-8,  388;  differentiation  of,  sum- 
mary, II,  288-90,  388;  differentia- 
tion of  animal,  II,  339-44;  osseous 
development,  II,  347-51;  muscu- 
larity, II,  364;  muscular  colour, 
II,  365-9;  heart-motor  apparatus, 
II,  374;  differentiation  and  Inte- 
gration in  animal,  II,  376-9,  383; 
wood  formation,  II,  567-92;  re- 
sum6  of  wood  formation,  II,  592-7. 

VaucJieria,  reproduction,  I,  279,  289. 

Vegetative  System,  co-ordination  of 
actions  In,  I,  578. 

Vegeto-alkalies,  physiological  ef- 
fects of,  I,  54-5. 

Velocity,  of  moving  bodies,  II,  219- 
20. 

Tertebrata:  size,  I,  139;  size  at 
birth  and  maturity,  I,  144;  axial 
structure,  I,  165;  embryonic  de- 
velopment and  self-mobility,  I, 
175;  functional  differentiation,  I, 
206,  591;  reparative  power,  I,  219, 
223,  589;  homogenesis  universal, 

I,  271;  distinctive  traits,   I,   392, 

II,  35;    distribution    in    time,    I, 
408;   classiflcatory   value,    I,   446; 
embryonic  mammalian  respiratory 
system,  I,  456;  einbryological  pre- 
adaptation,  I,  461;  evolution  and 
vertebral  column,  I,  470;  rudimen- 
tary organs,  I,  473;  evolution  and 
varied   media,   I,   479-85;   size  of 
head  and   vertebra,   I,   512,   537; 
segregation     and      evolution     of 
vertebrae,    I,    515;    fertility    and 
development,  I,  583,  598-9;  Weis- 
mann    on    reproductive    cells,    I, 
635;  limb  locomotion,  II,  15;  adap- 
tive    segmentation,      II,     117-23, 
125-7,    223,    602,    605-7;    supernu- 
merary  vertebrae,   II,    123;   bilat- 
eral symmetry,  II,  203-6;  Internal 
organic  symmetry,   II,  208;  gene- 
sis of  rudimentary  axis,  II,  212-6; 
natural   selection   and   genesis   of 
structure,   II,   216,   227;   origin   of 
notochord,   II,   216-8;   spinal   seg- 
mentation,  II,   218-22,   224;    skull 
development,  II,  222,  227;  r€sum6 
of    axis     development,     II,     224; 
Cope  on  author's  theory,  11,225-7; 
nerve  differentiation,  II,  304;  sen- 


sory organs,  II,  318;  air  chambers, 
II,  334;  osseous  differentiation,  II, 
344-55;  activity  and  muscular  col- 
our, II,  365-9;  heart-motor  appa- 
ratus, II,  374;  cost  of  genesis,  II, 
436;  agamogenesis  unknown,  II, 
445;  growth  and  genesis,  II,  454; 
heat  expenditure  and  genesis,  II, 
468-9,  474;  Owen,  theory  of  skele- 
ton, II,  548-66;  evolution  of  verte- 
brae, II,  563-6;  origin  of  type,  II, 
598-600. 

Vestiges  of  Creation,  I,  491. 

Vibrissae,  function  of,  I,  75. 

Vitalism,  hypothesis  examined,  I, 
114-7. 

Vittadini,  C.,  on  silkworm  disease, 
I,  622-3. 

Viviparous  genesis,  I,  271,  274-5, 
278. 

Voice,  correlated  sexual  traits,  I, 
371-2. 

Volcano,  definition  of  life  and,  I, 
85,  89. 

Volvocinca:  unicentral  development, 
I,  163;  individuality,  I,  245;  disin- 
tegration of  genesis,  I,  276,  587; 
spherical  aggregation,  II,  24;  sym- 
metry, II,  137,  187;  fertility,  II, 
441. 

Vomiting,  alimentary  canal  devel- 
opment, II,  328. 

Vorticella:  secondary  aggregate,  II, 
90;  symmetry,  II,  188. 

WALLACE,  A.  R.:  "The  Origin  of 
the  Human  Races,"  I,  553;  the  ex- 
pression "  Survival  of  the  Fit- 
test," I,  530;  his  association  of 
natural  with  artificial  selection,  I, 
609;  co-adaptation  in  giraffe,  I, 
615;  skin  sensitiveness,  I,  646. 

Wasp:  co-ordination  of  instincts  in 
Mason-,  I,  574,  679-80;  genesis  of 
worker,  I,  654-7. 

Waste,  animal,  I,  69,  213-5,  228;  re- 
lation to  activity,  I,  196,  220-1;  in 
plants,  I,  213,  220. 

Water:  properties,  I,  7,  9;  colloidal 
affinity  for,  I,  28;  organic  change 
from,  I,  29;  organic  need  for,  I, 
147;  proportion  in  mammalian 
adult  and  foetus,  I,  154;  motion 
through,  I,  156;  organic  develop- 


SUBJECT-INDEX. 


663 


merit  and  environment,  I,  -173, 
177,  479;  terrestrial  organisms  in- 
habiting, I,  400;  adaptation  of  or- 
ganisms to  change  of  media,  I, 
479-85;  vegetal  tissue  differentia- 
tion, II,  253;  molecular  rear- 
rangement, II,  359;  colloidal  con- 
traction, II,  361-2. 

Water-weed,  American,  invasion  of, 
I,  399. 

Watts,  Dr.,  on  The  Principles  of  Bi- 
ology, I,  ix. 

Wax,  foliar  deposit,  II,  260-1. 

Weber,  on  tactual  discriminative- 
ness,  I,  602. 

Weight:  relation  to  environment  of 
organic,  I,  174,  177;  varying  as 
cube  of  dimensions,  I,  151,  II, 
434,  470. 

Weismann,  Aug. :  reproductive  tis- 
sue in  Medusce,  1, 281;  in  Daphnidce, 
I,  290;  his  theory  of  the  differ- 
entiated germ-plasm  and  its  fun- 
damental units,  I,  357,  622-3, 
628-30,  633-44,  646,  II,  618-9,  622; 
the  alleged  differentiation  and 
plant-phenomena,  I,  359-60;  and 
regenerative  processes,  I,  360; 
false  joints,  I,  362;  implied  com- 
plexity of  determinants,  I,  370; 
theory  inadequate  to  explain  cor- 
relation of  sexual  traits,  I,  372; 
and  variations  in  peacock's  tail 
feather,  I,  372-3,  695,  II,  618;  his 
view  of  natural  selection  as  sole 
factor  in  organic  evolution,  I,  559; 
the  doctrine  of  panmixia,  I,  561-3, 
612,  632,  649,  667-9,  671,  685, 
689;  arguments  against  inherit- 
ance of  acquired  characters,  I, 
612-3,  651-65,  669-71;  blindness  of 
cave-animals,  I,  613;  current  ac- 
ceptance of  his  views,  I,  631, 
690;  cannot  explain  the  process 
of  natural  selection,  I,  651;  the 
degradation  of  the  little  toe  in 
man,  I,  652,  669,  673;  caste  gra- 


dations of  social  insects,  I,  654, 
658-65,  670,  675,  678-84,  685;  food- 
seeking  instinct  in  Amazon  ants, 
I,  660,  670;  the  co-adaptation  of 
co-operative  parts,  I,  663^1,  670, 
674,  675,  676;  tactual  discrimina- 
tiveness,  I,  665,  672;  intra-selec- 
tion,  I,  676-8;  effect  of  nutrition 
on  fertility  of  blow-fly,  I,  678-9. 

Whale:  weight  of  brain,  I,  599; 
rudimentary  limbs,  I,  668-9,  685, 
693. 

Wheat,  adaptive  variations,  II,  298. 

Whistling,  definition  of  life  and,  I, 
112. 

White-Cooper,  Mr.,  on  inheritance 
of  abnormal  vision,  I,  306. 

Willow,  nutrition  and  growth,  I, 
294. 

Wilson,  E.  B.:  composition  of  chro- 
matin,  I,  260;  separation  of  seg- 
mentation spheres  of  Amphyoxus 
ovum,  I,  691. 

Wind:  and  vegetal  bilateral  sym- 
metry, II,  142;  and  inner  vegetal 
tissue  differentiation,  II,  275-9, 
285,  288,  388;  and  proliferation  of 
Bryophyllum,  II,  295;  and  vegetal 
sap  movement,  II,  583,  584,  587; 
resume,  592-6. 

Wolff,  C. :  vegetal  fructification  and 
nutrition,  I,  283,  II,  179-80;  vege- 
tal vascular  system,  II,  283. 

Women  (see  Man). 

Wood  (see  Plants). 

YEAST:  fermentation,  I,  38;  fertil- 
ity, I,  581,  II,  440;  linear  aggre- 
gation, I,  587,  II,  23. 

ZEBHA  marks  in  horses,  I,  314. 

Zoology,  classification,  I,  124-5, 
380-9. 

Zoophytes,  structural  indefinite- 
ness,  I,  173. 

Zoospores,  unit-life  of,  I,  185. 

Zygote,  of  conjugating  Algce,  I,  283. 


THE   END. 


SOUTHERN 

UNIVERSITY  OF 

LIBRARY 


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ill  111  ii  HIM  i 

001  185  792     7 


